4 Sep
1998
4 Sep
'98
4:33 a.m.
On 4 Sep 98 at 13:12, Philip.Belben(a)powertech.co.uk wrote:
Larry and Tony are discussing a blown capacitor...
Oops. I meant 6V 9W
> 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.
Hmmm. Now I'm confused (as always). Wouldn't that mean you'd have to
leave the "power good" (orange ?) line connected to the motherboard otherwise
the PSU would shut down ? Was why I mentioned the higher W rating.
DO connect the "voltage sense" line(s) to
the load as well as +5V and
ground.
Philip.
Ahh this seems like the source of my confusion about this. What is the
difference between the "power good" and the "voltage sense" lines ?
For example, most of computer PSUs I've seen have up to 12 lines connected
to the MB sometimes segmented into 2 groups. One of these groups has a
uniquely color-coded line ( orange ?) which I took to be the PG line. the
conectors for the peripheral devices have 4 lines 2 of which are usually black
(ground or neutral) and there's usually a red which I believe is "hot or
positive" Is the 4th line (usually blue or yellow) the "voltage sense" line
?
BTW is there any color-code convention for wire like the one for resistors and
other components ? Green is always ground in most applications for example.
I also have an IBM 8557 which is a SCSI mchn that has 4 heavy-guage red and
black connected to the PSU as well as a connector with 7 smaller guage wires
( 2 blu, y, wh, or, bla, brn). The heavy guage are obviously power but what are
the rest for ? The orange I take it is PG ,the black neg. or return. The device
plugs are red, bla, bla, blu. no smaller perp. plug. I guess the 3 1/2" FDD
takes power from the SCSI bus.
ciao larry
lwalker(a)interlog.com