Intro: Variable Lab Power Supply
I do a lot of work with low voltage electronics which often require various voltages. I was getting sick of constantly set up a series of batteries and then selecting the appropriate resistor just to test a single small part of a circuit. Ideally, I wanted to be able to dial in to a voltage simply and quickly and test. So that is what I made, a variable lab power supply for testing electronics.
This variable power supply provides anywhere between 1V - 10.7V DC. It is really easy to make one slight adjustment to allow a range of AC voltages, but I never use AC, so I didn't bother. It is also really easy to provide 0V-36V, but I hardly ever need above 10V, so I left it with the smaller components that I had readily available.
There are several other lines available off of the var psu including: 12V, -12V, 5V, -5V, 3.3V, GND, Variable (1-10.7V), and a mains-outlet extension (115V AC). The mimimum current limit on any of the lines is 2 amps, the max limit is 20 A on one of the lines. I forget the exact limits on each particular line. These limits will be based on the power supply unit (PSU) you choose to use in this project.
To all the comments I am sure will ensue, yes, I could have used a PSU and just installed binding posts, but I rather liked the geometry of this box. The size was right if I some day want to mount it to the workbench (when I get a real workbench) and there was plenty of space to install the fancy bells and whistles that I wanted (namely, the panel meters and the switch along with the variable line).
Anyways, this is a fantastic addition to your electronics workbench setup!
Step 1: Materials
- 8 binding posts ($0.18/each) Tayda - RadioShack
- Volt meter ($7.95) Adafruit
- Current meter ($9.95) Adafruit
- Temp meter ($9.95) Adafruit
- 3-prong outlet
- 10K Linear Potentiometer ($0.50 - $0.95) Tayda - Adafruit - RadioShack ($3.19)
- Knob for potentiometer ($0.19) Tayda
- Switch (varies greatly)
- Indicator LED (any LED will work) [optional]
PSU (computer power supply unit) one of the small ones will work, you should be able to find it for $5-25, easy.
- it is nice if you have a spare one around
- look near IT centers or electronics stores, they might have an old one for you
- search eBay, amazon, google for key words like "psu", "atx power supply", etc. (you dont need a full atx psu, look for the rectangular small ones, not the big square ones. Although they will work better if you want one)
- RadioShack supplies ATX PSU's if you want to go with one of the bigger ones.
- Wire (multiple colors) I always prefer solid core ($2.50/25 ft.) Adafruit
- Gorilla glue for mounting the front panel
- Prototyping board
- LM317T ($0.23) Tayda - RadioShack
- 10 uF electrolytic capacitor ($0.01) Tayda - RadioShack
- 1 uF electrolytic capacitor ($0.02) Tayda - RadioShack
- 0.1 uF Ceramic capacitor ($0.01) Tayda - RadioShack
- 1N4001 Diode (x2) ($0.01) Tayda - RadioShack
- 220 ohm resistor (1/4 W) ($0.01) Tayda - RadioShack
- Any spare LED's of your choosing (any color, any voltage, etc) Tayda has a concise, cheap collection (starting at 2 cents!). Otherwise, RadioShack caries a wide selection if you don't want to order online.
- A fan or two to cool the unit (the size of the fan should fit the size of your case).
- Solder + Soldering iron
- Case (I am using an old NetGear box, it didn't work anymore so forget about how much value it is compared to the product) Just find any old box sitting around. Search behind computer repair center's for discarded anything.
I have included RadioShack as an alternative for buying parts online. It is definitely cheaper to order online (way cheaper). But Hey, if you want your parts pronto, there aren't too many stores around that offer these parts (except Fry's if you are lucky to live near one).
Step 2: Panel Design
Prepare a model of what you want the front-plate of the box to look like. I used CAD software (Ashlar-Vellum's Graphite) to design all the components I wanted included. I started by taking the measurements of the panel meters (volt-meter, amp-meter, and temp-meter) along with the size of the potentiometer, the switch, the indication LED, the 3-prong outlet, and the binding posts. I measured the space I wanted between the binding posts.
After drawing a few sketches by hand, I made a fully documented CAD file with all the measurements. I used the .dxf equivalent file and went to my local FABlab (If you don't know what a FABlab is, look it up immediately! They are way cooler than my ible!) and laser cut a faceplate from an interesting scrap plastic I found. It was strong, durable, hard to cut, non-conductive, and slightly flexible, and a sleek black! All the properties of the ideal material for a face-plate!
With my stunningly gorgeous new face-plate I proceeded to work on the interior of the box.
Step 3: Open the Box!
Gut the old box (unless it happened to be a power supply unit). You will need to install the new power supply into the box as tightly as it fits (I wasn't able to spare a millimeter!) Make sure that none of the leads or capacitors in the power supply are touching the case if it is metal. Use electrical tape or foam or some other insulator to prevent conductive contact.
Once the new PSU is installed, either glue it in or screw it in depending on what your box is. Tabs of gorilla glue or hot glue work fine (they are non-conductive). You shouldn't need much to hold it tight. I happened to have screw blocks to tether my PSU to. Lucky me!
Next it will be time t o plan out where you want all the wires to go to match the face-plate previously designed..
Step 4: Mounting the Face-Plate
We are going to use that handy-dandy gorilla glue. I love this stuff.
First though, lets modify the front plate of the box as it already is. My front plate was (for all intents and purposes) sheet metal to start with. I took some measurements of my face plate and then designed a couple of support framework to cut around such that I could cut the front plate off with only leaving a few tabs behind (see picture).
Take a dremmel and a metal cutting wheel and have some fun watching sparks. Wear safety glasses! It is really hard to avoid the spray, and they hurt and irritate you. Its worth the time. Maybe gloves too. I used a pair of pliers to hold the metal plate while cutting it. This is A) to prevent metal shards from spraying your hand, B) so that you don't get burned, the plate will get hot! and C) to protect my hand in the case that the cutting wheel breaks off (as they often do). Gloves would also work.
Next, place the face-plate onto your newly cut front frame. It should fit nicely, if not, cut it down or something.
Now here is where I should have done things differently, slightly. I wish I would have attached wires the the insides of the back of the binding posts before affixing them into the front-panel. But I got them anyways! (This is a hint to do that)
Now, gorilla glue the beautiful face-plate with all its components attached to it to the metal frame you just cut. Make sure the glue doesn't get anywhere that you may still need to solder or anywhere where the metal plate needs to be re-affixed to the rest of the box. So perhaps be cautious with the amount of glue you use and do it in multiple steps.
Step 5: Wiring Schematic
First, I made a sketch of the total schematic and transferred it to the very crude picture below. The current meter in combination with the variable voltage meter was the hardest part (it isn't that bad at all) because It requires rethinking how you run the ground line. See my schematic for the best explanation (lower left). Since the current-meter needed to be in-line with ground, I just inserted it before the ground binding posts connected with the PSU ground. I excluded the fans and indication LED's from this ground line so that they didn't contribute to the current draw shown.
The particular meters I got all required to have a ground and 5V line to power the LEDs (this ground was also excluded from the metered-ground), so each of these lines needed to be routed to the PSU as well. Additionally, the volt-meter had a sense line to measure the voltage across the variable voltage line, so connect this right to the "VAR" binding post.
Each of the remaining binding posts were then hooked up to their appropriate PSU lines. This should be pretty straight-forward.
Finally, the variable voltage circuit needed to be designed. I had a lot of designs that I could have gone with, but then it occurred to me that why not just keep it simple (stupid). I pretty much exclusively work with low voltages (between 1 and 10 V). So why not just use the cheap components I had sitting around. I used a LM317 in the circuit shown below. I am not going to explain it because I would butcher the explanation. I am sure there are plenty of people on ibles that can help design a better version :) Just follow the one I made if you want something simple!
A quick chart of which color wires go where (use in conjunction with the first picture for this step)
- Blue = -12V. Connect to the -12V binding post.
- Yellow = +12V. Connect to the +12V binding post. Also connect to the yellow wire for the V(in) of the LM317.
- Red = +5V. Connect to the +5V binding post. Also connect to the fans, LED's and source red wires for the panel meters. (only the thin red wire for the current-meter, not the thick one)
- White = -5V. Connect to -5V binding post.
- Orange = 3.3V. Connect to 3.3V binding post (can also be used to power LEDs)
- Black = ground (0V). Connect to GND binding posts. Also widely used for rest of schematic (see picture)
- Purple = 5V (standby). This line is active (5V) whenever the PSU is plugged into the wall. So the LED I have hooked up to the purple wire will always be on, even when the switch is in the off position. You don't have to do anything if you dont want to with this line.
- Green = Power On. This is the key switch. For any PSU to turn on (most), you need to short the green line to ground to turn the rest of the PSU on. So this is where we put the switch. (direction / polarity doesn't matter)
- Grey = power good. Just ignore this line completely.
Step 6: Finishing Touches
If you want to, you can spray paint the box before you reassemble it. Or you can add backlighting inside the case (I threw in a blue LED just for fun). Make sure the fans are working. Make sure the binding posts don't get pushed back. Make sure the whole thing runs before putting it back together!
With that said, everything should work!
Now I have a wonderful variable lab power supply sitting right on my desk (which is actually a door turned sideways with paper on it). It fits nice and snugly with my soldering station, lamps, and all the other electronic tools that I need. Next will be to build myself a waveform generator and I will have everything I need!
Enjoy! Please post pictures if you decide to make one! There are lots of designs out there, but mine is flashier (than most)! :)