Variable ATX Bench Powersupply - FabLab Edition

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Introduction: Variable ATX Bench Powersupply - FabLab Edition

About: I study (civil) electronics engineering at the VUB in Belgium. I have a passion for making things, both useful and cool.

A good power supply is very useful when working with electronics, but it can be pricey. However, you could have one laying around in the form of an ATX PSU (Power Supply Unit). They are the most commonly used power supplies for computers. In this instructable I'll show how I converted my ATX PSU in a variable benchtop power supply. I have got some ideas from many similar projects: https://www.instructables.com/id/Encyclopedia-of-AT...

I combined ideas of several of those projects I found, to give my PSU the most options as possible.

*As I mainly use breadboards for my projects, I added female headers to my powersupply. I also made crocodile clamps that fit in those headers. For appliances that draw more current, I added banana jack terminals. And for powering my arduino a USB port was added.
*For the powering of some DC motors I wanted a variable powersupply, so I added a variable output with an LM317 (a voltage regulator, http://en.wikipedia.org/wiki/LM317 ), controlled by a 10-turn potentiometer.

*Lastly I added fuses as safety measure. An ATX PSU should have a short-circuit protection built in, but this way we are completely sure we don't destroy it.

Wanting to add as many features as possible, I also made a case for it. I chose for plexiglass, as it is nice to see the cable management inside (I am a cable management freak). The plexiglass was laser-cut at FabLabXL, a FabLab (http://en.wikipedia.org/wiki/Fab_lab) in Brussels, Belgium.

As for heat management and safety I don't recommend installing everything in the PSU itself.

WARNING
As we are dealing with a high voltage power supply, I recommend only attempting this if you have a basic knowledge of electricity, and its dangers. Inside the PSU there are some BIG capacitors, that - when charged - will kill you upon touch; even if the PSU is off! Always let them discharge for a few days before opening the PSU. As stated before this is why I don't recommend mounting everything inside the PSU, but making a seperate enclosure. I am not responsible for injuries or deaths.


Step 1: Basic Information + Parts

The ATX PSU has a lot of wires coming out of the back. They can be recognized by their color (http://en.wikipedia.org/wiki/ATX ).

  • Black: ground (GND)
  • Orange (+ brown): 3.3V
  • Red (+ pink): 5V
  • Yellow: 12V
  • Blue: -12V
  • Purple: 5V Standby.
  • The brown and pink wires are sensing wires (they verify if the output is 3.3V or 5V respectively).
  • The green should be connected to GND (black) to turn on the power supply.
  • The grey wire is "power good", and gives +5V if all wires are connected properly (i.e. green connected to black, borwn sensing wire to orange wires and pink sensing wire to red wires).

To give a stable output, the PSU needs a minimum load on its highest current wire, in my case the 5V line (see the label on the PSU). So a 10 Ohm 10W power resistor is added between 1 red and 1 black wire.

1) Basic parts

  • 10 Ohm 10W power resistor
  • 2x 220 Ohm resistor
  • Red & Green LED
  • Switch
  • Banana jack terminals (x6)
  • Female headers 1x4 (x7) (you can cut them from longer strips)
  • USB port
  • Screw terminals (optional)
  • Cable connectors (optional)
  • Fuse holder (x6)
  • Fuses (I choose 5A for the normal ones, 1.5A for the variable)
  • Crocodile clips and header pins (optional)

2) Variable circuit parts

  • LM317 (voltage regulator)
  • 1uF capacitor
  • 100nF capacitor
  • 1.2k Ohm resistor
  • 10k Ohm 10Turn potentiometer
  • 1N4007 (or similar) diode
  • Heatsink for LM317 (CAN NOT be mounted against the case!)
  • Screw terminals (x3)

3) Case

  • Plexiglass 4mm (about 0.5m x 0.5m)
  • Threaded rods M4 18cm (x6)
  • Washer M4 (x24)
  • Nut M4 (x24)
  • Handle
  • ATX screws (x4)

Step 2: Assemble the Basics

To make use of the fixed output voltages of the PSU (3.3V, 5v, 12v and -12V) we just need to connect some wires. Ofcourse we will add a switch, some indicator LED's and connectors to make it really useable.
Always make sure to use heat shrink tubing and tape where possible, we don't want short circuits.

* The first step is to cut off all connectors and bundle the wires of the same color. I recommend opening the case to blow out all the dust, but as mentioned before: be careful! Try not to touch the components and certainly don't touch the leads of the big capacitors!
* Take the switch and attach the green wire to one leg, and a black wire to the other leg. I used cable connectors, but you could just solder it.
* Now solder the resistors to the anode (long leg) of the LED's and add heath shrink tubing. Next, solder both cathodes (short legs) to a black wire. Solder the purple wire to the resistor coming from the red LED anode and the grey wire to the resistor coming from the green LED anode. (Look at the schematic)
* We also need to add a dummy load: a 10 Ohm 10 W power resistor. Simply solder it between a red and black wire coming from the PSU and make sure to insulate the connections. Attach it to a heatsink (or the PSU itself), because it gets hot!

* For the power wires, take a look at the schematic for a detailed overview. The basic idea is (“>” = 1 wire):

  • GND >>>> Banana >> Header
  • 3.3V >>> Fuse (5A) holder >> Banana > Header (2 orange wires and the brown sensing wire)
  • 5V >>> Fuse (5A) holder >> Banana > Header (2 red wires and the pink sensing wire)
  • 12V >> Fuse (5A) holder >> Banana > Header
  • -12V > Fuse (1.5A) holder > Banana > Header
  • Variable > Fuse (1.5A) holder > Banana > Header
  • 5V > Fuse (5A) holder > USB port positive side

* Add cable connectors to the 3.3V, 5V, 12V and -12V wires coming from the PSU and attach them to one side of the fuse holder. Again, you could just solder them together. Do the same with a short wire that will be connected to the variable circuit in the next step. 4 black wires can directly be connected to a banana terminal.
* Now we need wires that connect our fuses to the banana terminals. Take wires of about 5 cm and add cable connectors to both ends of the wire. You could also simply solder them together.
You can cut off pieces from the PSU wires, they should be long enough.
* Next we will prepare the header pins. Take a strip of perfboard (prototyping board) and place the headers on regular intervals. Take 2 header strips for ground. Now solder them to the perfboard. The perfboard makes it possible to easily mount them inside a case, and to ensure an even spacing.
* Now solder the banana terminals to the headers with a wire of about 5cm.
* Solder the usb port on some perfboard and add a screw terminal. Connect a black wire from the PSU to one side. Connect the other side to the fuse. Make sure to check your connection here before plugging in a USB device! Compare it (with a multimeter) to a USB wall adapter you have laying around.

I used 2 wires per color, which should give me enough current. If you want more current, use more powerful fuses and more wires. It is very important to attach the brown wire (3.3V sensing) to the orange wires (3.3V) you use! Also connect the pink wire (5V sensing) to the red wires (5V).

I also made some crocodile clips that fit in the headers. Simply connect a male header pin to the crocodile clip using a length of wire.

Now your PSU should be working. The only thing left to do is add the variable output and make a case for it!

Step 3: Make the Variable Circuit

For the variable output, we need some characteristics of the LM317 voltage regulator first. The output voltage is defined by this formula: Vout = 1.25*(1+(R2/R1)). The resistors are indicated on the schematic.
I used a 10 turn 10k Ohm potentiometer I had lying around, and adapted the other resistor to it. I really recommend a 10 turn potentiometer for an accurate setting of the output. Because we can only go as high as 12V (it will be a bit lower in reality), Vout should be 12V. Because R2 is 10k Ohm, we know: R1 = 10000/((12/1.25)-1) = 1162 Ohm. As the LM317 will not be able to give the full 12V I rounded it up to 1.2k Ohm, which will give us a theoretical maximum output of 11.7V. The lowest output will be 1.25V.
Now we can start building the circuit!

* Solder the small circuit (see schematic) together on a perfboard and replace the LM317 connections with screw terminals. Also add screw terminals for input 12V, variable output and ground wires. Make sure the polarity of the diode and electrolytic capacitors are right.
* Cover the bottom of the perfboard with hot glue and glue on another piece of perfboard.
* Solder a wire to each of the LM317 legs, and don't forget heat shrink tubing.
* Connect the wires coming from the LM317 to the screw terminals, do the same for input (+12V wire from PSU) and ground (PSU wire).
* The only terminal left - output - should be connected to the fuse for the variable output. Use the wire we made in the previous step. You could leave out the screw terminals and solder it directly.
* Now attach the LM317 to a separate heatsink! The exposed metal on the LM317 is not ground, but is Vout, so we need to make sure the heatsink doesn't come in contact with any wire that is not Vout. The enclosure of the PSU should be grounded, so that means we can not use it as a heatsink for the LM317.

The heatsink is important, as the LM317 can get pretty hot: P (power) = (Vin-Vout).I. This means that with an output of 1.25V and with a current draw of 1.5A it will give about 16 Watts of power. The reason I extended the leads of the LM317, is to keep it a further away from the rest of the circuit. This way, the heat that it produces, will not influence the resistance of the resistor R1, and the output will be stable.
An LM317 can output a maximum current of about 1.5A, so put a 1.5A fuse here.

Step 4: Make and Assemble the Case

For the case, I used plexiglass of 4mm, which I cut with a lasercutter from a FabLab. If you don't have access to a lasercutter, you could always use a saw (preferably a jigsaw) and a drill. Wood could be an alternative for the plexiglass. Because I used many components, it's not a good idea to cram everything inside the enclosure of the PSU. It will be nearly impossible to make all the holes, and air will not be able to circulate and cool the PSU.

* Start by designing everything on paper to make sure everything will fit. It is much easier to correct mistakes in cardboard than it is in wood or plexiglass. The design is up to you, but I added my design in svg format.
* Don't make the case too small, we want enough space for the wires. I made mine 10cm longer than the PSU.
* Once you have a design you like, draw it on the computer and export it as dxf files for the lasercutter. If you don't plan on using a lasercutter, you can skip this step.
* Cut your pieces using a lasercutter (or jigsaw).
* Now drill the holes for the threaded rods (M4) and for the handle. Put thread in the holes for the rods (optional) using a tap (http://en.wikipedia.org/wiki/Tap_and_die). Remember to drill a smaller hole when using a tap.
* Cut 6 threaded rods of 18cm. Add washers and nuts. They will connect the both sides of the case.

Screw everything in place and add some hot glue if necessary (LED's, USB and switch).
Secure the PSU to the back panel with ATX screws (they were used to connect the PSU to the PC case).
The potentiometer is installed by removing the turn knob from the body and replacing it, once through the hole.
Also make sure the heatsink from the LM317 is not touching the PSU enclosure! I attached it to the plexiglass case using some velcro. I installed the small circuit board in the same way.
The big advantage of using cable connectors is the easy of connecting everything together in the case.
Finally, tape the unused wires together or cut them of. Make sure to add tape to the ends to avoid short circuits

Step 5: Test Your PSU & Enjoy!

We're done! The only thing left to do is to test the PSU. If everything works fine, you can securely tie the nuts.
Now you can enjoy your homemade variable benchtop power supply unit!

6 People Made This Project!

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128 Discussions

Capacitors can be discharged quickly and safely by shorting the leads of each for 3 seconds with an insulated screwdriver or insulated wire.

0
user
DimebagG

Question 2 months ago

I got 24 pin atx smps and found that it switches on just by connecting green cable to ground. Why do I need to put power resistor on highest amperage line? it gets very hot. Any other option instead of dummy resistor or without it, is it fine?

1 more answer

The load is necessary for a good regulation of the powersupply. You can read the comments below for more info. I verified it myself, and it makes a difference.

0
user
THERJ

1 year ago

Thanks for the detailed DIY. I am wondering if it is crucial to use fuses as I have seen several DIY where they have not been used. what kind were used in this DIY?.

Also, I am having some trouble with the variable circuit, Can the pot be directly soldered onto the board?

What is the reasoning behind adding glue to the bottom of perfboard and gluing another one on the bottom?

what is the Pot connected to on the variable circuit Board?

You state that the LM317 heat sink should not be mounted against the case. Does this mean it cannot be inside the PSU and has to be ouside the case?

Thanks in advance!

1 reply

The fuses are extra protection, they are simple glass ones.

Pot can be on the board, but it is easier to mount if it's connected via a wire. It's the variable resistor symbol in the schematic. It should indeed be outside the case, or isolated with some thermal pads.

The glue and extra pcb are for mounting.

Hello, this is definitely a great project and so I'm busy building my own :).

Though, I'm making the variable output so that my Vout max would be plus minus 37V.

Therefore I took for R2 a 10kOhms potentiometer and for R1 360 Ohms.

So far, I don't have a resistor of 360 Ohms so I used several resistors to achieve this value. 3 x 100 Ohms in series with 2 x 30 Ohms.

Now comes the problem, one of those resistors 100 Ohm starts to heat enormously because smoke comes 2 seconds after applying power (37VDC).

After replacing the resistor by a new one, it also burns out.

Is there possibly an explanation for this?

Thanks for reading.

3 replies

There certainly is an explanation, electronics is not magic (although it sometimes seems like it) ;)

First, in the case you omit the lm317: the power dissipation in the resistors is too high. You can calculate it: P = UI = U^2 / R.
So let's say your potentiometer is at 0 ohms, that means you have a 360 ohms resistor between 37V and ground. So P = 37^2 / 360 = 3.8 Watt (!)
Normal resistors only allow a power dissipation of about 0.25 Watt, so it will heat up and burn out.

In the case you do connect the lm317, this shouldn't happen. Since 0 ohms on your potentiometer will give you 1.25V output, you power dissipation will only be 0.004 Watt. That means you made a mistake in your wiring. I would suggest to rebuild it, and take another look at the pinout of the part.

I hope it helps, good luck!

Hi, I didn't make any mistake in wiring but though I checked my print with a magnifying glass and found there was a connection made between the adjust pin of the LM317 and my GND on the circuit board. This was made because of very little rests of solvent and thin. Even after cleaning this remained. I'm glad there was no danger for my components and myself of course :-). Anyway thanks for answering my reply, indeed, 0,25Watt resistors shouldn't be used for what I needed. I created a plus minus 240 Ohms resistor by putting 2 x 470 ohm 0,5Watt parallel ;-).

Additional information: same result even when taking out the LM317.

0
user
Rhasta

1 year ago

Hi, and thanks for posting this.

My PSU seems to have two 12V rails and they're both 18A. The 5V is 15A and the 3.3V is a whooping 24A. (screenshot)

1) Should I put the dummy load on the 3.3V.

2) Can I connect the two 12V rails together so they can share a load higher than 18A?

2a) If I can merge the two 12V, do I have to put the dummy load on the 12V instead?

3) If I put the dummy load on the 3.3V, should I make it lower than 10ohms?

4) As someone suggested, can I put a useful load instead of a dummy one, like a fan, even though it would use much less power?

Thanks. Love this site.

IMG_20170406_183836.jpg
2 replies

That's a beefy powersupply :P

1) Yes, put it on the 3.3V line

2) Not sure, it depends on how the powersupply is internally wired. I would keep them seperate and put an extra banana jack. Also, 18 A is already very high for the wiring, so it's probably not a good idea to actually draw that current :P

3) You could, but 10 Ohms is probably fine

4) Sure, as long as there is a significant load it's fine ^^

Good luck!

3.3V rail with a 10 Ohms for the dummy load it is then.

As it turns out I'm going to need 16V and 24V for some motors so I have decided to use the 2nd 12V rail for a buck/boost converter and skip the LM317 part.

Thanks again.

Hey!

Great project, thought I would give it a go, but still learning. Was a little confused about this: " I used a 10 turn 10k Ohm potentiometer I had lying around, and adapted the other resistor to it." when reviewing the schematic and the images, it looks like you only used two of the three connections, which one did you omit?

I have:

http://www.digikey.ca/product-detail/en/bourns-inc...

Thoughts?

Any help would be greatly appreciated!

Thanks!

1 reply

Potentiometers always have 3 pins, and depending on how you want to use them you can use 2 or 3 of them.


The value that is written on the potmeter (10k here) is the resistance you will measure between pin 1 and 3 on the image below. Pin 2 is called the wiper and will "glide" along the resistor. When measuring the resistance between 1 and 2, you will see that it changes between 0 Ohm and 10k Ohm. The same can be done by using pins 2 and 3. You have now made a variable resistor, which is exactly what we need.

When using all 3 pins, you form a voltage divider, look on wikipedia for some more information.

Good luck!

Really nice project, and awesome looking case! Don't mind if I build something heavily inspired by this :-) Only thing I miss is a small display showing variable voltage.

One question, though: Which wire gauge (AWG) would you recommend for this?

1 reply

I used the wires from the PSU itself, they are 18 AWG. Feel free to copy the design, that's the reason I shared this project :)

0
user
cazino

1 year ago

Hi Thomas, I am in the process of building your well explained design but I have a couple questions. a). I do not have a pink wire in my PSU...only one wire colors are grey, Green, violet, blue and Brown. What do I do in this case?. The other question is related to the -12v output. Do I need it, what is it used for? I mean having already a + 12 v output. Sorry if the question is too basic, I am just starting. Tks in advance

1 reply

Hello!

I'm glad you like the instructable :)
It is possible that you don't have a pink wire. In that case, just don't use it :p It's not an issue.
The -12V can be used if you want to power something with a so called symmetrical powersupply. Opamps often require +12V, GND and -12V. Connect it now, so you can use it if you need it in the future.

Welcome to the wonderful world of electronics ;)

Ths

fonte.JPG