"robb" wrote in message
...
"robb" wrote in message
...
i have a micro-controller board with 3 big 40DIP ICs, 10 or so
16
DIP support ICs. with 32v/8v/5v transformer supply

i believe the 32v shorted with 5V line

So....where does one start to diagnose such a *big* problem ?


I can only work on this in short segments of time and am usually
tired ... so i am moving backwards.

Anyway the revisit.

The last diagnostic consensus was to remove voltage regulators
and inject a good 5v power into the 5v lines and look for
warming components to get a fix on problems to repair.

Since my 5.2V *aged* lantern battery power supply did not impress
many i cobbled a simple 5v power supply from a 18 VAC/2.5A wall
wart, a 100V/1.5A bridge and 7805 5V regulator plus some
resitors and a 6V bulb to test.

I connected the 5v (through 450 Ohm) to the PCB 5v and 0v lines.
i measured ~60 mA curent nothing warmed ...i idecremented the
resistance by 100 for each iteration of testing the measured
currernt increased slightly until i made it to (100 Ohms) and
then the current spiked to ~+ 1.5A and the 7805 fried ?

but still no heat on the board anywhere, no trace no component no
IC ?

do i need to do something different with power supply test ?
thanks for any help,
robb

Do you mean that the 7805 got hot and shut down ? One of those should never
"fry", because they are SOA protected. I suspect that something else
happened there, because if you do the math, it is impossible for 1.5 amps to
flow through a 100 ohm resistor, with only 5v driving it ... Apart from
that, most variations of the 7805 are only rated to 1 amp, although there
are exceptions.

When you cobbled together your test supply, did you mount the regulator on a
heatsink, and most importantly, did you place a 4u7 cap, paralleled with a
0u1 cap directly across the output and ground pins of the regulator, as
close to the device as you could get them? This is *very* important, to
prevent the regulator bursting into vicious ultrasonic or higher
oscillation. It may be just that you reached a current level where this
happened, and the meter didn't know what to do with what it was measuring,
so just displayed some meaningless nonsense. These regs do get very hot when
they oscillate. Although not strictly necessary, it's considered good 'belt
and braces' to put a 0u1 cap between input and ground pins as well.

The more I think about the 'problem' that this board has, the more non
'real-world' it seems. I can't really see any reason why three unrelated
fuses, on three unrelated rails, should have blown, unless there was a
serious and easily visible problem. The only thing that did occur to me was
input voltage. You're not by any chance firing the thing up on UK 240v
power, and its transformer is actually rated for 100 or 110v ? That would do
it ...

Arfa