ATX PSU Knowledge required

[TheHippySeal]

Hey Guys. I'm doing the ATX PSU mod. My query is as follows. The Power on pin, green wire pin 16 on ATX I think. That wire, it is shorted to ground to turn it on. Now, what I want to do is use a darlington driver IC(what I have) to sort the two pins when anouther 5V input power source switches on.

I'm confused as to what the Power on pin, green wire, actually is. As if it was a 5V pin, it would be short circuited when grounded I think. As the input to the darlington driver would have to have current limiting there would have to be a resistor in there somewhere.

[Rerouter]

it is the power_on or enable input for the atx supply, from memory it cannot require more than 40mA to turn an atx supply on while pulling that pin low so your darlington pair should be fine, just limit the base current to your darlingtons, which you should do anyway, more so as to not exceed its specs than to limit the pins current.

[TheHippySeal]

Ok, I did a little drawing. I think I know what it meant when it said it had a pull up resistor on the power on line. Everything behind the > is siimulating what's there.
It should work right? Don't want any power supplies exploding. It's an old one so I'm not sure whether it has short protection.

Yer, I'll just look up the current to switch the transistors in the driver IC datasheet.

[SeanB]

You do not need the 1k resistor, as this is basically a TTL input, and has internal current limiting and a built in pull up resistor. The 100R base resistor should be around 10k, and you can get by with just using almost any NPN transistor instead of the darlington to drive it, it does not need much current sunk to turn the ATX supply on, it is a transistor it drives inside the box, not a relay. just remember the master and slave PSU's need a common ground connection.

[TheHippySeal]

Yer thanks. I was just simulating the pull up resistor. I only have a dalington IC hanging about to use, no transistors. You're saying it needs 0.5 mA to work. Yer it's probably something around that. Hmm. Should I connect the ground wires on both PSUs then? Have I missed something important there?

[EEMarc]

I don't know what you are trying to do here. Normally, the PSU is used to power an enable circuit.

Usually the purple +5VSB or 5 volt standby is used for turning the power supply on. This gives roughly an amp or two at all times even when the PS-ON is not enabled. Mine has 2.2 amps.

Then you can connect your turn on circuit to the +5VSB. When you would like to turn the power supply, use your Darlington to pull the PS-On down as an open collector with no pull up. The power supply has an internal pullup on the PS-ON line to the +5VSB internally.

If you are trying to drive a second power supply for additional power, you will need to make sure that the ground voltage levels are at the same voltage level else the PS-On circuit won't work correctly. Alternatively, you can use an optoisolator to drive the PS-ON for the second supply.

[TheHippySeal]

Ok. This is what I will be doing then. Didn't see anything about minimum switching current in the ULN2003A datasheet. Might test it or just try around 10K. Thanks for the help everyone.

[Rerouter]

just know that if you are going to tie your grounds together, all atx supplies have there negative tied to case earth, while you can remove the earth strap and have it floating, but this depends a little on the internal designs, this concern comes from the fact you generally dont want earth loops

[SeanB]

With that array you do not need a base current limiting resistor, it is built in. 500ma max, minimum is limited by the leakage of the switch when off. 1mA is about the minimum you can switch, as the leakage is under 100nA or so at 5V typically.

[TheHippySeal]

I don't really know very much about what you speak of. I happen to have some opto isolators. I was planning on completly mixing the two power supplies up tho. For example using the 12V to power a graphics card.

[TheHippySeal]

@SeanB Ok I'll make it a bit lower resistance

[Psi]

You could also use a simple 4 pin optoisolator.

Collector to psu green wire
Emitter to psu ground

Then you have a simple led anode and cathode to turn the powersupply on/off

[TheHippySeal]

It's not isolated if powering any device which shares it ground with the other, right? Don't pcie devices share the motherboards ground? I dunno if all of them even have external power connectors. Sata devices do also.

[Psi]

Yeah, in your case the optoisolator would not be isolating anything.
Since you would have the led ground connected to the ATX ground.

But it would work just fine, no other components needed.
Well, other than a resistor for the led.

A transistor would be cheaper, but if you happen to have a opto lying around it would work fine.