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Dose anyone know how to make a DC 20v - 3.5A power supply and if so can it be made from an ATX computer power supply Answered


It can't be made from a computer ATX power supply, they have a max output voltage of 12 volt. Plus it would be hard to mod a switchmode power supply. You need 20 volt transformer and a bride rectifier for dc voltage. Or option two is you get a 24 volt transformer and lm317 to drop 24 volt down to 20 volt. You can find the circuit diagram here Datasheet As long your transformer volt is at lease 3 volt higher than your output voltage, LM317 will work. Hope this help out.

One of my PSU's have an max voltage output of 12.2v. Just to let you know... Also, I have seen some people modded their PSU to get twice higher than its rated voltage, i still don't know how to do that...

It depends on how far they modified it. If they replaced the transformer, checked and replaced as needed the parts after it so they had high enough voltage rating (possibly schottky diodes and capcitors), then altered the feedback loop so the target was the higher voltage, that could be effective.

If they just altered the feedback loop, then shooting for such a large change in voltage without changing the transformer would mean pretty noisy output from the PSU, and possibly heat issues if the PSU weren't rated for quite a bit more than the load current.

Why not just buy the right PSU for the job? Sure, modifying and reusing things is great when you can, but the whole point of our modern world is to have what we need available to get the job done right instead of hacking *everything* just to meet basic needs.

An LM317 is not capable of 3.5A. The general idea of using a linear regulator to drop the extra voltage is possible but not with LM317.

You can you need a bypass transistor . So what is a LM317 is for a paper weigh? You can use to drop the voltage. Look at page 17 on that link that to data sheet they show a bypass transistor in use. Or the other way of doing it using a resistor to drop the voltage down to 20 volt from 24 volt. Using v/i=r (24-20)/3.5= 1.5 ohm P=VI 20*3.5= 70w so using a resistor it get quite hot, so um yeah I'm not sure how you are going to drop the extra voltage down.

True, you can use more parts than just a LM317, but at over a dozen watts lost that is a lot of heat to deal with. IMO, better to just use a PSU that was meant to produce 20V. Using a resistor it won't get any hotter than using the transistor, because either way it is a loss of exactly the same amount of power, I'm not sure what made you think the LM317 plus complimentary parts is more efficient because it isn't. Granted, a transistor facilitates easy heatsinking to a large sink it's mounted to, but so do power resistors of the appropriate rating. Also, if using a transistor with LM317 and/or other complimentary parts, the voltage drop isn't necessarily 3V anymore. Switching PSU aren't necessarily hard to modify. They all work based on the principle of feedback to change pulse width or frequency (usually the former). That means in the feedback path you do one of two things, either change the feedback voltage divider ratio (resistors) or pull the voltage up with a tap into a higher voltage portion of the circuit.

To clarify, if one were going to use a lossy way to drop this much voltage, a regulated output from LM317 or other methods is superior to using a resistor, but since either method uses parts quite a bit larger and more costly than just the 50 cent LM317, several other factors come into play. For example, if it is a static load and one had to use the same transformer, putting a resistor between it's output and the regulation circuit input would also help to drop voltage prior to the loss of the regulation stage. It's not a pretty way to do it, I'd sooner take some windings off the transformer before that if it were somehow the only transformer on earth which might be used instead of another with lower voltage rating, but it is possible.

A lm317 can work for this project...You can add 2n3055 transistors to increase amperage. Their are plans all over the internet for this. And also as mentioned you can look up the datasheet from ECG.
You can also google free diy power supplies schematics/plans. Their are plenty of diagrams in these places as well.
Their are a couple other vatiable power supplies lm regulators out their as well.I have the shortcuts on my other computer otherwise i would post the shortcuts here.


8 years ago

And the whole idea of Instructables is to do the exact opposite. But the "basic needs" bit, yes, I agree with that. I want to do the exact opposite (almost), 12vDC-5vDC for my car cd player :(.

That was to AC-DC's last comment btw, clicked the wrong button :(.