# Can I modify a PC AT switching power supply to put out higher amps?

What I have is a 250W PC AT power supply. It has 12V 8A, 5V 30A. I don't need the 5V, so I was thinking maybe I can use a booster to kick the 5V up to 12V and add some amps to the 12V side.

So if my math is correct, 5V * 30A = 150W and 12V * 8A = 96W. If I can combine these into just 12V, I should get 20A and that's way more than I need, I only need about 12~15.

I see some DC->DC boost converter in that amp range want to start at 6V for the input, I'll only have the 5.

Q. is there a way to modify this old AT PS so that it outputs about 15A on 12V? Maybe a boost converter, maybe there's a way to modify it internally, IDK.

• Not going to be easy. Can't you just use any ATX power supply instead? That would give you much more power on the +12V rail. Commented Dec 4, 2020 at 9:48
• You would need a lot of experience to pull this off. Finding a 150 W, 5 V to 12 V boost converter will be a challenge and you will need a lot of experience with boost converters if you're going to make your own. Then you can't just put two 12 V outputs in parallel, boost converters and Power supplies aren't designed for that. The supplies will start to work "against" one another instead of cooperating. TLDR: in principle this might be possible but it has so many pitfalls that no sane EE would even consider it. Just buy a 12 V supply with the power that you need. Commented Dec 4, 2020 at 9:53
• Modifying the supply is in principle possible if you have the full schematic and if you're experienced with the designs of SMPS and if the SMPS is build such that such a modification can be done without overloading components. Again, it will be a lot of work and no guarantee for succes so again, no sane EE would even consider modifying the supply. Just buy a 12 V supply with the power that you need. Commented Dec 4, 2020 at 9:59
• The answer is "yes" only if, by "modify", OP means disassembling the AT PSU, dessoldering its components, rewinding some of its transformers, replace a lot of parts, and put all the remaining parts and the rewinded transformers in a redesigned PCB. Commented Dec 4, 2020 at 13:41
• Are you asking if you can stick an external boost converter to the 5V line and not modify the internals? Because that may work Commented Dec 4, 2020 at 18:29

The transformer and the internal circuitry design of the PSU will not allow you to simply get >8A from the 12V output.

So if my math is correct, 5V * 30A = 150W and 12V * 8A = 96W. If I can combine these into just 12V, I should get 20A and that's way more than I need, I only need about 12~15.

It's not that simple. After boosting the 5V to 12V, you can get 11A at best with a properly designed step-up converter. However, you shouldn't connect this 12V output to the main 12V output as they are not ideal sources. Plus, as they are being regulated by individual converters, there may be some other problems regarding stability.

Q. is there a way to modify this old AT PS so that it outputs about 15A on 12V? Maybe a boost converter, maybe there's a way to modify it internally, IDK.

Simply no. Removing all the secondaries of the main transformer and re-winding an individual secondary with thicker wires, replacing the secondary rectifier diodes with higher-current ones, adding parallel output capacitors (due to increased output current) then modifying the control circuitry (e.g. tricking POWER_GOOD signal) may be a solution. But this still requires knowledge of SMPS design and especially the PSU's internal circuitry.

The power calculations are not enough. They seem to allow what you want but before thorough investigations you do not know if 12V output has the rest what's needed for 15A

• transformer wire thickness
• other wire thicknesses including the connector
• rectifier rating
• output filter

In addition often the 5V output only is actually regulated with feedback and pulse width modulation. 12V output is what it happens to be based on transformer's turns ratio, the regulation of 5V and all voltage drops in the wires and components.

In theory it would be possible to change the regulation happen from the 12V output but that needs exact schematic as the minimum. It does not increase the wire thickness nor the ratings of the parts. I can recommend the attempt only as hobby project to test if one understands how a PSU actually works. To use it for something needs careful tests does the operation stay acceptable with different loads and do the temperatures of the parts stay acceptable.

I do not believe the original manufacturer could afford so much unused capacity. You probably must remake the transformer, change many parts and reinforce the wiring. But that's only a guess.

Inserting a 5 to 12 volt booster means inserting a part which dissipates a part of the energy. The calculations should take it into the account. Combining the output of the booster and the existing 12V to one is tricky - impossible without careful design of the booster for that idea. Having 2 separate 12V outputs isn't a problem if you can divide the load.