Im turning a pc power supply into a 12v psu to run a car amp, i got it running fine using a few 12v rails and a few grounds together and connected the green wire to ground to sawitch it on.
i gradually increased the wiring until i had all the 12v rails and most of the grounds connected.
all was fine so i cut the rest of the wires short (removed the atx block) to tidy it all up. as soon as i did this the psu refuses to run, it starts for a split second and then is dead.
i had the same problem with another atx psu (brand new).
the thing i cant understand is that theres nothing connected to anything else in the block itself, every pin is isolated from everything else so surely it shouldnt make the slightest difference cutting the unused cables???

 

One possibility is that the 3.3V may have a voltage sense wire connected to one of the output wires at the connector. If you disconnect that sense wire (generally a thin orange wire) then it will shut down.

 

One possibility is that the 3.3V may have a voltage sense wire connected to one of the output wires at the connector. If you disconnect that sense wire (generally a thin orange wire) then it will shut down.

Nothing is connected to anything in the block though.
Do I need to drop a load across the 5v rail?

 

Nothing is connected to anything in the block though.
Do I need to drop a load across the 5v rail?

I think there would have been two orange wires going to one socket - a thick wire and a thin wire - so those two wires would be connected at the socket. The thin wire 'senses' the voltage on the thick wire and passes it back to the regulator in the PSU. If the sense wire is disconnected then the 3.3V rail cannot be correctly regulated.

 

I always put a 4.7 Ohms 10W resistor on the 5V rail, can you post pictures of the inside of your psu, also what chip is it using, and any pictures of wiring?

I've done hundreds of these and make them Variable too.

 

i tried a 10 ohm resistor across the 5v rail and it didnt make a difference, il have a look inside again and get some photos,
ps i have 2 atx psus with exactly the same issue

 

I always put a 4.7 Ohms 10W resistor on the 5V rail, can you post pictures of the inside of your psu, also what chip is it using, and any pictures of wiring?

I've done hundreds of these and make them Variable too.

i tried a 10 ohm resistor across the 5v rail and it didnt make a difference, il have a look inside again and get some photos,
ps i have 2 atx psus with exactly the same issue

 

See pin #12 here: https://reprap.org/wiki/PC_Power_Supply#Pinouts_2
That brown sense wire must be connected to the 3.3V output.

 

See pin #12 here: https://reprap.org/wiki/PC_Power_Supply#Pinouts_2
That brown sense wire must be connected to the 3.3V output.

bingo!!!
although in my case the thin orange wire appears to be the 3.3v sensing and the brown is connected to ground, dont know why but 1 psu is now running and without a resistor on the 5v rail too.
thanks for the help guys