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I am looking for a cheap way to get +24V DC max 3A from computer power supply. I prefer easy soldering solution like DIP, and standard components that do not need to be purchased over internet. Any ideas, please?

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If the PSU has two 12V connectors (and it should), maybe you could just use them connected together? –  AndrejaKo Dec 8 '10 at 11:15
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If you connect two +12V supplies together in parallel, you will get a +12V supply that can source more current. –  W5VO Dec 8 '10 at 11:59
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3 Answers 3

up vote 10 down vote accepted

You can open the supply and look for the TL431 chip. There will be two resistors forming a voltage divider from the 5V rail to the 2.5V that the TL431 compares against. If you calculate the right resistor values, you can set this divider to output half the voltage it currently does. Then you can get the output up so that the +5V rail becomes +10V and the +12V rail becomes +24V. You will probably need to install new output filter capacitors of higher voltage rating as well. And if you modify an ATX supply this way, please cut off the ATX connector and install something else, so it won't plug in to a motherboard.

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I did exactly this with one of my supplies. You'll also have to disable the overvoltage protection, if it has any. In my case it was just an SCR so bridging gate-anode stopped it from tripping off. Before disabling that, mine was limited to about 13.5V. I made mine adjustable, using a potentiometer. See: youtube.com/watch?v=mK_rIs7uFlo –  Thomas O Dec 8 '10 at 17:38
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I have done this in the past using 2 supplies in series. The ground for the supply is the wall ground so to make it work I had to take one of the supplies apart and mount the board on nylon bolts to isolate it from the case. Then that supply floats and can be put in series. Not quite sure what happens around the current limit of the supply.

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The ground on most computer power supplies should be completely isolated from any mains conductor, including earth/safety ground. You can safely parallel two supplies together. –  Thomas O Dec 8 '10 at 17:39
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But that does not seem to be the pratice, check some out, it has been some time since I did. –  russ_hensel Jan 6 '11 at 15:08
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I am not sure you can get 3A out of it, but there are both +12V and -12V leads in ATX. You have -12V on blue wire, 0V on black, and +12V on yellow. So it's 24V betweeen blue and yellow.

If you need +24V from black wires, you'll have to arrange a boost converter and draw about 6A from the 12V. You'll likely want to use more than one lead then.

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Is there any reason why it wouldn't be possible to connect two 12 V grounds together and use two +12 V wires as a 24 V source? –  AndrejaKo Dec 8 '10 at 11:46
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Yes, there is a reason it will not work. That would be putting two 12V sources in parallel, which would still give you 12V. You would need to put the two supplies in series, but that is probably impossible since the grounds are connected internally. –  W5VO Dec 8 '10 at 11:57
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@W5VO I meant, use the two +12 V wires as + and - connectors, that is to put them in series. Still, I missed the internal GND connection. –  AndrejaKo Dec 8 '10 at 12:04
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Be aware that the -12 V rail (and all negative rails in modern PSUs) are relatively very weak. It is not likely able to sink 3 A. –  Nick T Dec 8 '10 at 14:46
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-12V usually uses a 7912 regulator. That can pass a maximum of 1A, but probably much less in practice. –  Thomas O Dec 8 '10 at 17:36
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