Disconnect the PSU's 20 or 24 pin connector from the motherboard, short the green pin (PS_ON) to any black pin (GND) and it should turn on, if it doesn't it's faulty.
Monitor the grey pin (PWR_OK) it should go from 0V to 5V within about half a second to tell the motherboard that the PSU deems itself to be working within limits, if it goes low the motherboard will shut off the PSU.
It already turns on via the button. Fans run and the DVD LEDs light up. So it has 12v and 5v. DVDs will also open and close and spin up. No LED showing disc activity, though.
It doesn't shut it down.
As I said, the only strange thing (other than the motherboard appearing totally dead) is the PS fan doesn't run until the reset button is pressed. Then stops when released.
I was trying to eliminate the motherboard from the equation, not much point buying a new PSU to check with, if it's not the problem ...
Powercut != brownout. Most computer power supplies will happily run on any thing from 100 V to 250 V AC or DC.
This abilty of SMPSU's to run on almost anything confused the heck out of me when the supply dropped to about 125 V at 0400 one morning An ice storm brought the distribution lines down in half a dozen or so places and snapped a few poles. Some bits of kit were on and quite happy, others off.
Perhaps so, but it's not usually just one or the other. It most likely drops a random amount unevenly, possibly getting under the 100V for part of that time.
How come the mini SMPSUs in my LED lighting make the LEDs go noticeably dimmer when the voltage drops a bit? (Like 10-20V) Yet they are rated 85-265V. Or are they actually just dimming for a fraction of a second before compensating, and my eye detects this as dimming? I thought the way SMPSUs worked, the output wouldn't vary one bit?
The most common situation round here is that one of the three phases goes open circuit and neutral floats up to a voltage mid way between the two remaining live phases. So 240 vac drops to around 200v. LED lights and PC's don't even notice this but filament bulbs glow dim orange.
Decent ones don't. The cheap and nasty ones might. Most will work OK with anything above about 100v just drawing more current to compensate.
The fact that the PS fans don't run at switch on seems odd. But do when you press the reset button.
You must have some weird supplies there. I thought they were all similar to what I have. There's a substation across the road from me which takes 3 phase 11kV in. It outputs 3 phase 240V. The neutral is grounded at the substation with a big stick. Each house (they alternate between phase 1, 2, 3 as you go along the street) gets a phase and the neutral, supplied to the house with an armoured cable. A 100 amp live carrying a phase, surrounded by the armour which is neutral (and ground). My neutral can never be anything but zero volts, because of the big stick. Mind you, I guess if the supply to the substation lost one of the three 11kV phases, some weird s*it might happen to th voltage on my 240V phase. But I'd expect breakers or something to detect that and shut it off before someone gets unusual voltages.
Perhaps I'm just seeing a very temporary dimming, the human eye isn't that accurate. If they can take 85V, then they can't dim much by dropping from 240 to 220. It's just I thought it would be impossible for a SMPSU to do that. In its simplest form, it's charging a bulk capacitor to 340V DC, or less if your input AC is lower. Then that's passed in a controlled manner by an IC and transistor to the lower voltage output capacitor (in my case 70VDC for the LEDs). Surely if the bulk capacitor lowered in voltage due to a brownout, the duty cycle for transferring power to the output capacitor would just increase to compensate?
yes, a delay on the cpu/motherboards fans I could understand, if the SMM takes a while to initialise, but not generally the PSU fan.
If you unplug all the PS connectorss from the MB, disks etc and turn it on does the PS fan blow, or does it need a load to do anything ?.
Yeah, very unusual symptom. You should state the model number of that PS and the motherboard.
Not a load, but it does need the signal from the motherboard to turn on. That can be kludged from the +5 standby line.
Won't work as it stands. You need to short a particular pair of pins on the big connector.
I wouldn't be without my cheap Chinese PSU testr. Saves a lot of time and effort.
The +5VSB pin (purple) isn't involved in turning on a PSU, just short the PWR_ON pin (green) to any GND pin (black) ... unless it's an older DELL.
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