# How-To: Variable DC Power Supply

Hobbyist electronics projects need robust, reliable power supplies for prototyping and testing. I learned how to build this circuit from the Basic Analog Circuits class at ITP taught by Eric Rosenthal, but took it several steps further in building a solid enclosure and integrating a voltage meter. Now it lives on my desk, ready to power most small projects I’m working on, ranging from 0 – 24 volts and up to two amps. You can learn all about how to make one yourself using this Make: Projects tutorial.

The mini volt meter from Adafruit is what brought this project to the next level. All of the components on this three-digit, seven-segment display are packaged into a small size. All you need to do to integrate it into any project is attach a positive and negative lead to whatever you wish to measure.

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## 34 thoughts on “How-To: Variable DC Power Supply”

1. Anonymous says:

Nice little power supply project, Michael! Incorporating the voltmeter was a good move.

A couple of notes:
The minimum output will be 1.2V, not 0, as that’s the preset output of the LM317.
The pot in the Adj return leg let’s you increase the regulated output, but not decrease it.

For 2 Amps (really only 1.5, per spec), I would recommend a much bigger heat sink, and better ventilation; maybe even a mini fan.

Whenever the LM317 is regulating the output to a value below the raw supply voltage, say 5 volts out, it has to drop the difference across the regulator. If your bulk supply is around 30 volts as this appears to be, that’s 25 volts dropped across the LM317, at 1.5 amps, or 37.5 Watts dissipated.

This case (TO-220, or LM317-T) has a thermal resistance from the junction inside to the case of 4°C/Watt. If you could hold the case to 25°C (an *infinite* heat sink), the junction would be at (37.5*4 + 25) = 175°C. It’s rated for a maximum of 125°C…

The poor device will get smoking hot!

Dave

1. Michael Colombo says:

Thanks Dave! I’ll certainly be careful when using it, and that should go for anyone else who attempts this project. I wasn’t planning on drawing too much current from it, but am now considering adding a larger heat sink and ventilation fan in a future iteration. Thanks again for a great comment.

2. Eric Marcoullier says:

I see the bridge rectifier in the circuit diagram and in some of the photos, but it’s not included in the parts list. Does it come with one of the other parts, like the step-down transformer or voltage regulator?

1. Michael Colombo says:

Yes there is a bridge rectifier – the part number is BR805D – I’ve updated the tutorial with it. Thanks.

3. Anonymous says:

One more comment: the 5K variable resistor could be changed to a (more expensive) multiturn part to provide better resolution in setting the output voltage.  You could buy it new from Digikey (3545S-1-502L-ND – \$26 – ouch!) or find one surplus (http://www.halted.com/commerce/ccp18376-3w–10-turn–wirewound–plastic-shaft-pot-5k-502-0120-rep527.htm – \$16 – better)

4. Anonymous says:

Leuk ding, mijn zoon heeft net een Dehler Sport Sprint aangeschaft, waarin ook een starre 12 volt accu zit.
Wat is de prijs.
4 november 2011 viert hij zijn verjaardag.
s.faber8@kpnplanet.nl

5. Sean Michael Ragan says:

I have no sophisticated EE insights into this project, Michael, but I wanted to say that I love the way the case looks.  Sure, go ahead and pretend like you didn’t do it on purpose…

6. samuel says:

i need write up on variable power supply of 10amps 0-20v and construction steps

7. samuel says:

i need a circuit diagram for 15amps 0-20v variable power supply

8. prodctodc says:

it is a good idea. waiting for you success.

9. max says:

Hello,

Can you guys help me out. I Just made my power supply with a rating of 1.25-30v and I was wondering if how can I connect my 3 wire mini voltmeter (DC 4.5-30v). My power supply reaches a max voltage of 32v and I don’t know if the voltmeter can handle that voltage.

10. Chris says:

The link to the tutorial doesn’t seem to work. Is it down?

11. snarkyboojum says:

12. Martin Daigeau says:

I can’t seem to find the actual project steps. I’d love to create one of these for myself, please help me find steps.

13. Suchit says:

For future readers who are unable to access the project steps from the link above, here is the actual tutorial: https://makezine.com/projects/0-24-volt-2-amp-bench-top-power-supply/

15. Sam Lasiste says:

Hey guys. I’ve got some problems in making my power supply. Hope anyone can help me.
Okay, I’ve got my circuit all done, but what I don’t understand is that the output voltage is fixed at approximately 30V. I’ve checked almost everything, from the condition of the LM317, the potentiometer, the connections (open or short circuits), and everything seems fine. Can anybody share a solution or suggestion to solve the problem? Thanks.

16. Ace says:

If I were to put a fuse, where is the best place to put it in the circuit?

1. Andrew Terranova says:

A fuse on the AC input line will provide protection against an internal short circuit. You could fuse the DC output too, but chances are if you try to power something that needs more current than the power supply can provide you will see the voltage dropping before you cause any real damage to your circuit.

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### Michael Colombo

In addition to being an online editor for MAKE Magazine, Michael Colombo works in fabrication, electronics, sound design, music production and performance (Yes. All that.) In the past he has also been a childrens' educator and entertainer, and holds a Masters degree from NYU's Interactive Telecommunications Program.

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