Re: How to turn on an AT-style power supply?

From: alpha_uma (none_such_at_home.com)
Date: 07/26/04 

Date: Mon, 26 Jul 2004 06:38:23 GMT

"www.ChantCd.com" <hormel77@hotmail.com> wrote in message
news:8vZLc.142658$a24.110602@attbi_s03...
> (I was trying to use an ATX supply before, but it seemed more complicated,
> so...)
>
> There is a switch on this AT-style power supply that I have now (of
course).
>
> I stripped off the ends of one of the hard drive connectors, making sure
> they
> don't touch when I hit the power switch.
>
> I hit the switch, and nothing happens.
>
> Do I have a bad power supply? Or do you have to have a load of some kind
for
> it to work? I'm just trying to turn it on with nothing - so I can measure
> the voltage
> with a VOM. Starting at the beginning, as it were.
>
> The other day, I was diagnosing a friend's bad PC - it turned out to be a
> bad power supply. (ATX)
> The way I found out? I just plugged the AT power supply to the hard drive
> (which wasn't spinning up, BTW) and turned it on - voila! The hard drive
> spun up. By this I guessed that the power supply was bad (turned out to
be
> the case).
> My point is, I had just 1 hard drive as a load, and the thing powered up
> just fine.
> (I'm pretty sure it's the same power supply, too)
>
> Matthew
>

Please note that Matthew is talking about an AT power supply, not an ATX
power supply.

I have gone through a dozen or so AT PSUs in ages past, and none of them
needed any external load to be activated. If you connect the AT PSU to the
AC wall socket, and then push the power button that comes with it, the fan
of the power supply should spin right away. If it does not, then the AT PSU
is not good (or at the very least, its fan could be "dead"). If the AT PSU
fan is spinning, use a voltmeter to verify the voltages on the yellow line
(12V) and the red line (5V). If they check out fine (could be anywhere from
11.8V to 12.4V, and from 4.8V to 5.3V respectively), then power off the PSU,
and connect the HD. Then power up. If the HD does not seem to be spinning
up, then the problem is likely with the HD.

As for an ATX PSU, shorting the green wire (pin 14) and a black wire (such
as pin 16) is equivalent to toggling the "soft" power switch on a typical
ATX computer case. In addition, most ATX PSUs these days would require some
minimum load before they would power up properly (I've seen some exceptions
in earlier days but that is another story). To test an ATX PSU, at the very
least, connect an external system fan to the PSU or a floppy drive (if you
have one handy) before shorting the green wire to the black wire. It it does
not turn on, the PSU is probably bad (or at the very least, the fan inside
the PSU is dead).

I recommend reading the following pages to fully understand the differences
between an ATX PSU and an AT PSU and the differences between the two form
factors:

     http://www.pcguide.com/ref/power/sup/index.htm
     http://www.pcguide.com/ref/power/sup/parts_Motherboard.htm

Al-U

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