4 Sep
1998
4 Sep
'98
1:12 a.m.
Larry and Tony are discussing a blown capacitor...
> I'd replace it. A 250V _AC_ capacitor, class
X. You should be able to get
> that from a good parts company (alas, being in another country I can't
> help you there).
>
> Then try again. Put the PSU on dummy load if you like (I certainly would)
> - a 6V 6W bulb on between the +5V output and the 0V pin. Let it run for a
> few minutes - taking care not to touch the PSU board as there's mains and
> rectified mains all over it. If it runs for (say) 5 minutes on dummy load
> it's safe to try it in the computer again.
[...]
You have referred before to this dummy load 6V 6W
device. If my
memory of Ohms law hasn't failed me completely this works out to
a 1 amp limit. I also imagine this has to be on the power good line.
Is there any reason you use this configuration ? I have some old 6V 6W
bulbs . Would these work just as well ?
Eh? 6V 6W bulb is exactly what Tony suggested.
To be more specific, many power supplies have regulator circuits that
expect some current to be flowing in the load. These PSUs will push out
the wrong voltages, or shut down altogether, if you power them up with
nothing on the output.
Therefore, when testing power supplies, connect some sort of load device
- it doesn't matter what as long as it draws roughly the right amount of
current - to the output. Most computer PSUs regulate the voltage on the
5V rail and hope all the others will follow, so this is the best place
to connect the load.
Light bulbs make good dummy loads because (a) they light up when the
current is flowing and (b) they are relatively small, cheap and easy to
obtain compared with (say) resistors of similar value.
12V bulbs (e.g. car running lamp (0.3 A) or indicator lamp (1.75 A)) are
good for loading 12V lines should you need it.
6V bulbs are good for loading 5V lines. 6V 6W flashlight bulb is good
for small and medium PSUs; for larger power supplies I use a 6V 24W car
headlamp bulb (yes, my truck has 6V electrics). 12V bulbs also work at
5V, but remeber they have a much lower resistance at 5V than they do at
12.
DON'T use the "power good" line to connect the dummy load. This is a
digital output - usually TTL level - which can only drive a few mA.
DO connect the "voltage sense" line(s) to the load as well as +5V and
ground.
Philip.