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bizzare atx psu problem

Gaspode

New Member
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 switch 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???
 

gophert

Well-Known Member
Most Helpful Member
You need about 0.5 amps on one of the 5v rails as a minimum load for reliable start up. You got lucky on the first startups as some peo0le do sometimes. Add a 10ohm, 10watt resistor between 0 and a 5v wire. Solder one of your cut wires back to the 5v source on the pcb and connect to any ground (Black) wire.
 

gophert

Well-Known Member
Most Helpful Member
Any of the 5v rails? It ran fine more than a dozen times with nothing on the 5v rails.
Any. Like I said, reliability is sporadic when minimum load is not met. Could have been a change in temperature, humidity or the price of tea in China that hour.
 

Gaspode

New Member
Any. Like I said, reliability is sporadic when minimum load is not met. Could have been a change in temperature, humidity or the price of tea in China that hour.
LOL it confused the hell out of me.
What value resistor do I need to drop across a 5v rail?
 

Gaspode

New Member
I didnt cut them all the way back incase i needed to get at them again, wise descision!
Is 22 ohm too much? I have 1 lying around,
 

gophert

Well-Known Member
Most Helpful Member
I didnt cut them all the way back incase i needed to get at them again, wise descision!
Is 22 ohm too much? I have 1 lying around,
That should be worth trying since it is starting some times with no load. V5/20 ohms = 0.25 amps or 1.25 watts. Make sure the resistor is fat enough to handle it (2 watts or more is safest).
 

gophert

Well-Known Member
Most Helpful Member
The other wire ( GREEN ) on the ATX block must be grounded out... This is the wire the motherboard uses to self power! I use an on/off button on this wire.
See OPs post #1 - first sentence...

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 switch it on.
 

Gaspode

New Member
right i have 2 atx power supplies, connected the 12v to the amp (and made sure it switches on) and put a 10ohm resister across a 5v rail and they both still do the same thing, ie turn on for a split second then off again, do i need a load on a 3.3v rail?

and yes iv connected the green wire to ground to switch the psu on
 

gophert

Well-Known Member
Most Helpful Member
Make sure the fan is plugged in and spinning. Many PSUs shut down if they don't detect the pulse of the spinning fan.
 

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