Easy variable voltage power supply


Sometimes, to supply the necessary voltage in a project, i needed to buy a lot of batteries, or i had to look for a proper power supply for hours and it is not necesary, beacause now i have that awsome variable voltage power supply that can supply from 1,25 to 29 volts and 500mA, but if you decide to make your own, it would be able to deliver 3A.

It is easy to make and only cost me 4$.
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Step 1: Materials

-A 30-32v power adapter. (Mine is salvaged from an old printer)
-A DC-DC buck converter. (1$ on ebay)
-A LED panel voltmeter. (2$ on ebay)
-An audio spring terminal. (0.40$)
-A potentometer, the same value as the one of the converter, mine is 10K (0.60$)

Step 2: No project box needed

I know that kind of power adapters have a lot of empty space inside that we can use to place our components, so i'll open it to se if it can be the project casing.
If you use a transformer with a rectifier circuit, you'll need a different box.
As you can see un the pics, there's enough space for all the components.

Step 3: Electrical connections

The wiring is pretty easy, you can see it in the diagram.
What you'll need to do first, is replace the small multi-turn potentiometer with the big one, it'll make the voltage controlling easier.
Then connect the voltmeter and the spring terminal in parallel at the converter output.

Step 4: Preparing the case

Picture of Preparing the case
Open three holes in the case. one for the voltmeter, other for the terminal and one for the potentiometer.
Then glue them all to the case and cover the connections with tape to avoid short circuit risk.
I didnt have any knob on hand, i recomend you to use one for a better appearance.

Step 5: Final steps

Now solder the 30v output of the adapter to the converter input and cover it with more tape.
Then close the adapter with everything inside and glue the case with a strong glue as epoxi or cyanocrylate.
mlocke made it!1 hour ago

I used different packaging but the componants and circuit is exactly the same as your 'ible. Thanks for posting.


Really nice Instructable, also very portable which can be useful.

I've been thinking about making one of these but maybe with the power supply built in just so I can carry it around and use it as a power supply. Probably just chuck a 9V battery in there; get a bigger enclosure if needs be.

gkal made it!6 days ago

Excellent instructable. Case was hard to close but finally did it. Put the connectors to the side. Thank you!!!

akumar15910 months ago
Hey this looks great and want to make one, but quick question i have a old laptop charger and on the details its got

INPUT 100-240V ~ 1.4A 50/60 HZ
OUTPUT : 19V-- 4.74A

Im a bit of a noob at all of this but what output voltage would i be able to get or is this determined by the D-DC buck and the potentiometer?
iq201 akumar1593 months ago
This is odd. My computer adapter has the exact same specs. When I use this adapter with my "pinomelean" power supply cranked up to max, I get 100% of the voltage (19.2V) at the output. There is zero drop in voltage. (That's a good thing.) I checked with 3 different multimeter. I am using a multi-turn pot, instead of a single turn, but I don't think that matters.
pinomelean (author)  akumar15910 months ago
You can get 17.6v as maximun.
iq201 pinomelean4 months ago
Is that because the buck converter is 92% efficient?
pinomelean (author)  iq2014 months ago
Buck converters are always working, so they output a maximun of Vin-1.5v
dbyrd263 months ago
The terminals are your positive and negative outputs I believe.
pinomelean (author)  dbyrd263 months ago
tgaffney13 months ago
step up or step down converter
pinomelean (author)  tgaffney13 months ago
Buck=step down, but you can use an step up/boost converter if you want.
swapnil20253 months ago
what are these terminals for??? check out the links even check out this video plz reply fast..need ur help
pinomelean (author)  swapnil20253 months ago
I don't know what those terminals are for, mine didn't have them, maybe for an on-off swich or something.

I can't really tell why the voltmeter isn't working.
Try to connect an external voltmeter and see if the converter works correctly, if it outputs the whole range of voltages, better buy a new panel voltmeter.
But if the output voltage doesn't work, replace the buck converter.
swapnil20253 months ago
output is coming uptill 5v but after that there is a sudden drop in voltage to 2v even if the resistance is increased and the display doesnt work it just starts blinking after 5v?plz reply early
pinomelean (author)  swapnil20253 months ago
Check the voltmeter measuring range, some ones can't measure under 3v.
Is your converter a buck one? What's your power supply voltage?
be sure you have connected the pot in the right way.
i i bought this digital voltmeter display

and this dc-dc buck convertor

my input power supply is around 20v for the buck convertor

is there a specific way to attach the pot...i mean about two end terminals ..?????
pinomelean (author)  swapnil20253 months ago
Connect the center pin to the center pad, ans the other two pins to the other two pads.
swapnil20253 months ago
should i decrease my potentiometer value..???
swapnil20254 months ago
how to decide the value of potentiometer.. as my dc to dc buck convertor is different??
pinomelean (author)  swapnil20254 months ago
If it has it's own pot, only replace it (same value)
swapnil20254 months ago
how to decide the value of potentiometer.. as my dc to dc buck convertor is different??
swapnil20254 months ago
how to decide the value of potentiometer.. as my dc to dc buck convertor is different??
swapnil20254 months ago
How to check whether LED voltmeter panel (display) is working or not..???
pinomelean (author)  swapnil20254 months ago
Connect it to a regulated power supply with a known voltage.
Sayedur4 months ago
Great Instructable. Thanks for sharing.
iq2014 months ago
(removed by author or community request)
pinomelean (author)  iq2014 months ago
When you adjust a voltage at the pot, only a little current flows, but when you conect a load, the resistance on that side drops ad the output volage changes.
See this video ( for more information, this guy made the videos where i learnt my first electronics, dont miss it!
iq2014 months ago
Do you find your buck converter gets hot? Should the buck converter have a heat sink on the switching component? Or should you put holes in the box for ventilation?
pinomelean (author)  iq2014 months ago
In my case, the power supply is limited to 500mA, and the converter can handle 2A without heatsink (but with ventilation) and 3A with it.
If you are foing to use with more than 1A you should use a heatsink.
iq2014 months ago
This is a great tutorial. Based on your foundation, I ordered components I need from Ebay to fit my personal needs. I did a few things differently, but the fundamental concept is the same. Thanks!
swapnil20254 months ago
i have a smalll ac adapter output pin is different... and even tell whether the ac adapter will work with ur project??can i mod that output pin??

visit these links to check out images..
pinomelean (author)  swapnil20254 months ago
Mine was the same, i opened it and only used gnd and +32v cables.
If you don't want to open it , cit the gns and +32v cables and solder them to a converter.
sammydogjj6 months ago
Thanks also how do you find the value of the potentiometer needed? That's all!

Just look at the value of the one you're removing. In the picture from the auction pinomelean listed it says W103 on the side, which is 10k.
psycholist6 months ago
Thanks for the tutorial!

I have a question about a possible modification. There may be an obvious reason why it won't work, but I don't see it, so maybe other eyes can pick out the flaw.

On the original circuit there is a power transistor. The item mounted to the L-shaped piece of metal by a screw. Whether it is stepping the 110v DC down to 32v or the 32v down to 16v I'm not sure. Since it already provides voltage conversion shouldn't it be possible to just change the voltage it is supplying?

On the existing circuit there should be 2 resistors connected to the gate of the transistor. These would be acting as a voltage divider. Altering the voltage seen by the gate alters the voltage output by its emitter. By swapping one of these resistors for a potentiometer it should be possible to alter the supplied voltage. This would eliminate the need for buying an extra conversion circuit board. Or I'm missing something, which is certainly possible.
pinomelean (author)  psycholist6 months ago
I think the mosfet is actually switching the 220Vdc crectified from the 220AC mains, then a transformer in flyback configuration steps down the voltage to a 32v and a 15v rails, the output voltage depends of the wire turns in the secondary of the transformer.
the output is rectified to 32Vdc, and then we use a variable DC-DC converter to obtain the voltaje we want under 30V.

If i'm wrong, tell me, because im interested in dc switching power supply working and design.
Perhaps you are correct with the one you have. I've got one here with a bridge turning 110vac into 145vdc, then a mosfet stepping that down to 32vdc. The problem I face is that almost everything else is smc, so there's no reasonable way for me to replace the resistor with a pot.
sammydogjj6 months ago
Could you tell me the model of dc dc buck converter?
pinomelean (author)  sammydogjj6 months ago
I dont know thw model i bought it here:

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