As the title suggests; when connecting the PS_On signal directly to ground in order to switch on an ATX PSU, is there a required amount of resistance?
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To try to clarify my particular problem; I have a fully modular ATX PSU (a Seasonic Platinum Series) but am not using it to connect a motherboard, so have no need of the bulky motherboard cable. I decided to order a custom-made cable compatible with the modular connectors on the PSU itself, with a wire connecting PS_On to ground (via a 2-pin header so I can connect a suitable switch or LED if I want). However, when I connect the pin directly, although the PSU switches on, it almost immediately switches off again.
The PSU is not at fault, as connecting the regular motherboard cable using the paper-clip trick works just fine and the PSU starts providing power normally. The only difference I can see between the working and non-working cables is that the non-working cable uses much shorter wires and of a finer gauge (AWG22 at about 5-6cm as opposed to AWG18 at around 60cm). So I'm wondering; is my cable failing to work because of a lack of resistance?
If so, I'd appreciate anyone that can also clarify how I would work out what type of resistor I would need to add to correctly complete the circuit.
Best Answer
I suspect your problem is lack of connection for the sense lines. If you take a normal power supply, with just separate insulated wires in a bundle, and look at the motherboard connector, you will notice that several pins have two wires crimped together. The larger gauge wire is the actual power wire and the smaller gauge wire is the sense wire.
The purpose of the sense wire is to overcome voltage loss due to resistance in the wire harness: the power supply increases the 3.3V (for example) so that at the other end of the wires, you get true 3.3V. These may also be a sense wire on ground.
If the sense wire is not connected, the power supply detects a fault and shuts down. If you cannot tell which wires are sense in your original harness, you will need to connect together ALL +5V wires, connect together ALL +3.3V wires, connect together ALL +12V wires, and connect together all ground wires.
Edit: I just checked the ATX specification and it appears that there is probably only a sense line on the +3.3V. Try just connecting together all of the +3.3V (orange) wires.
Additional edit: the answer to the OP's specific question is no, no resistor is required, as shown by the fact that the power supply works with its original cable harness.