Old Computer Power Supply

Oh great explanation, it makes sense when you know that kind of stuff. Anyway, I’m going to the electronic store to buy a 10 Ohms 10W resistor today and I will give it a try. I will keep you posted! :smiley:

Hang on, the computer power supply is still plugged into the motherboard? Well, there is your 5V load.

Hang on, the computer power supply is still plugged into the motherboard? Well, there is your 5V load.

I still use my motherboard for now because it’s the only way the power supply starts but I’m trying to make the power supply start without it.

I tried with a 10 Ohms 10W resistor I bought this morning but surprisingly it’s still not working. As you can see on the photo, I plugged the resistor with a red and a black wires and I also have a jumper wire from the grey wire to a brown wire. When I plug the green wire with the white one, the power supply starts for a second and stops. I don’t understand what’s wrong, probably wrong wiring. Someone as an idea?

2014-08-16 14.25.43.jpg

Not so obvious, but just try plugging it between the black and orange (3.3V) wires.

On ATX type power supplies, I just jumper PS_ON# to common (ground) so it turns on without the motherboard … no load resistor required.


black and green?

Tom… :slight_smile:

Yes … jumper PS_ON# to COM.

The load resistor is there because the power supply regulates the 3.3V line (5V on older AT power supplies) and the other supplies are just along for the ride. IE, they are in the range because of the relative number of windings on the secondary.

But without sufficient loading, ringing occurs and while 3.3V will be regulated, the other voltages may be incorrect.

So a load resistor on the 3.3V line on an ATX supply, and a jumper on PS_ON# to turn it on.

I tried many ways to plug it but what is weird is that the wire’s colours of my connector doesn’t look like an ATX so, I attached an image of an Compaq connector (apparently what I have because the colours are the same and the pin 8 is inexistent). When the power supply is connected to the motherboard, I use the green (+5V AUX) and the white wire (PS/ON) to start the power supply. When it’s not connected to the motherboard, I put a jumper wire from a brown wire (+3.3V) to the grey wire (0V AUX) and when I connect the green wire with the white one the power supply starts for a seconds and stops (With and without a load from a red wire (+5V) to a black wire (0V)).

All videos and tutorials I saw are with ATX connectors and it seems so easy… This article http://www.experts-exchange.com/Hardware/Components/A_3940-Use-a-Compaq-200-Watt-Power-Supply-PSU-as-a-second-power-source.html is the closest I found to my connector but still not the same because on the pictures I can see a purple wire and I don’t have this wire.

My power supply is a PDP116P from compaq…


Don't go by the color, go by the position on the connector.

But with a non-standard supply from Compaq, all bets are off. I see PSON, the question is does it get pulled low, or high to turn the supply on?

Older computer power supplies only watched the 5V line for out-of-range. Newer ones watch all the lines, so without a load they may shut down.

From your description, it makes sense that you would jumper +5V AUX (green) to PSON (white). Note that if pin 16 was labeled PSON#, that would imply you need to pull it low to activate (inverted logic).

It appears that +5V AUX is at +5V while the power supply is off and for 2 seconds after turning on.

Instead of jumpers, try using just 2 diodes as follows:

[color=green]+5V AUX [/color] ---|>|--- PSON
[color=red]    +5V[/color] ---|>|--- PSON

Hopefully this will keep the power supply on.

@dlloyd I tried the same wiring I had before but with 2 diodes from the +5V AUX and +5V to the PSON but the power supply still starts only a second. :frowning:

For now, I will probably continue to use it with the motherboard (even if it takes much place on my workbench) but I will also search for a standard ATX power supply to reuse from an old computer, I think it should be easier this way.

eoto88, did you try that with a 10 ohm resistor on the 3.3V line?

@polymorph I tried the same wiring as @dlloyd suggested me but with a load of 10 Omhs 10W from 3.3V to COM... Still starts only a second. :(

I will continue to use it with the motherboard until I find a better power supply but I'm still open to your suggestions, maybe it could help someone else someday...

OK. Without being there to try some things, I'm not sure what else to suggest. Except maybe stick a scope on the outputs and see if any of them jump up substantially above what they are supposed to be. in normal use, most or all of the voltages will be loaded.