Computer PSU mystery

[MarkD]

I'm trying to fix a secondhand motherboard to my computer. The existing psu has 12pins. The new(secondhand) board has a 20 pin socket to accept power from the psu. I've tried a couple of (actually 3) psu's borrowed from other pc's and none of them work, ie the cooling fans do not spin when connected to the power. All of them work when back in their original computers. Does the psu have to be fully connected before the cooling fans will spin?

What's going on?

I've been trying to test the psu's before taking the pc apart and connecting everything. Do I just have to bite the bullet?

Thanks in advance...

Cheers, Mark, M/cr

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[Harry Bloomfield]

MarkD brought next idea :

A Switch Mode Power Supply, which is what a typical PC power supply is, needs some sort of load on it or it will shut itself down. An adequate load would be something like a hard disk, plugged into one of the four pin connectors. I'm not that familiar with the 20 pin jobs, so these might well need something more, such as certain pins to be linked.

[Rob Morley]

12 pin is AT format, 20 pin is ATX. They're not really interchangeable - you can get adaptors to plug them together, but the case will need some modification or bodging as well. Switch-mode PSUs will only work with at least a minimum load across the output. In ATX PCs the power is controlled by the motherboard - if it's not fully hooked up you'll need to momentarily short out the pins that would be connected to the switch on the front panel.

[Sparks]

That sounds like an AT style motherboard, where you have a switch on the front that will turn off the power supply via the mains side (Big CLICK switch that you can see is on or off without it being plugged in (Or if it is a push button type, you can feel it latch on or off)

This sounds like an ATX power supply, these work differently to AT ones. On the motherboard, there should be two pins probably marked PWR SW or something that will mean power switch - these need to be momentarily shorted to get the power supply to turn on.

Or maybe the motherboard is faulty!?

Sparks...

[MarkD]

Ahhhh, that explains it....

...been driving me potty that one.

Thanks guys!

;-)

Cheers, Mark, M/cr

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[nightjar]

Chuck the old PSU and buy one to ATX specs. Personally, I doubt that there will be much, if anything, worth saving from the original machine. Even when I absolutely *had* to use the case from an AT machine (the bloke I was upgrading it for had had furniture specially built for it and nothing else would fit) I ended up cutting a hole for a new fan as the cooling wasn't good enough without.

Colin Bignell

[Colin Wilson]

What make is the "new" board - Dell use proprietry PSU connectors, and they often won`t work with "normal" power supplies.

[MarkD]

SNIP...

Yes, I hadn't realised that the two styles of MoBo treated power differently. What I hadn;t done is connect up all the hardware, switches etc. Momentarily short the power/switch pins eh? Watch out for a smal mushroom cloud in the direction of Manchester....

I'm quite prepared for the Mobo to be faulty after all but what I couldn;t work out is why none of the psu's were working.

Cheers, Mark, M/cr

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[MarkD]

SNIP...>

the ones I have been trying are atx...and i'm very closwe to junking the whole lot but the idea was to create a cheap second pc to use the internet on with a cheap 20ukp wireless card installed.

The irony is the computer never worked properly in the first place. TXPro2 motherboard that never recognised the wireless network card and wouldn;t reinstall windows properly....hence the attempt at installing a new board bought fresh from a computer fair....

Thanks again, all...

Cheers, Mark, M/cr

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[Colin Wilson]

Holy crap is that all you wanted it for ? - check out

[MarkD]

Heh heh, some good proces there....

M/cr

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[RedOnRed]

Traditionally, with PSU's, if you just connect mains power into them without connecting the PSU's power leads to say the motherboard etc...then the PSU will blow. It needs to dispel it's incoming power some where.

[Andrew Gabriel]

Hum, I've done this tens, maybe hundreds of times, and never had a PSU 'blow' in the process. A few AT PSU's will shut down with no load, and most have poor regulation on the +12V and -12V rails until you are drawing at least a few amps from the +5V, but this doesn't harm the PSU (although if you have connected something to those rails like a disk without drawing enough load on the +5V rail, you might have problems with the disk drive -- this was quite a common scenario).

ATX PSU's will just provide the low current 5V standby rail if you plug them in with no motherboard connected. That's effectively part of the ATX spec, as with the PS_ON pin disconnected, the rest of the PSU will remain switched off.

[Kinell]

"RedOnRed" wrote in news:d5tl8v$2f3$ [e-mail]:

Nonsense.

[Kinell]

MarkD wrote in news: [e-mail]:

That suggests that the original PC, PSU and mobo are AT standard, whereas the replacement PSU and mobo are ATX standard.

AT PCs had a conventional off/on mains switch on the front panel.

ATX PCs are powered into standby mode when connected to mains and use a 'soft' power on switch that tells the mobo to tell the PSU to come out of standby and start up. The power on switch needs to be a momentary close that shorts a pair of contacts on the mobo.

Given the low price of PC cases, a replacement may be an easier prospect than mackling the existing case.

[RedOnRed]

That surprises me, i've known quite a few (rookie) techies to do just what you've been doing and not get away with it.