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How to turn on an AT-style power supply?

W

www.ChantCd.com

Jan 1, 1970
0
(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
 
D

Dave Platt

Jan 1, 1970
0
www.ChantCd.com said:
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.

Bingo. PC switching supplies have a minimum load (usually referenced
to the +5 line, but some supplies may have a minumum load on +12 as
well). If you don't have enough of a load on them, they won't
regulate and (as a safety issue) usually will not start up.
 
H

Hans Summers

Jan 1, 1970
0
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.

They need a minimum load. I have had a 100% success so far whenever I have
tried it, by using a 10 ohm resistor across the +5V and Ground. This takes
0.5A. Power is 2.5W so you need to choose a resistor rated for at least this
power consumption, I suggest 5W or more. If no suitable resistor is
available, I've also used a car headlamp bulb which works just as well.

Hans
http://www.HansSummers.com
 
A

alpha_uma

Jan 1, 1970
0
www.ChantCd.com said:
(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
 
R

Roger Hamlett

Jan 1, 1970
0
alpha_uma said:
kind to

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.
I have seen a lot of AT supplies, where powering them up without a load,
will blow the input fuse. These have a thyristor crowbar, set so if the 5v
rail goes above about 6v, the thyristor is switched on, and shorts out the
input. This is so quick, that unless you happen to be watching the fan at
the moment the power button is pressed, you will see nothing (the blades
usually 'twitch' about 1/4"), and once it is done, the supply will be
dead, until the input fuse is replaced. No supply of this sort should be
operated without any load at all. However most will operate OK, with a
fairly 'minimal' load.
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

Best Wishes
 
W

William P.N. Smith

Jan 1, 1970
0
alpha_uma said:
I have gone through a dozen or so AT PSUs in ages past, and none of them
needed any external load to be activated.

Some may need a load, some may not, some may need a load to stay in
regulation, some may blow fuses (as Roger pointed out). It's probably
a good idea to have some minimal load just to ensure that you aren't
chasing load-related problems.

FWIW, I've got an AT supply running a VHF marine radio (as 12V load
only), and the 12V supplyis really noisy and unstable without a 5V
load. I added some random 5V load and it works fine.
 
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