Variable DC Power Supply for <$15

Step 2: The Variable Voltage Regulator.

Start by making the variable voltage regulator. Refer to the schematics below.

Step 3: Adding the Digital Readout

Adding the digital readout is really simple! Just open up your multimeter and solder your output to the test leads. Add a switch to the battery. And that's it! You're done :D
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stewie00563 years ago
correct me if i am wrong but in the schematic you drawn the pot is just going to be a 5k resistor because you didnt attach the wiper to anything.

would you care to explain that for me?

one more thing. would a tl780-05c voltage regulator work?

If you have a 3 pin pot, attach the wiper to the upper leg, the same leg that attaches to pin 1 of the LM317.

I'm pretty sure that's just a standard symbol for variable resistor.
In comparison to some other similar instructables, I'd say the wiper terminal just needs to be connected to ground (ie. connect the wiper terminal to the bottom (in terms of the circuit diagram) terminal of the 5k variable resistor.
Jonny1247 months ago
I'm no expert but I think the max of the lm317 is the max of the power supply witch is also the max of the whole thing
Dwmiller7172 years ago
I want to build the variable power supply. Can I build it with a 30 volt power supple? Will I have to change all the other parts? Thanks Dave
Hi there,

This is a very cool instructable! I already bought all the parts, except for the capacitors. I am quite a novice with electronics so please be patient… :)

“0.1 uf Mylar Cap (~$0.10)” – Is this 0.1 uf Radial Electrolytic capacitor? In the pictures included in the pdf file it is a 0.47 uf 50v capacitor, will this also work?

“1 Axial Electrolytic Cap (~$0.20)” – In one of the comments “silencekilla” said that is should be a radial capacitor, is this correct and what should the value be? 1 uf ?

What was the output voltage of the power supply?
im sorry if i missed this but what voltage power supply is used?
the links are broken so i cannot see :(
and also would the resistors regulators and such change if i were to use a different voltage power supply?
(sorry i do not know the purpose of these things and what they do)
LM317 Voltage Regulator ($0.50)
0.1 uf Mylar Cap (~$0.10)
1 Axial Electrolytic Cap (~$0.20)
220 Ohm Resistor (~$0.05)

thanks in advanced
How do you know what capacitors and resistors to use?
burzvingion6 years ago
While this will work well for small stuff, neither the "wall wart" nor the voltage regulator will provide much more than an amp, whereas bench supplies that cost $$$ generally provide upwards of 10, 20, or even 30 amps. Personally I like the idea of turning an old ATX switching power supply into a multi-voltage bench supply. You can find guides for it almost anywhere, and the power output is very nice. Granted, you're stuck with the standard +/- 5, +/- 12, and 3.3v levels. So for $15, not too bad, especially when you need an obscure voltage.
Hmm. Don't forget you can mix them to get non-standard voltage, ie 3v + 5v or 12 + 3v or 12 + (-5v) or etc. etc.
I agree. In fact, on my bench I've got a variable supply and a fixed supply. PC supplies need a dummy load of a few watts, eg a resistor or indeed, an old motherboard. My latest incarnation is to hack an old hard drive to get the 4 pin power plugs and the bit of printed circuit board round them and make a plug that fits into one of the 4 pin power sockets coming out of the supply of the computer that sits on my test bench. Then bring out the voltages to banana sockets on a face plate on the front. The module is thus self contained and can be moved to different PCs. 12V brake lamp from a car for the short circuit protection on the 12V and 6V 500mA torch globe for the 5V protection. I'm finding I use the fixed 12V/5V a lot more than the variable voltage power supply, though both are really useful.
wii5523 years ago
I built that exact circuit, and when i have no load, i get up to 15 volts ( i am supplying 20), but when i attach a small pager motor, the voltage becomes only 4 volts and i only get 12 milliamps of current. What is wrong?
ummm.... thats a radial capacitor not an axial capacitor...
hitachi84 years ago
how much power can i draw from this little thing ? what if i put a big heatsink on the transistor ?
neardood5 years ago
Rrom experience, i know that when you try to draw 1 Amp or above from the voltage regulator, it gets HOT. So, I reccomend putting a fitting heat sonk on youre voltage regulator.
I have many heat sinks, but i haven't come across a heat sonk for a while. Do they still make those? =)
scratchr4 years ago
you inspired me.
I dont know much about this , but is a LM317t good enough ? What is the difference ?
It will work.
TOCO4 years ago
is anyone else having a problem with the links? I checked all of them in firefox and they all come up 404 not found. I just updated firefox to the newest version.
godofal4 years ago
these instructables are the best in my opinion, cheap, easy, and smart!
Mark685 years ago
I need to have a ac/dc that is adjustable from like 50 vdc - 30 vdc. Can anyone point me to a website that might help me make something like that. In other words I want to be able to plug it in to the wall and have the ability to go from 30 dc to around 50 dc. Any suggestions would be very helpful, specially a website that I could get schematics from. Thanks
does it also work with a LM340T-12
I found a better one lm337(it makes up to -20V(change -20V witch ground and you got +20V))
hi.... this device looks really compact and neat. but if i want to incorporate a feedback loop such that the output current or voltage can be regulated and controlled depending on my feed back parameters, how do i do it?
gilbert20485 years ago
this is a great idea, although i would have gone with a higher voltage, probably 24V
recon5066 years ago
ummm. i'm kind of a noob at soldering on PCB's. could anyone explain to me how you would solder this project onto a PCB?
icinnamon (author)  recon5066 years ago
I actually used a prototyping board. Are you asking how to solder? Or what to solder it to?
what im really asking is if i need to etch my board
icinnamon (author)  recon5066 years ago
Yes, a breadboard works. Any board works... it just depends how neat you want it. For the best looks, go with a PCB. For okay looks, go with a prototyping board. For temporary use, go with a breadboard. You only need to etch a PCB.
I'm new to semi-conductors and diodes and I wanna ask u just how ppl determine what size capacitors, resistors, use in these circuits? I'm already familiar with residential and industrial electrical systems, but I'm really trying to learn about the inner workings of computer components and such as I know there is a world of difference.
what are the codes for the supplies you have listed. i'm trying to fibd them @ all electronics corp. but i cant find the electrolitic cap, the resistor, and the mylar cap on there
icinnamon (author)  recon5066 years ago
You can't really link to them (you'll see why in a second). Go to and click on capacitors on the right and then click mylar and select the correct value. Do the same for axial/radial electrolytic. Then choose Resistors from the right side and pick the appropriate value. Have fun!
ummm, i cant find an axial cap of your specs. from the pics it seems you used a radial cap
icinnamon (author)  recon5066 years ago
sorry i meant radial :)
i just looked up the pot you said to use, and it has 3 leads. on your scematic it only shows it with 2 leads. wtf??? (plz help)
icinnamon (author)  recon5066 years ago
How much do you know about electronics? If you know a bit, you should really invest in a multimeter (they start at $7). So basically just use a multimeter to see which lead is which. I believe the lead closest to the know is the common and the other two are the variable ones. Remember, you only use 2 out of the three leads. But truly, the best way to find out is to use a multimeter...
so i use the variable leads?
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