Introduction: Switched Mode Dual Variable Power Supply

Picture of Switched Mode Dual Variable Power Supply

A variable power supply is one of the most important pieces of equipment to have on the electronics workbench. It's only a matter of time before the voltage or current required in a circuit isn't practical for battery power.

Bench-top variable power supplies available today are typically transformer-fed linear voltage regulators which are both simple and inexpensive to manufacture. However these supplies are also large, heavy, and inefficient for most of their output voltage range. Many linear designs cannot operate anywhere near their rated output current when large Vo-Vi is required, but for low power applications they provide stable and noise free output.

Switched-mode power supplies are more than 90% efficient through almost the entire output voltage and current range, require much less space for heat sinks and transformer cores (90% less in medium to high current designs), and are as much as 5 times lighter than an equivalent linear power supply. But these advantages come at the expense of ripple, noise, and transient response; the three parameters that linear power supplies excel at.

I was working with some high power LED designs recently that required 2.5V to 9V and forward current between 1 Amp and 2 Amps. My LM317-based lab supply couldn't run more than a few minutes without tripping the thermal overload due to VI and Pmax limitations. This was a pretty hefty supply but it was getting too hot to operate reliably. So I decided to build my own 100W dual variable switched supply that could drive 2 Amps at an output voltage between 1V and 20V.

I wanted the regulation specifications to be competitive with the LM317 but my application did not require an extremely low ripple/noise figure. Current limiting and overload protection were important so independent voltage and current adjustments were required. And it would be nice to include a V/I meter for each supply for convenience.

This project will illustrate how to construct the 100W dual output switched mode variable power supply I have for about $150 using off-the-shelf modules and a prefabricated enclosure available on amazon.com or ebay.com. This power supply is compact, weighs less than 3 lbs, and deliverers professional appearance and performance competitive with commercial switched power supplies.

I've used this supply for a lot of heavy duty circuits including a DC motor controller and a 50W prototype audio amplifier with great results. I would not recommend this supply for precision Op-Amp or Radio Frequency circuits but for just about everything else it has worked really well.

Step 1: Review the Power Supply Design

Picture of Review the Power Supply Design

The Switched Mode Variable Power supply was designed using off-the-shelf modules that could be wired together using simple tools and basic soldering and wiring techniques. Two modules require modification so that front panel controls can be used instead of the PCB mounted multi-turn potentiometers included with the modules. These modifications are covered in a later step.

Power Supply Specifications

Input: 120VAC (+/- 15%) 60Hz 1A Full Load

Output 1: 1.2V - 20V @ 2 Amps

Output 2: 1.2V - 20V @ 2 Amps

Load Regulation: 0.5% Full Load

Line Regulation: .001% Full Input Range

Noise/Ripple: 20mV RMS, 100mVpp

General Circuit Description

AC power is connected to the supply via an IEC 320-C13 AC input module. AC safety ground is bonded to the power supply case and feed through to switching power supplies 1 and 2. The case of the power supply is grounded to the AC mains circuit. DC output ground is electrically isolated and independent of the AC mains ground.

Switching power supplies 1 and 2 are energized and de-energized through an illuminated DPST power switch. These supplies provide the constant 24V DC needed for DC-DC converters 1 and 2, the cooling fan, and the V/I displays. DC-DC converters 1 and 2 provide controlled output voltage and current to the power supply binding posts.

The output voltage and current set points are determined by two 50K Ohm and two 100K Ohm single-turn potentiometers. The DC positive power connection can be disconnected from the circuit by flipping the output power switch to the off (Down) position.

Two panel meters provide direct readout of the voltage set point and the current being consumed by the circuit attached to each power supply. The panel meters use a shunt type current sensor inline with the DC ground conductor. Power for each meter (< 20mA each) is taken directly from the 24V switching power supplies.

All power supplies are current and thermal overload protected and include last resort short circuit protection via fuses on switched power supply AC input and converter DC output. Cooling for the power supply is forced air via a 27 CFM fan using 24V @ 100mA drawn from switching power supply 2.

Design Tradeoffs

In order to keep total cost around $150, single-turn potentiometers were used instead of precision multi-turn potentiometers. Setting the output voltage is easier with 10-turn precision pots but a well made set would have increased the cost of the supply by $40. I decided to live with the fiddly nature of a general purpose single-turn pot for setting the output voltage. My applications do not require exact voltages. Close enough is good enough.

In order to keep costs low and simplify module wiring, I did not use DC-DC converters with an external voltage sense feature. This results in a slight degradation in load regulation (0.5% instead of 0.1%) due to the current shunt used in the current meter.

The 60mm x 60mm fan I used is overkill for this design and a bit louder than I would have liked. Lower CFM fans from Delta Electronics were non-stocked at Mouser so I decided to accept the overkill. With the supply installed on the instrument shelf I hardly notice the fan noise among all the other fan noise going on in the lab.

The current limit control is usable for only about half of it's range due to the 5A current rating of the DC-DC converter. I could have used two resistors to scale the current control to use the full rotation but did not feel the wiring complexity was worth the effort. I usually start with minimum current on a new circuit and slowly increase the current limit until a stable output voltage is achieved. I might add the scaling resistors at a later time if I think it's needed.

Step 2: Review the Parts List

Picture of Review the Parts List

Obtain the parts listed for the power supply project. Everything can be purchased from Amazon, eBay, and Mouser as of August 2015. All prices are current as of August 2015.

I keep an inventory of plastic cable ties and a variety of screws, nuts, and washers. I used a few of these in the finishing of the power supply and didn't list them in the parts list because their single unit cost is extremely low. Have on hand some 4" cable ties and a few pieces of #6 and #10 screws/nuts/washers.

With respect to the IAASR SimCase product there are a number of options available on the iaasr.com website. Be sure to choose the color you want, select fan/power/ac-input, and choose the 24V fan option.

Step 3: A Word About Electronics Enclosures

Picture of A Word About Electronics Enclosures

I love to see a useful project professionally finished in a nice enclosure. A well designed enclosure improves the durability and appearance of a DIY project and enhances the "I made that" pride in craftsmanship a builder earns from the work. However, many builders have the following complaints with the enclosure products available on the market today:

1. Project boxes cost more than the value they add, and sometimes more than the parts they enclose.

2. Poking holes of various shapes and sizes in an enclosure is hard work. If not done properly, the appearance of an expensive enclosure can be destroyed.

3. Designing the front and back panel layout is time consuming and not as much fun as designing and building the circuit.

4. It's difficult to find an enclosure that is the right size and shape for a particular type of project.

When I started looking for an enclosure for the dual switch-mode power supply project, I was shocked at the prices manufacturers were asking for a simple project case. The basic grey on grey cabinet without a single hole cut in it was $100 and up! If I was going to spend that kind of money, I better have a fully equipped machine shop to do the job right. But then the sizes available were either too large or too small, too deep or too tall. I didn't want unpainted aluminum or battleship grey. None of the manufacturers in Mouser or Digikey had anything that fit my design in an affordable, easy to build way.

While searching through Amazon and eBay I happened to discover IAASR (www.iaasr.com) and their line of SimCase and HexCase enclosures. These are purpose-built enclosures with holes already cut and parts already installed for specific use-cases. When I saw the IAASR SimCase I said "That's exactly what I need!". The SimCase product is designed by IAASR to house a DIY power supply. It includes an EMI shielded mild steel enclosure, the AC input module, an illuminated AC power switch, a fan, and ventilation holes validated with thermal analysis software... for $49. That's a layout I didn't have to design, parts I didn't have to research and order, and holes I didn't have to cut which would save me a huge amount of time. IAASR offers their enclosures in 5 standard colors and 15 custom colors which means your project can look cool like you imagined it would instead of like a low-bid government job.

But that's not all. I contacted Shiraz Macuff, CEO, about the front panel design. He says, "Send me a layout and we'll cut the holes before we ship at no extra charge". That's service you can only get when you order quantity 10,000 from any other manufacturer. I ordered quantity one from IAASR. It turns out that IAASR is disrupting the enclosure market with purpose-built products that save time, add value, and can be mass-customized to meet the requirements of the DIY, prototype, and small-medium volume manufacturer. IAASR enclosures can make your DIY project seem more like a professionally designed kit. And you don't have to worry about accidentally mutilating your enclosure with a power drill.

In this article, I am including the design drawings and assembly steps for a generic enclosure. But I strongly encourage you to use the IAASR SimCase product indicated in the parts list instead of trying to make do with the generic cases sold elsewhere. You will enjoy the building experience much more when you can focus on the assembly work and not have to put up with the dull, dirty, and sometimes dangerous fabrication work. Shiraz and his team can save you a lot of time.

Step 4: Prepare the Enclosure

Picture of Prepare the Enclosure

The power supply project described in this article requires an enclosure with the following minimum dimensions:

7" Wide x 3.5" High x 6" Deep

Although the enclosure can be constructed of any rigid material (plastic, aluminum, etc.) I recommend using a material that can provide some EMI shielding and AC ground fault protection. In this design I used a painted steel enclosure from IAASR which had the holes cut and AC input, AC switch, and fan already installed. I removed the components for illustration purposes showing the product being fully assembled.

Attached below are the detailed shop drawings needed to fabricate the enclosure front, back, and bottom panels.

The shop drawings are full-scale and can be used as a template for transferring the layouts to the enclosure. When cutting holes, I strongly recommend protecting the panels with two layers of painters tape to prevent accidental scratches and tool marks from marring the finish.

If using a power drill for round holes, be sure to use a thick piece of wood at the back to avoid bending/cracking the panel and to act as a drill stop. Square openings can be cut and smoothed out with a Dremel tool cutoff wheel. Curved openings in steel can be rough cut with a Dremel tool cutoff wheel and finished up with a Tungsten Carbide cutter.

If you purchase the SimCase enclosure from IAASR you can skip this step.

Step 5: Install the Case Feet

Picture of Install the Case Feet

1. Remove the plastic case feet from the package and verify all mounting hardware is present.

2. Install three plastic feet as shown in the illustration above. Do not install the left rear foot (next to the AC input module position) yet.

3. Cut one piece of green #18 AWG wire 2" long, and one piece of green #18 AWG wire 4" long. Strip and tin 1/4" from each wire end.

4. Insert one end of the 2" and 4" green wire into a #8 Ring Terminal and solder the wires to the terminal.

5. Solder a 0.25" Female Quick Disconnect connector to the free end of the 2" green wire.

4. Scrape off the paint inside the case around the foot screw hole so that the ground cable ring terminal makes metal-to-metal contact with the case.

5. Install the last plastic foot as shown in the illustration above making sure that the ground cable ring connector is installed first, then the lock washer, and finally the hex nut.

Step 6: Install the AC Input Module

Picture of Install the AC Input Module

Note: If you purchased a case from IAASR the AC Input Module is already installed. Skip to the next step.

1. Insert the AC Input Module oriented as illustrated in the diagram above.

2. Fasten the AC Input Module to the case with two #8 machine screws and hex nuts.

3. Firmly tighten the machine screws but do not over-tighten.

For reference, the AC Input Module datasheet is included below:

Step 7: Install the AC Power Switch

Picture of Install the AC Power Switch

Note: If you purchased a case from IAASR the AC Power Switch is already installed. Skip to the next step.

1. Insert the AC Power Switch as illustrated in the diagram above.

2. Push the AC Power Switch into the case cutout until the top and bottom retaining clips snap into place.

Step 8: Install the Cooling Fan

Picture of Install the Cooling Fan

Note: If you purchased a case from IAASR the Cooling Fan is already installed. Skip to the next step.

1. Hold the fan cover against the outside fan opening and thread a single #8x1" machine screw through the cover and into the case.

2. Determine the direction of fan flow from the datasheet and orient the fan so that it's exhaust side is facing the fan cover.

3. While holding the screw in place, slide the fan (wire leads facing up) over the machine screw and thread a #8 hex nut onto the screw until both the fan cover and fan are held loosely against the case.

4. Line up the cover and fan so that each machine screw can be threaded through the cover and fan.

5. Insert the remaining three machine screws through the cover and fan.

6. Thread a #8 hex nut onto each machine screw until all four corners of the cover and fan are held loosely in place against the case.

7. Tight each machine screw until firm. Do not over-tighten.

For reference, the fan datasheet is included below.

Step 9: Wire AC Input, AC Switch, and Ground

Picture of Wire AC Input, AC Switch, and Ground

1. Connect the ground lead quick disconnect to the center lug of the AC input module as illustrated in the diagram above.

2. Cut one piece of white #18 AWG wire 1.5" long, and one piece of black #18 AWG wire 1.5" long. Strip and tin 1/4" from each wire end.

3. Solder two 0.25" Female Quick Disconnect connectors to the black wire.

4. Solder two 0.25" Female Quick Disconnect connectors to the white wire.

5. Connect one end of the black lead to the left lug of the AC input module. Connect the other end to the bottom left lug of the AC Power switch. Refer to the diagram above to verify that the black lead is connected properly.

6. Connect one end of the white lead to the right lug of the AC input module. Connect the other end to the bottom right lug of the AC Power switch. Refer to the diagram above to verify that the black lead is connected properly.

Step 10: Install Front Panel Binding Posts

Picture of Install Front Panel Binding Posts

The binding posts from Vktech are ruggedly constructed and include a lot of mounting hardware which makes them a good value for the money. However the center conductors are about 1/2" longer than necessary which can waste a lot of space in the cabinet. To keep the supply compact, the binding posts must be modified as indicated in the above diagram and the following steps:

1. Remove all the hardware from the binding post and pull off the rear plastic insulator.

2. Unscrew the Red and Black post caps several turns and push down firmly from the end each cap to make sure the center conductor is seated all the way down on the front plastic insulator.

3. Using a Sharpie pen and a ruler, measure and place a mark on the metal center conductors 1/2" from the front plastic insulator (refer to diagram above).

4. Using a cutoff wheel and a Dremel tool, cut through the metal center conductors at the marks to remove the top portion of the center conductors (refer to diagram above).

5. Insert the front portion of the binding post into the case (refer to above diagram).

6. Slide on the rear plastic insulator, followed by two flat washers, and a lock washer on each center conductor (refer to above diagram). All hardware is included with the Vktech binding posts.

7. Thread a hex nut onto each binding post and hand tighten while sliding the insulators back and forth until they are seated properly in their holes. Do not tighten fully yet.

Step 11: Install Front Panel Output Switches

Picture of Install Front Panel Output Switches

1. Remove the outer hex nut, lock washer, and flat washers from the SPST toggle switch.

2. Hand tighten the inner hex nut until snug against the switch body.

3. Install the large flat washer with the tab facing toward the switch body as illustrated in the diagram above.

4. Insert the SPST toggle switch into the bottom left hole in the front panel.

5. Orient the SPST toggle switch so that the two solder lugs are closest to the bottom of the enclosure as indicated in the diagram and photo above.

6. Install the small flat washer onto the toggle barrel at the front of the enclosure as indicated in the diagram above.

7. Thread the hex nut onto the toggle barrel until hand tight.

8. Hold the switch body in position and firmly tighten the hex nut. The switch body should not move when the toggle switch is operated. If it does, tighten the hex nut until the switch body does not move.

Repeat the above for the SPST toggle switch on the bottom right of the front panel.

For reference, the SPST toggle switch datasheet is included below.

Step 12: Install Potentiometers

Picture of Install Potentiometers

1. Thread a hex nut onto two 50K Ohm potentiometers and hand tighten until snug.

2. Insert the 50K Ohm potentiometers into the positions indicated in the above diagram.

3. Install a flat washer onto the 50K Ohm potentiometer shafts.

4. Thread a hex nut onto the 50K Ohm potentiometer shafts until hand tight.

5. Hold the potentiometer body in the position indicated in the above diagram and firmly tighten the hex nut. The potentiometer body should not move when the shaft is turned throughout it's full range of motion. If it does, tighten the hex nut until the potentiometer body does not move.

Repeat the above steps with the 100K Ohm potentiometers.

For reference, the potentiometer datasheet is included below.

Step 13: Install V/I Meters

Picture of Install V/I Meters

1. Insert the V/I displays half way into the cutouts provided.

2. Using fingertips or a screwdriver, depress the plastic retaining clips on the display bezel so that they clear the panel cutout.

3. While keeping the plastic retaining clips depressed, press the display into the cutout until the clips snap into place. Use care not to bend the panel while installing the displays.

Note: On some V/I displays the plastic retaining clips are too thick or too rigid to allow the display to be easily installed without bending the front panel. The best solution is to trim some of the plastic from the retaining clips until the display can be installed with reasonable force.

Note: On some displays the plastic retaining clips are too far back from the front bezel which causes the displays to fit loosely in the front panel. The best solution is to hold the display against the front panel while running a small bead of hot glue along the left and right side of the display (inside the case). Use caution to avoid getting glue on the outside front panel.

Step 14: Install Control Knobs

Picture of Install Control Knobs

Insert the Red and Blue control knobs onto the front panel potentiometers as indicated in the above diagram.

The enclosure is now complete with all attachments and controls. The next section will describe how to install and wire the power supplies and converters.

For reference, the knob datasheet is attached below.

Step 15: Prepare the DC-DC Converters

Picture of Prepare the DC-DC Converters

The DC-DC Converters used for this project accept a wide range of input voltages (5V - 32V) and convert that to a variable voltage between 1V and 20V with adjustable current limit between 0.1A and 3A. The DC-DC converters are operated in step-down switch-mode from a 24V DC input. The DROK converters are compact, easy to use, and >95% efficient for most of their range.

Adjustment of the output voltage and current limit is accomplished with two multi-turn trimmer resistors. In order to bring these adjustments to a potentiometer on the front panel, it is first necessary to remove the trimmer resistors. The quickest way to do that is to carefully cut them off of the board with a small pair of wire cutters. This might seem extreme but the PCB is very thick and the trimmer resistors are soft and easy to cut through. When the trimmer resistor body is removed, there will be three small component leads sticking up that can be easily desoldered. I believe this method is faster and results in less chance that pads and traces will be damaged from excess heat. I used the cut and release method on both converters with no problems. If you have a vacuum powered desoldering station by all means give that a try. Start with DC-DC Converter #1:

1. Notice that on each board the parameter that the trimmer resistor adjusts is indicated in white letters. On a blank piece of paper, draw an outline of the board and make a note of which trimmer is the voltage adjustment (CV) and which trimmer is the current adjustment (CC). On the DC-DC converters used for this project, the voltage trimmer was on the outside of the board and the current trimmer was next to the voltage trimmer.

2. Starting with the outside trimmer, use a small pair of wire cutters to cut a small groove into the outside corner of the trimmer body. Use one hand to hold onto the trimmer resistor while cutting with the other hand. Use only the force necessary to keep the wire cutter blades in contact with the trimmer body. Let the scissor action of the wire cutters do the work. The goal is to cut through the plastic body of the trimmer. Don't try to cut too much at one time.

3. When the corner gap is deep enough, begin cutting a groove into the adjacent corner next to the inside trimmer. When the second groove is deep enough, cut through the side of the trimmer body. There may be a crunching sound as the wire cutters reach internal ceramic components. Do not be concerned. Only the trimmer is being damaged.

4. Starting with the opposite outside corner of the trimmer, begin cutting a groove into the trimmer body.

5. When the groove is deep enough, cut through the short side of the trimmer body. Don't attempt to cut through the metal adjustment screw.

6. At this point, the trimmer body will crack and separate in half. Remove the ceramic disk and brass adjustment hardware.

7. Using needle nose pliers, straighten the three wires sticking up from the remains of the trimmer body.

8. Carefully cut away the remaining bottom of the trimmer body leaving only the three wires sticking up from the PCB. Do not cut these wires as the remaining length will aid in removing the wires from the PCB.

9. Repeat steps 2 through 8 for the remaining trimmer resistor.

10. Desolder the trimmer wires and clear as much as solder as possible from the pad holes.

By working carefully and slowly, it is easy to remove the trimmer resistors without harming nearby components or the PCB.

11. Remove the 10A output fuse and replace with a 3A fuse.

12. Cut 4" lengths of #28 or smaller stranded hookup wire. Choose wire with different color jackets to make identification easier when the wires are soldered to the potentiometers in a later step.

13. Strip 1/4" insulation from both ends of each wire and tin the ends with solder.

14. Solder each wire to the DC-DC converter solder pads as illustrated above.

Repeat the above steps for DC-DC Converter #2.

Note: In the illustration above the potentiometer pads are labeled 1, 2, and 3. These numbers (and wire colors) will be referred to when soldering the converter leads to the potentiometers. The jacket color chosen in this step is arbitrary. Any color can be used as long as the builder remembers to match the instructable colors with the actual colors so that the potentiometer leads are soldered to the correct pin.

Step 16: Attach Enclosure Standoffs Using Screws and Insulating Washers

Picture of Attach Enclosure Standoffs Using Screws and Insulating Washers

The IAASR SimCase product comes with 9 standoffs and 18 machine screws that work perfectly with the DROK DC-DC converters. Remove these standoffs from the case for use with mounting the converters.

If using a different enclosure, the converters will require eight #8 hex standoffs and 16 #8 machine screws.

Insulating washers are used ensure no exposed traces on the DC-DC converters come in contact with the enclosure and AC safety ground. They also add some height to the standoffs so that there is enough clearance for the PCB heat sink.

1. Insert one insulating washer on a #8 machine screw.

Note: The washer may be a tight fit on some screws. If the washer cannot be easily installed on the machine screw, increase the diameter of the washer slightly with a push drill.

2. Insert the machine screw and washer into a mounting hole on DC-DC converter #1.

3. Insert one insulating washer onto the screw where it exits the opposite side of the PCB.

4. Thread the hex standoff onto the screw until hand tight.

5. While holding the standoff with a pair of pliers or adjustable wrench, firmly tighten the screw.

6. Repeat steps 1 through 5 for the remaining PCB mounting holes.

Repeat the above for the DC-DC Converter #2.

Step 17: Install DC-DC Converters

Picture of Install DC-DC Converters

1. Line up the holes in the front of the enclosure with the hex standoffs of DC-DC Converter #1.

2. Insert a #8 flat washer onto a #8 0.5" machine screw.

3. Tread the machine screw through the bottom of the enclosure and into the hex standoff.

4. Hand tighten the machine screw.

5. Repeat steps 2 through 4 for each remaining hex standoff.

6. When all 4 machine screws and flat washers are installed, firmly tighten each screw.

7. Repeat steps 1 through 6 for DC-DC converter #2.

Step 18: Build Front Panel Cables

Picture of Build Front Panel Cables

1. Cut a piece of 22AWG stranded red wire 4" long. Strip 1/4" insulation from each end and tin with solder.

2. Solder a #10 Ring Connector on one end of the 4" wire wire.

3. Cut a piece of 22AWG stranded red wire 3" long. Strip 1/4" insulation from each end and tin with solder.

4. Solder a #10 Ring Connector on one end of the 3" wire.

5. Cut a piece of 22AWG stranded red wire 5" long. Strip 1/4" insulation from each end and tin with solder.

6. Cut a piece of 22AWG stranded red wire 6" long. Strip 1/4" insulation from each end and tin with solder.

7. Strip 1/4" insulation from each lead attached to the 3-pin display connector and tin with solder.

8. Solder a #10 Ring connector to both Yellow leads attached to the 3-pin display connector.

9. Strip 1/4" insulation from each lead attached to the 2-pin display connector and tin with solder.

Step 19: Wire the Front Panel Power Connections

Picture of Wire the Front Panel Power Connections

Use the diagram above in parallel with the following instructions to complete the output power connections on the front panel.

1. Remove the hex bolt and lock washer from the positive (Red) Power Supply 1 binding post.

2. Install the Ring Terminal of Cable A followed by the lock washer onto the positive (Red) Power Supply 1 binding post.

3. Thread the hex bolt onto the positive (Red) Power Supply 1 binding post and tighten firmly. Do not over-tighten the hex bolt.

4. Solder the free end of Cable A onto the lower pin of the Power Supply 1 SPST power switch.

5. Remove the hex bolt and lock washer from the positive (Red) Power Supply 2 binding post.

6. Install the Ring Terminal of Cable B followed by the lock washer onto the positive (Red) Power Supply 2 binding post.

7. Thread the hex bolt onto the positive (Red) Power Supply 2 binding post and tighten firmly. Do not over-tighten the hex bolt.

8. Solder the free end of Cable B onto the lower pin of the Power Supply 2 SPST power switch.

9. Solder one end of Cable C to the upper pin of the Power Supply 1 SPST power switch. Route the other end of Cable C to the output connector of DC-DC Converter 1 but do not attach yet.

10. Solder one end of Cable D to the upper pin of the Power Supply 2 SPST power switch. Route the other end of Cable D to the output connector of DC-DC Converter 2 but do not attach yet.

11. Remove the hex bolt and lock washer from the negative (Black) Power Supply 1 binding post.

12. Install the Ring Terminal of Cable E followed by the lock washer onto the negative (Black) Power Supply 1 binding post.

13. Thread the hex bolt onto the negative (Black) Power Supply 1 binding post and tighten firmly. Do not over-tighten the hex bolt.

14. Remove the hex bolt and lock washer from the negative (Black) Power Supply 2 binding post.

15. Install the Ring Terminal of Cable F followed by the lock washer onto the negative (Black) Power Supply 2 binding post.

16. Thread the hex bolt onto the negative (Black) Power Supply 2 binding post and tighten firmly. Do not over-tighten the hex bolt.

17. Plug the 3-pin connector of Cable E onto Power Supply 1 display module.

18. Plug the 3-pin connector of Cable F onto Power Supply 2 display module.

19. Insert the Red wire from Cable E and the Red wire Cable C into the DC-DC Converter 1 output terminal block positive (+) position and firmly tighten the terminal block screw. Do not over-tighten the terminal block screw.

20. Insert the Black wire from Cable E into the DC-DC Converter 1 output terminal block negative (-) position and firmly tighten the terminal block screw. Do not over-tighten the terminal block screw.

21. Insert the Red wire from Cable F and the Red wire Cable D into the DC-DC Converter 2 output terminal block positive (+) position and firmly tighten the terminal block screw. Do not over-tighten the terminal block screw.

22. Insert the Black wire from Cable F into the DC-DC Converter 2 output terminal block negative (-) position and firmly tighten the terminal block screw. Do not over-tighten the terminal block screw.

Step 20: Wire the Front Panel Potentiometers

Picture of Wire the Front Panel Potentiometers

Use the diagrams above in parallel with the following instructions to complete the potentiometer connections on the front panel.

1. Solder the CV leads from DC-DC Converter 1 to the 50K voltage adjust potentiometer. Solder pins 1 through 3 on the DC-DC Converter PCB to pins 1 through 3 on the potentiometer as illustrated above.

2. Solder the CC leads from DC-DC Converter 1 to the 100K current adjust potentiometer. Solder pins 1 through 3 on the DC-DC Converter PCB to pins 1 through 3 on the potentiometer as illustrated above.

3. Solder the CV leads from DC-DC Converter 2 to the 50K voltage adjust potentiometer. Solder pins 1 through 3 on the DC-DC Converter PCB to pins 1 through 3 on the potentiometer as illustrated above.

4. Solder the CC leads from DC-DC Converter 2 to the 100K current adjust potentiometer. Solder pins 1 through 3 on the DC-DC Converter PCB to pins 1 through 3 on the potentiometer as illustrated above.

Step 21: Dress Front Panel Wiring With Plastic Cable Ties

Picture of Dress Front Panel Wiring With Plastic Cable Ties

Double-check front panel wiring. Using small plastic cable ties, dress the front panel cabling for appearance.

Step 22: Wire the 24V Power Supplies

Picture of Wire the 24V Power Supplies

1. Remove one screw above the barrier strip from each 24V power supply as indicated in the above illustration.

2. Install a plastic P-Clip using the screw just removed oriented as indicated in the above diagram.

3. Build cables A, B, C, and D as indicated in the diagram above. Strip 1/4" insulation from both ends of each wire. Solder all connectors and tin all bare wire ends.

4. Attach cables A and C to 24V Power Supply 1 as indicated in the above illustration. The Black wire of cable A connects to the barrier screw marked 'L'. The White wire of cable A connects to the barrier screw marked 'N'. The Black wire of cable C connects to the barrier screw marked '-V'. The Red wire of cable C connects to the barrier screw marked '+V'.

5. Attach cable D to 24V Power Supply 2 as indicated in the above illustration. The Black wire of cable D connects to the barrier screw marked '-V'. The Red wire of cable D connects to the barrier screw marked '+V'.

6. Place 24V Power Supply 1 and 2 back to back as illustrated in the diagram above.

7. Connect cable B between 24V Power Supply 1 and 2 by threading the wire through the P-Clips. The Black wire of cable B connects to the barrier screw marked 'L' on both supplies. The White wire of cable B connects to the barrier screw marked 'N' on both supplies. The Green wire of cable B connects to the barrier screw marked 'G' on both supplies.

Step 23: Install the 24V Power Supplies

Picture of Install the 24V Power Supplies

Lift both 24V Power Supplies and place them in the enclosure as indicated in the diagram above. Verify that the power supply modules align with the enclosure mounting holes but do not fasten the power supplies at this time.

Step 24: Complete Wiring of the 24V Power Supplies

Picture of Complete Wiring of the 24V Power Supplies

1. Plug the 2-Pin connectors G and H from Step 18 into the V/I displays.

2. Connect cable C in Step 22 from 24V Power Supply 1 to DC-DC Converter 1. The Red wire connects to the 'IN+' screw terminal of the DC-DC Converter as shown in the diagram above. The Black wire connects to the 'IN-' screw terminal of the DC-DC Converter as shown in the diagram above.

3. Connect 2-Pin cable C to DC-DC Converter 1. The Red wire connects to the 'IN+' screw terminal of the DC-DC Converter as shown in the diagram above. The Black wire connects to the 'IN-' screw terminal of the DC-DC Converter as shown in the diagram above.

4. Connect cable D in Step 22 from 24V Power Supply 2 to DC-DC Converter 2. The Red wire connects to the 'IN+' screw terminal of the DC-DC Converter as shown in the diagram above. The Black wire connects to the 'IN-' screw terminal of the DC-DC Converter as shown in the diagram above.

5. Connect 2-Pin cable D to DC-DC Converter 2. The Red wire connects to the 'IN+' screw terminal of the DC-DC Converter as shown in the diagram above. The Black wire connects to the 'IN-' screw terminal of the DC-DC Converter as shown in the diagram above.

6. Connect the Cooling Fan cable to 24V Power Supply 2 as shown in the diagram above. The Red wire connects to the barrier screw marked '+V' on 24V Power Supply 2. The Black wire connects to the '-V' barrier screw on 24V Power Supply 2. The Cooling Fan Blue tachometer wire is not used.

7. Secure the 24V power supplies to the enclosure with four #8 machine screws, washers, and lock nuts.

7. Dress all wires and secure with cable ties.

Step 25: Testing Before Power-On

Before plugging in and powering on the completed power supply for the first time, perform the following checks:

1. Using a digital VOM set to Ohms, measure the resistance between the 'L' and 'N' binding screws on 24V Power Supply 1. The VOM should read very high (>10K Ohms) or infinite resistance. If the VOM reads low resistance or a short circuit, verify that all AC wiring is correct using the power supply schematic diagram. DO NOT CONNECT THE POWER SUPPLY TO AN AC OUTLET UNTIL THE MEASUREMENT IS CORRECT (VERY HIGH OR INFINITE RESISTANCE).

2. Using a digital VOM set to OHMS, measure the resistance between the 'L' and 'G' binding screws on 24V Power Supply 1. The VOM should read very high (>10K Ohms) or infinite resistance. If the VOM reads low resistance or a short circuit, verify that all AC wiring is correct using the power supply schematic diagram. DO NOT CONNECT THE POWER SUPPLY TO AN AC OUTLET UNTIL THE MEASUREMENT IS CORRECT (VERY HIGH OR INFINITE RESISTANCE).

3. Using a digital VOM set to OHMS, measure the resistance between the 'N' and 'G' binding screws on 24V Power Supply 1. The VOM should read very high (>10K Ohms) or infinite resistance. If the VOM reads low resistance or a short circuit, verify that all AC wiring is correct using the power supply schematic diagram. DO NOT CONNECT THE POWER SUPPLY TO AN AC OUTLET UNTIL THE MEASUREMENT IS CORRECT (VERY HIGH OR INFINITE RESISTANCE).

If any of the above measurements are not correct and all wiring has been verified, do not proceed and do not connect the power supply to an AC outlet. Contact the 24V AC Power Supply representative for further instructions.

4. Make sure the power supply output SPST switches are in the OFF (Down) position.

5. Using a digital VOM set to OHMS, measure the resistance between the Positive (Red) output binding post and the Negative (Black) output binding post of power supply 1 (Left Side). The VOM should read very high (>10K Ohms) or infinite resistance. If the VOM reads low resistance or a short circuit, verify that all DC-DC Converter output wiring is correct using the power supply schematic diagram. Check that the output binding post is properly seated in the front panel and that there are no bits of wire or solder touching the power supply enclosure or other circuit connections. Do not proceed until the resistance is within the indicated range (>10K Ohms).

6. Using a digital VOM set to OHMS, measure the resistance between the Positive (Red) output binding post and the Negative (Black) output binding post of power supply 2 (Right Side) . The VOM should read very high (>10K Ohms) or infinite resistance. If the VOM reads low resistance or a short circuit, verify that all DC-DC Converter output wiring is correct using the power supply schematic diagram. Check that the output binding post is properly seated in the front panel and that there are no bits of wire or solder touching the power supply enclosure or other circuit connections. Do not proceed until the resistance is within the indicated range (>10K Ohms).

7. Put both power supply output SPST switches in the ON (Up) position.

8. Using a digital VOM set to OHMS, measure the resistance between the Positive (Red) output binding post and the Negative (Black) output binding post of power supply 1 (Left Side). The VOM should read very high (>10K Ohms) or infinite resistance. If the VOM reads low resistance or a short circuit, verify that all DC-DC Converter output wiring is correct using the power supply schematic diagram. If after verifying that circuit wiring is correct, do not proceed. Contact the DC-DC Converter representative for further instructions.

9. Using a digital VOM set to OHMS, measure the resistance between the Positive (Red) output binding post and the Negative (Black) output binding post of power supply 2 (Right Side). The VOM should read very high (>10K Ohms) or infinite resistance. If the VOM reads low resistance or a short circuit, verify that all DC-DC Converter output wiring is correct using the power supply schematic diagram. If after verifying that circuit wiring is correct, do not proceed. Contact the DC-DC Converter representative for further instructions.

10. Attach a power cord to the power supply AC Input Module. DO NOT PLUG THE POWER CORD INTO AN AC OUTLET.

11. Turn on the AC Power Switch.

12. Using a digital VOM set to OHMS, measure the resistance between the power cord hot and neutral conductors. The VOM should read very high (>10K Ohms) or infinite resistance. If the VOM reads low resistance or a short circuit, verify that all AC wiring between the AC Input Module and the AC Power Switch is correct using the power supply schematic diagram. DO NOT CONNECT THE POWER SUPPLY TO AN AC OUTLET UNTIL THE MEASUREMENT IS CORRECT (VERY HIGH OR INFINITE RESISTANCE).

13. Using a digital VOM set to OHMS, measure the resistance between the power cord hot and ground conductors. The VOM should read very high (>10K Ohms) or infinite resistance. If the VOM reads low resistance or a short circuit, verify that all AC wiring between the AC Input Module and the AC Power Switch is correct using the power supply schematic diagram. DO NOT CONNECT THE POWER SUPPLY TO AN AC OUTLET UNTIL THE MEASUREMENT IS CORRECT (VERY HIGH OR INFINITE RESISTANCE).

14. Using a digital VOM set to OHMS, measure the resistance between the power cord neutral and ground conductors. The VOM should read very high (>10K Ohms) or infinite resistance. If the VOM reads low resistance or a short circuit, verify that all AC wiring between the AC Input Module and the AC Power Switch is correct using the power supply schematic diagram. DO NOT CONNECT THE POWER SUPPLY TO AN AC OUTLET UNTIL THE MEASUREMENT IS CORRECT (VERY HIGH OR INFINITE RESISTANCE).

Step 26: Attach the Top Cover

Picture of Attach the Top Cover

Place the enclosure top over the power supply chassis and line up the holes in the top cover with the holes in the base. Secure the top to the base with the screws supplied by the enclosure vendor.

Step 27: Power-On Testing

Picture of Power-On Testing

1. Verify that the back panel AC Power Switch is in the Off position.

2. Verify that the front panel Output SPST switches are in the Off (Down) position.

3. Verify that the front panel Voltage and Current controls are rotated fully counter clockwise.

3. Plug the power supply power cord into an AC outlet.

4. Turn on the AC Power Switch

The AC Power Switch will illuminate. If it does not illuminate, verify that the AC Outlet is energized and that the power cord is fully plugged into the outlets at both ends. IF THE AC POWER SWITCH DOES NOT ILLUMINATE, REMOVE THE POWER CORD FROM THE AC OUTLET.

5. The Cooling Fan will turn on and the front panel V/I displays will illuminate. If the cooling fan or V/I displays do not come on, turn off the AC power switch and disconnect the power cord from the AC outlet. Remove the top cover and verify that the fan or display wiring is correct. If the fan or display wiring is correct, contact the fan or display representative for further instructions.

6. The front panel V/I displays should indicate approximately 1.00V and 1.50V for the output voltage and 0.00A for the output current. If the V/I display indicates approximately 20V with the voltage controls fully counter-clockwise, turn off the AC power switch and disconnect the power cord from the AC outlet. Remove the top cover and verify that the voltage potentiometer wiring is correct. If the potentiometer wiring is correct, contact the DC-DC Converter representative for further instructions.

7. Rotate the Power Supply 1 (Left Side) voltage control clockwise. The V/I display should show the output voltage increasing as the control is turned clockwise and decreasing when the control is turned counter-clockwise. If the V/I display does not change in value as the voltage control is rotated, turn off the AC power switch and disconnect the power cord from the AC outlet. Remove the top cover and verify that the voltage potentiometer wiring is correct. If the potentiometer wiring is correct, contact the DC-DC Converter representative for further instructions.

8. Rotate the Power Supply 2 (Right Side) voltage control clockwise. The V/I display should show the output voltage increasing as the control is turned clockwise and decreasing when the control is turned counter-clockwise. If the V/I display does not change in value as the voltage control is rotated, turn off the AC power switch and disconnect the power cord from the AC outlet. Remove the top cover and verify that the voltage potentiometer wiring is correct. If the potentiometer wiring is correct, contact the DC-DC Converter representative for further instructions.

Optional Load Testing

9. Rotate all front panel voltage and current controls fully counter clockwise.

10. Verify that both output SPST switches are in the Off (Down) position.

10. Attach a 10 Ohm, 20W resistor between the Positive (Red) and Negative (Black) output binding posts of Power Supply 1 (Left Side).

11. Flip the Power Supply 1 output SPST switch to the ON position.

12. The V/I display should show an output current of approximately 0.10A

13. Flip the Power Supply 1 output SPST switch to the OFF position.

14. Rotate the voltage control until the V/I display reads 10V.

15. Flip the Power Supply 1 output SPST switch to the ON position.

16. The V/I display should indicate a reduced output voltage and between 0.10A and 0.20A.

17. Slowly rotate the current control until the V/I display reads 10V and approximately 1.00A.

18. Flip the Power Supply 1 output SPST power switch to the OFF position.

19. Remove the 10 Ohm, 20W resistor from the output binding post of Power Supply 1.

20. Attach a 10 Ohm, 20W resistor between the Positive (Red) and Negative (Black) output binding posts of Power Supply 2 (Right Side).

21. Repeat steps 11 through 19 for Power Supply 2.

22. Turn off the AC Power Switch and disconnect the AC Power Cord from the AC wall outlet.

The Dual Switched-Mode Power Supply is ready for use.

Comments

mikepeg14 made it! (author)2015-09-21

Well I got my case and some of the parts in last week and assembled and tested the first half of the PSU. It worked great with one exception. The case that IAASR supplied to me had a deeper fan so I had to mount it on the outside of the case and it was also a 12v version ( found that one out the hard way. oops.). I had to order a 24v fan and now that it and a couple of other parts arrived, I present my version of your power supply unit. It works Great! Thanks again for the thorough instructable. It made this project a breeze and one that I hope to use for many years to come.! I encourage everyone who is interested in this one to give it a go.

netzener (author)mikepeg142015-09-21

OH MY!!! You... Are...Totally... Awesome!

Apologies regarding the fan. I notified IAASR about the 24V fan requirement just to make sure they were aware of it since their production standard has a 12V fan installed. I suspect that my notice did not get caught in manufacturing fast enough. Shiraz has updated the manufacturing build list and should have the fans in stock by now. You know how it is when you're doing something new.

I love that black case. And it looks like everything fit as expected. Not that I'm surprised but sometimes variances creep up in unexpected places.

Man yours looks totally cool and professional! Maybe I should have done the black case :-) Great job!

netZener

netzener (author)netzener2015-09-21

To further illustrate the Switched Mode Dual Variable Power Supply that mikepeg14 and I have working successfully in the lab, below is an image of the Blue version powering a Voltage Source circuit which is one of the building block for my next article. Mike has the Black version running which as you can see is very slick.

The Switched Mode Dual Variable Power Supply isn't a device that just sits on a shelf. It's a very useful instrument that is powerful, compact, and light-weight. It's something you can use every day if you are an electronics experimenter. I'd love to see a picture of the Red and White versions. If anyone has an opportunity to build one, post an "I Made It" here so Mike and I can see how it turned out.

netZener

RobinA47 (author)2017-09-22

Is there short circuit & over load protection ?

Thanks

utiofuse (author)2017-07-21

Hi, thanks for sharing your project! I found the 58v 3a fuse only on mauser but the shipping cost is too high. It's ok if i'm going to use a 32v 3a fuse?

netzener (author)utiofuse2017-07-21

Yes. That fuse will work as long as it fits on the PCB. The fuse voltage rating is the maximum voltage that a fuse can clear a fault against and still meet it's specified clearing time. The output of the converters is only about 20V and the input voltage from the switching supplies is only 24V. So your 32V fuse is still safe if something goes wrong with the DC-DC converters. You have a worst case safety margin of 133%.

NetZener

utiofuse (author)netzener2017-07-22

Thank you so much for solving my dubts. One last question; what is the size of the fuse? I see that there are different forms but in the description of the converters I can't find the measures of the fuse.

Iqbal Samin (author)2017-06-05

Can you please give me the link to your DC-DC converter board?

netzener (author)Iqbal Samin2017-06-05

Sure. As of June 5th, 2017 the US Amazon link is:

https://www.amazon.com/gp/product/B00JKG57T4/ref=o...

The product description is:

DROK DC/DC Automatic Boost Buck Converter Module 60W Constant Voltage/Current Car Voltage Regulator DC5-32V to 1.25-20V

The Amazon ASIN number is:

B00JKG57T4

All of the above is subject to product profile changes made by DROK and Amazon so check in with me if you are having trouble finding the modules.

NetZener

Iqbal Samin (author)netzener2017-06-05

Thank you so much!

I have another question, how hot does it get at max output current? Is the heatsink good enough?

netzener (author)Iqbal Samin2017-06-05

Maximum output current for this power supply project at full voltage (20V) is 2A so the DC output power per converter is 40W. DROK claims "up to" a 96% conversion efficiency which would leave 1.6W to be dissipated by the power inductor and switching transistors in the converter. However, assuming at least a 90% conversion efficiency for a worst-case input voltage/output-current, then about 4W would be dissipated. From my experiments with this power supply design, the converters are more efficient than 90% and the heatsinks get warm (43 DegC) at full power with the fan off after about 5 minutes of operation. With the fan running they are only about 5 DegC above ambient temperature. The heatsinks are sufficient for convection or forced-air cooling in a commercial grade environment (0 DegC to 85 DegC). I wouldn't recommend enclosing them in a cabinet without some kind of ventilation which is why I recommended the case from IAASR or a home-built one just like it.

NetZener

Iqbal Samin (author)netzener2017-06-05

I'm planning to make a switch mode power supply like yours, but I want the voltage and current control to go down to 0 and the voltage control up to 30V. I guess I need to find some other DC-DC converter.

Iqbal Samin (author)Iqbal Samin2017-06-05

and what is the minimum and maximum output voltage and current of this module you have measured? I'm asking this because what the seller says might not always be right.

netzener (author)Iqbal Samin2017-06-05

Both converters I bought back in 2015 were well within the manufacturers published specifications which are:

Input Voltage : 5-32V

Input Current: 8A (MAX) peak 10A (6A long-term work)

Quiescent Current: 4mA

Output voltage : 1.25-20V continuously adjustable

Output Current: 5A (MAX) (3A long-term work)

Output Constant Current: 0.2-5A

Working temperature: -40 to +85 degrees

Operating Frequency: 150KHz

Conversion efficiency: up to 96%

The output voltage on the power supply I built varies between 1.2V and 20.1V and I can draw up to 2A continuous. From my experience, the manufacturers data sheet is accurate.

All of the supplies in this project are overload and overtemp protected. Although the manufacturer states a peak output current of 5A, I would not recommend going there. The power supply project presented in this article is conservatively rated at 2A maximum continuous output current which is more than sufficient for most small to medium projects.

NetZener

DanielHutchinson200 (author)2017-05-21

Brilliant Build. excellent In-depth Intructable. thankyou so much for explaining in-depth how to change the pots to different ones I have been looking for a way to remove them. because people have been trying to remove them (On the Ltc3780 Model) by desoldering them directly from the bottem of the board and in doing so bracking the tracks/traces on board. when powering up the board to check to see if the modification was a success a FAULT light then lights up but now I can hopefully remove the pot by the above method safely thankyou for the brill advice

Daniel

I'm glad the article was helpful. Let me know if you were able to complete the modifications successfully.

NetZener

basm13 made it! (author)2017-04-26

Here is my version of one.. Its only a single but handles 8.5.amp and up to 12V.

netzener (author)basm132017-05-03

Most awesome! Excellent job. Very clean and professionally done. Looks like a commercial product and I bet you are very happy with it. There's nothing like looking at and using a device you built with your own hands. Enjoy! And congratulations. Thanks for sending the update!

NetZener

garyalan59 (author)2017-04-02

"Two modules require modification so that front panel controls can be used instead of the PCB mounted multi-turn potentiometers included with the modules."

Would strategic placement of the converter and appropriately placed holes in the case (such that a small screwdriver could be inserted) allow the use of the PCB mounted pots be a viable alternative?

I'm planning on making a single output version, as I don't have need for multiple output voltages. I'm using the case from a PC power supply and the 120 VAC fan that came with it. Do you foresee any modifications to the process?

netzener (author)garyalan592017-04-02

For a fixed output power supply you could certainly leave the on-board multi-turn potentiometers in place and adjust them with a screw driver through a hole in the case. It would be a good idea to mount the DC-DC converters close to the hole in the case so that an errant screw driver doesn't accidentally short something on the converter.

The only change I can think of from your description is that the 120V fan would need to be connected after the main power switch instead of to the 24V fixed supply as indicated in the project instructions. Other than that everything else can be wired as indicated.

NetZener

garyalan59 (author)2017-04-02

Help! Looking at various power supply modules and getting confused/frustrated at the many options available.

For example, the following are made by the same company, but have different layouts. Can't determine which is appropriate for this project.

https://www.amazon.com/dp/B019OLU3E8?th=1

https://www.amazon.com/dp/B019OMDP9C?th=1

netzener (author)garyalan592017-04-02

The first power supply you listed is the correct form factor. Part number B019OLU3E8.

There are a bunch of distributors for the same power supply design so you will see a several different brands offering the same power supply at different prices. And those brands seem to come and go every year. That can make sourcing parts from Amazon very confusing sometimes.

Let me know if you have any further questions or need assistance. I'm glad to help.

NetZener

amrus2011 (author)2017-01-22

I have to say that this is the best power supply instruction on the Internet that I have ever seen bar none. I want to build one of these but I live in Europe where the voltage is 230 @ 50Hz. I am relatively new to building electronics, so I am very ignorant about this. My question is...do I have to modify or change any parts to accommodate the higher mains input voltage, or can I build it as you have written?

netzener (author)amrus20112017-01-22

Many thanks for the kind comments. I'm glad you liked the article.

The AC-DC power supplies described in the article automatically adjust for input frequency between 50Hz and 60Hz, and input AC voltage between 90V and 250V AC. No changes in the instructions are needed other than the AC power cord will need the 2-pin/3-pin grounded plug style used in your region. The power supply project uses a power entry module with an IEC C-14 socket commonly used in office equipment worldwide. So you will need an AC power cord with a C-13 plug on one end and the appropriate euro style grounded plug on the other end.

If you build the power supply, be sure to send along a picture of your work. Instructables readers love that sort of thing, you know. Let me know if you have any questions are need assistance.

NetZener

amrus2011 (author)netzener2017-01-23

Thanks for the quick reply. That is fantastic news. Unfortunately, I am not able to use the enclose that you used shipped over here, so I am having to make do with what I can find that is close to what you and the others have used. Now it's time to shop for the components and get to work. One more question. For the wires connecting the pots to the DC/DC converter, would the wires from a CAT5e cable be sufficent?

netzener (author)amrus20112017-01-23

"Make do" is a good thing. Your character and ingenuity will shine.

You can use the #24 solid core wire commonly found in CAT5e cables for the potentiometer wiring. Keep in mind that solid core wire fatigues easily when bent. It will break with just a few hard bends, especially at solder joints. So once you get it soldered to the DC-DC converters and potentiometer lugs, avoid moving the wire too much until you have secured the wiring with cable ties.

NetZener

9A165 (author)2017-01-17

Sir can i connect two dc to dc converter same power supply and connect their out put in series to get -/+ dual output.is it possible

netzener (author)9A1652017-01-22

If all you need is a dual variable power supply with a common ground between the outputs, then a single power supply can be used to power the DC-DC converters.

The DC-DC converters are buck-boost type which use a common ground between the input and output terminals. It is not possible to use a single power supply and maintain return isolation at the DC-DC converter outputs. Electrical isolation between the DC-DC converter outputs allows the outputs to be used independently in circuits requiring different voltage and ground potentials, or stacked together for double the voltage output, or connected in a bi-polar configuration for use with op-amp or audio power amp circuits. I highly recommend dedicating a power supply to each DC-DC converter to make the dual variable power supply as useful and flexible as possible.

NetZener

Popart2 (author)2016-12-13

Very nice PSU, you did a good job. Nevertheless, when you sum up all the component costs there is no big difference with a factory build one! Just one DROK DC/DC Automatic Boost Buck Converter costs $16 and you need two of them...

netzener (author)Popart22016-12-13

Perhaps you are unfamiliar with "DIY Culture", more popularly known in the States as the "Maker Movement". Below are a few links that will help explain why Instructables.com exists, and why people like me publish articles like the Switched Mode Variable Power Supply:

https://en.wikipedia.org/wiki/Maker_culture

http://time.com/104210/maker-faire-maker-movement

http://www.techshop.ws/images/0071821139%20Maker%2...

You will find much more about DIY culture with a google search, but the movement is first and foremost "learning by doing in a social environment" and then "sharing the work with others" in the hope that they will appreciate it and perhaps learn how to do it too. Cost comparisons to commercial products are completely irrelevant because the Maker Movement is about making, not consuming.

Several members on Instructables.com shared that they successfully built the power supply described in my article. Probably several more made it and didn't share that they made it. No one built the power supply because it was cheaper than a commercial power supply. They did it for the satisfaction that comes from learning how to make something useful; The very human need to show others that we value something enough to do it with our own hands.

It could be easily argued that, outside of a factory, no one actually needs a variable power supply. Time and materials considered, it would be cheaper just to buy whatever it is you were going to make using the variable power supply, instead of buying the power supply and making something with it.

But that wouldn't be any fun and you wouldn't learn anything.

NetZener

mfalkvidd (author)2016-05-14

Drok seems to have several DC-DC converters, at least on Amazon. Which one do you recommend?

netzener (author)mfalkvidd2016-05-14

No problem! DROK has an Amazon store for US sales. Go to www.amazon.com and enter the following in the search bar at the top:

DROK 090747

That will bring up the converter I used with the description:

DROK® DC/DC Automatic Boost Buck Converter Module 60W Constant Voltage/Current Car Voltage Regulator DC5-32V to 1.25-20V Adjustable Current Step Up&Step Down Voltage Converter

NetZener

mfalkvidd (author)netzener2016-05-14

Sweet. Found it! Thanks a lot!

Filbert11 (author)2016-04-28

Note: I made sure to specify the 24 V fan to IAASR - but it still has the size issue. It was 1 inch thick - so the AC-DC converters do not fit. Just FYI. I would go without the IAASR Fan - and find one that is 10mm to 13 mm on your own.

netzener (author)Filbert112016-04-30

Thanks for the update. I'll let IAASR know about the fan sizing issue. I sent them a part number quite a while ago for the fan that conforms to the space but it may not have made it into their BOM.


The fan I used is a Delta 24V, 60mm x 60mm x 13mm, 27.72 CFM, 38dbA

Mouser Part Number 108-AFB0624VHC-F00
Delta Part Number AFB0624VHC-F00

The fan I used is overkill but works for my lab environment. A fan with lower CFM and dbA rating that would be better for a home lab. Unfortunately the Delta fans that are quieter are all non-stock at Mouser and Digikey. A different manufacturer than Delta might bring the best result.


NetZener

Filbert11 (author)netzener2016-05-01

As a "fan" of over-engineering (pun intended) - I just decided to mount the bigger fan on the outside. Works great. But I will also say that I purchased two fans from Amazon as suggested by another commenter on this post. Two fans for under $11 with free shipping and 2 day delivery!! The fit the mountng holes perfectly and only 10 mm (0.39") deep. And those little fans work great !! They kick a lot of air for their size. Here they are: http://smile.amazon.com/60x10mm-Blades-Cooling-Com...

OWH made it! (author)2016-04-21

It seems to work. Step 25 1,2,3,5,6 I got no ohm reading 8,9 got 2.8k 12,13,14

no reading. could not contact anyone so I plug it in. Results Step 27 6 got 1.18,

7 got 21.2 right side 8 1.2 19.0. optional test 12 left .12 right .11 16 left .14

right .04 17 left 1.04 right 9.94v 1.03A Is this OK?

127

netzener (author)OWH2016-04-22

Perfect! Excellent work. And a very nice job, too. Congratulations on successfully completing the project. I think you will find your power supply will provide good performance for many years. I use mine all the time in both Analog and Digital design work.

NetZener

Filbert11 (author)2016-04-05

Hey !! Thanks for the great instructable. I just got the 2 - 24V 2A Output Regulated Switching Power Supplies from Amazon - and noticed the following regarding
input/output isolation: As expected - I found there to
be no DC voltage between the ground/case and –DC or +DC terminals; and there was NO AC voltage between
–DC and +DC (just 40 ma of noise at a high freq). But there was a 60 Hz - 40 VAC voltage on both –DC and
+DC terminals with respect to the ground/case
- on BOTH power supplies. Does this sound right to you?? This would mandate that all downstream applications/connections be isolated from common ground !!

What would happen if I tied the -DC terminal to the chassis ground on this power supply? Would it short a 40 volt AC to ground - damaging the device? And - in the overall design of your instructable - do the DROK DC/DC boost/buck converters isolate the DC inputs from the Switching power supplies' outputs - effectively filtering out the AC voltage - such that it is OK to tie the - DC terminal of the output of the buck converters to ground??

Thanks for your responses !!!

netzener (author)Filbert112016-04-05

In a properly wired power power supply, there should not be a true AC voltage on any DC output (+ or -) with respect to the case ground.

What you are seeing might be just stray AC from the 120V wiring or the AC input over-voltage protection circuits. You can get odd readings when there is no load on the supply. A high impedance oscilloscope or volt meter might display an AC voltage that goes away when the circuit is assembled.

Check that the Hot, Neutral, and Ground leads are connected to the correct power supply terminal. It's easy to get the Hot and Neutral leads mixed up and that can result in very odd measurements.

To be sure, try the following test:

1. Verify that the 24V Switching Power Supply is disconnected from AC power.

2. Attach a 4700 Ohm 1/4W or 1/2W resistor between the DC output + and - terminals.

3. Connect the 24V power supply to AC power. Make sure the Hot, Neutral, and Ground terminals are connected correctly. The resistor will get very warm during the test so avoid touching it.

4. Measure the DC voltage across the output + and - terminals. The DC output should be around 24V. If not, immediately disconnect the power supply from AC power and send me another message.

5. Measure the DC and AC voltage between the case ground and the +/- DC output terminals. There should be almost no 60Hz AC present (you might see a very small amount of noisy AC but that's normal). If you see 40Vrms 60Hz AC, immediately disconnect the power supply from AC power and send me another message.

The DC outputs of the 24V power supplies are isolated from chassis ground. When you install the power supplies, the chassis and power supply case become part of the AC Ground Protection circuit. Attaching the -DC terminal of the power supply should not damage the supply but the power supply adjustable outputs will not be isolated and the supply will not work correctly. If you just want to test to make sure that the 24V Switching Power Supply is isolated, temporarily attach a 4700 Ohm 1/4W or 1/2W resistor between DC -V and the power supply chassis ground. Then measure the voltage across the resistor. There should be no voltage other than a little switching noise.

The DROK DC/DC boost/buck converters I received last year were manufactured with mounting holes that were isolated from ground. Yours should be the same. You can verify this by examining the mounting holes. If the mounting holes do not have a metal pad around them and do not have a plated hole, then your mounting holes are isolated. I used fiber washers on the top and bottom of the DROC DC/DC converters just to make sure that the mounting screws would not come in contact with any surface mount parts.

The DROC DC/DC outputs are electrically isolated from the case by the output binding posts.

When completed, you can connect the power supply outputs together in series to make a bipolar +/- 20V variable power supply. I have done this recently for a project with no problems. Just make sure the 24V Switching Supply outputs are connected only to the DROC DC/DC inputs. And the DROC DC/DC outputs are only connected to the meters and output jacks.

Let me know what you find out.

NetZener

Filbert11 (author)netzener2016-04-06

Netzener,

I think you nailed it when you said it was probably induced or otherwise stray AC voltage.

Here's what I found:

I took one of the power supplies apart and confirmed visually and w/ multimeter probes that the inputs & outputs were all isolated from each other. Even Ground and Neutral are isolated - though they get connected when plugged in thru the mains. I confirmed there are two transformers - an AC input step down transformer and the switching transformer on the outputs. To my surprise - the smaller one was for the AC mains.

I need to clarify that the 40 Volts AC / 60 Hz I previously saw on my scope was a Peak to Peak measurement. So - it was really only about 14 VAC RMS.

This time - I used 3 isolated multimeters. 1) measured the DC volts on the -DC to +DC terminals; 2) the AC volts Between either DC terminal (one at a time) and ground; and

3) the DC volts from either DC terminal to ground.

There was never any DC volts from either DC terminal to ground under any circumstances. That was as expected. Total isolation.

Then - I used various resistive loads across the DC terminals. 300 Ohms - 10 watt; 600 Ohms - 5 watt; and a 5700 ohn - 1 Watt. The DC voltage never waivered and performed very well. But there was still a 13.8 AC Volts RMS from either DC terminal to ground.

So (as you sugessted) - while using the 5700 Ohm load on the DC terminals - I connected either DC lead to ground thru a second 5700 Ohm resistor. And the AC dropped from 13.8 V RMS on both terminals to a 0.4 VAC RMS. And the DC voltage did not waiver !!!!! That means the second 5700 Ohm resistor was drawing only 71 MicroAmps of AC current - and it pulled the AC voltage down to near zero. That's only 28 micro-watts of power. The behavior was the exact same for both power supplies.

I think that proves that these power supplies have some sort of induced very low power AC voltage on the DC outputs - probably coming from stray magnetic fields from the 60 Hz input transformer !! WOW !!

Very interesting - and informative. Thanks a ton for working this out with me. I learned a lot !!!

netzener (author)Filbert112016-04-06

Great work! I was pretty sure you wouldn't rest until the mystery was solved.

In case you were curious: The small transformer connected to the AC input is commonly referred to as a Common Mode RFI Choke. Together with the capacitors attached to the AC terminals, the Common Mode RFI Choke prevents high frequency noise within the switching supply from being coupled into the AC power lines of your home and radiating RF interference all over the neighborhood. Something the FCC considers to be bad form.

Your comments and observations will probably help someone else trying to solve a similar mystery. Thanks for asking the questions and chasing down the answer.

NetZener

Filbert11 (author)netzener2016-04-07

Thanks for the info about Common Mode RFI Chokes !! I did not know chokes were used for that. I researched it - and it is valuable information !! {:>)

Filbert11 (author)netzener2016-04-05

NetZener - Thanks a ton for your fantastic and very quick response !! You offer a lot to think about and do - and I do not have all the parts for the build yet - only the 2 - AC to DC 24 Volt supplies. So far I have only tested the voltages of the AC to DC supplies with no load. I'll check out your suggestions that I can, and the rest when I get the other parts and keep you advised. There was one other guy on Amazon who noticed/posted that he saw a large 60 Hz AC voltage - 60 volts he said - from the DC terminals to ground (I only saw 40 VAC on my scope). Thanks again !!!

Filbert11 (author)Filbert112016-04-05

Here's a short follow-on. Above I meant to say 40 mV of noise - not 40 mA. Also - are the DC inputs of the buck converters isolated from their chassis !! If they are not – then the DC inputs could both have 40 VAC on them !!! Will that be an issue for the buck
converters?? Do I have to isolate the
buck converter chassis from the cabinent ?

netzener (author)Filbert112016-04-05

Any time. Glad I could be of assistance to you. Many thanks for your kind comments about the article.

I don't think you have anything to worry about regarding the AC voltages measured from the DC outputs with respect to chassis ground. The low voltage DC outputs are delivered from a switching transformer which is electrically isolated from the high voltage DC side of the power supply (the 120VAC input as actually converted to 170V DC via a bridge rectifier and filter capacitor). The AC input does have some over-voltage and RF suppression components (i.e capacitors) that connect between the Hot and Neutral terminals, and the Hot and chassis ground terminals. And many scope probe grounds are connected to the scope chassis ground which is itself connected to the 120V protect ground. Lots of people aren't aware of this until they try to measure 120V within a device and inadvertently connect their scope ground to the Hot lead. Instant sparks and breaker trip. Those same people then attach their scope to a 3-pin to 2-pin adapter to defeat the ground protect. Unsafe, but it helps avoid that mistake while making 120V measurements.

I think what people are seeing is a combination of capacitive coupling and ground circuit voltage differences. Scope probe impedance is typically 1Meg Ohms for a 1x probe and 10Meg Ohms for a 10x probe. You can pick up stray AC on impedance that high using your finger (always my favorite "is the scope working?" test). But when you connect a relatively low impedance, like 10K Ohms, across the same circuit the mysterious AC voltage goes away or is drastically attenuated.

As long as the power supply isn't faulty in some way, it should work fine. Try the resistor test to chassis ground and see if the stray AC voltage disappears. If it doesn't, your resistor will get hot and in that case there is something seriously wrong.

Thanks again for your comments!

NetZener

netzener (author)Filbert112016-04-05

No problem. I knew what you meant :-)

My reply above provides a more detailed answer. But in short, the 24V Switching Power supply DC outputs are isolated from chassis ground.

The DROC DC/DC converter inputs and outputs are isolated from chassis ground. Their mounting holes aren't connected to anything on the PCB but check with an Ohm Meter just to be sure that they haven't made a design change recently.

The DROC DC/DC converter outputs connect to the V/I meters and the output binding posts, which are themselves isolated from chassis ground.

When installed as described in the instructable, the DC connections are isolated from chassis ground and each other.

On the supply I built, I can connect the negative binding post of power supply 1 to the positive binding post of power supply 2 and have a variable bipolar supply for OpAmp circuits.

Let me know if you have further questions or need help.

NetZener

OWH (author)2016-04-05

I started getting the parts together. All the parts I have or on there way.

I am having trouble finding the 22 stranded wire and the 28 stranded wire.

Could I use 26 stranded wire for both?

OWH (author)OWH2016-04-05

Thanks for the info

netzener (author)OWH2016-04-05

If you have an old PC power cord laying around that you don't mind cannibalizing, you can strip the black PVC off of it and use the wire in it for the #22 stranded wire. Make sure it's the power cord with the round PVC jacket instead of the flat kind that looks like three wires molded together.

You can use #26 for the connections to the potentiometers. It might be a bit of a stretch getting the wire to fit into the hole. If it's too tight, just trim the end of the wire so that only about 1/16" (about 2mm) of wire sticks out beyond the insulation. Then solder the wire to the PCB pads. Avoid mashing too hard on the wire while soldering to prevent spreading the wire conductors and accidentally causing a short circuit between the PCB pads.

I strongly recommend #22 for the 120V AC portions of the wiring. It is important that the wiring between the AC input, switch, and 24V power supply inputs be large enough to withstand short circuit current long enough to trip the residential branch circuit breaker, should something go wrong inside the power supply. If the wire is too small, a short circuit in the switch or 24V power supply circuit ahead of the power supply fuse could melt or vaporize the wiring before your branch circuit breaker could trip leaving the power supply project in an unsafe condition.

#26 will definitely work under normal conditions, but will likely fail to protect you in the event of a wiring fault.

NetZener

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