Maximizing amps - 20/24 pin atx power supply board or DIY with terminal block

Check out this thread.  The aggregate amps, while high, are divided into several outputs.  Use the gauge of the output wire to decide the max amps per output.

Thanks Tracker John... I figured I had missed something somewhere on this topic.. Thanks again..

If you add up all the outputs at maximum current, you'll find they add up to more than the total P/S rating, 421 watts.  That is because while each output is capable of the stated maximum, total power out can't exceed the rated 350 watts.  So, you will want to insure you don't try to draw that much out of the supply, I'd probably recommend no more than a total of 300 watts.

There are plenty of these available to make their use easier.

Thanks GRJ and John H.. Really appreciate the responses.. This forum has always been good to help and assist others.. Thanks again..

I'm curious on how your power supply pushes 35A at 5v.  A gang of 5v regulators each capable of a few amps output on each output wire?  Are the output wires connected internally?  (I need to tear into a "retired" PC supply.)

Tracker John....

Not really sure how the 5v can push 35A.  I haven't opened the PSU yet, but I feel like all eight(8) wires are connected internally. I haven't done anything yet with the PSU, that's why I was asking the question on which way is the best way to maximize the amperage.  I've seen plenty of YouTube videos of the DIY bench power supply that uses the terminal block where they basically use all of the wires for the different voltage to maximize the amps/current.

However, I'm leaning towards the ATX breakout adapter boards to make things simpler/easier because I starting to agree with GRJ that it's probably not a good idea to try to max out the PSU.

Anyway, I've attached a photo of the PSU specs just to make sure I didn't read something wrong.

This Wiki Power Supply Unit description indicates the 12v and 5v come from single rails.  Some vintages of ATX PSUs have current limiting circuits.  YMMV, but if the cluster of 5v output wires are contiguous (test with a multimeter), you can probably gang them together for a high amp 5v source.  However, I wouldn't expect to get the total 5v output printed on the label.

Thanks Tracker John...

I hope to have time to open the PSU sometime today to check out the wiring and the voltage per wire using a multimeter. I don't have any jumper wires laying around so I'll need to make a small one to connect the green and black cables so it'll turn on.

I'll let you know what I find.

Thanks again...

@Tracker John posted:

I'm curious on how your power supply pushes 35A at 5v.  A gang of 5v regulators each capable of a few amps output on each output wire?  Are the output wires connected internally?  (I need to tear into a "retired" PC supply.)

There's no magic to it.  Here's a page describing a circuit in detail to produce up to 30A of DC.  This is a pretty old design for a linear power supply, but the idea is similar if you used modern even higher capacity parts.   This one is interesting in that it used multiple pass transistors to build a modular linear supply up to 30A.

High Current 12V-13.8V at 30A,25A,20A,15A Power Supply

Here's a more modern switcher design that would likely be closer to what you see in a PC power supply.

With FET devices that can handle as much as 2,000 amps and more, building a high capacity power supply is not that hard.

35 amps at 5 volts is only 175 watts. That's not a big deal.

Also, a computer power supply does not deliver that 5-volt source to the loads on a single wire. It is delivered to the mother board on several different wires.

Note that on this circuit card, there are six 5-volt source connections. Each one is capable of delivering just over 5 amps.

You should do the same if you use one of these power supplies on a layout. Split up the feeds your LED circuits and divide the load among those six outputs. You can light a lot of LED's with five amps!

Actually, Rich, if you open up that PC supply, you'll find that all of those 5V wires go to one big connection on the PCB inside.  Any of those wires can top the full current, but they might get a bit toasty.   The multiple wires are there just to spread the load over multiple wires, not because the actual power supply can't deliver the full current in a single wire if it were a large enough wire.

I understand, John. My point was that a modeler using one of these power supplies does not have to worry about anything INTERNAL to the power supply carrying a heavy, 30+ amp load. It's designed into the supply, as you pointed out.

EXTERNALLY, a modeler using one of these supplies along with that circuit board, can easily spread the load over the six provided 5-volt outputs.

Good point Rich, I can't disagree with that.  Having fuses on those outputs would also be handy...

I opened up the PSU and looks like all the wires are attached individually to the board.  I've attached 2 photos.

I did make one mistake, though.  I'd mentioned in my original post that there were eight(8) +5v red wires.  There are only seven(7).  Obviously, I miscounted earlier.  My bad.  There are five(5) +12v yellow wires and five(5) +3.3v orange wires.

As far as testing with a multimeter, the +12v measured +11.5v, the +5v measured +5.35v, and the +3.3v measured +3.23v. The measured voltage readings were basically the same for every yellow, red and orange wire.

NOTE:  I did the testing without a resistor, which I've heard can reduce the volts a little.

I need to purchase a resistor and re-test, but at least the PSU is still good and working!

@AnotherRREngr posted:

I opened up the PSU and looks like all the wires are attached individually to the board.  I've attached 2 photos.

Look on the other side of the PCB, all the wires go to one large land on the backside.  There is only one 5V source.  There used to be a dual-rail power supply that has two 12V rails, but even those seem to have faded to just full capacity on one output.

GRJ... I see what you mean...

Thanks for all the responses, inputs, and suggestions... This forum is great..👍

Years ago I was very curious why so many 5V and 12V wires, so I popped the PCB out to look at the other side.  Mystery solved.   I guess it's easier to just use a ton of smaller wires than one giant wire, so that's how they brought the power out.