"Arfa Daily" wrote in message
...

"robb" wrote in message
...
"robb" wrote in message
...
i have a micro-controller
i believe the 32v shorted with 5V line
So....where does one start to diagnose such a *big* problem
?


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.


Thanks for help Arfa,
I am describing from memory, the last thing i recall was
decrement too the 100 Ohm resistor and the amp meter's numbers
were jumping around and 1.5 is the largest number i caught with
my eye.

as for fried 7805 ... the 100 ohm resistor started to smoke and i
think i saw the 7805 let out a puff of smoke and now the 7805
does not produce output ? so i assumed it was fried

this all happened very quickly


When you cobbled together your test supply, did you mount the
regulator on a
heatsink,

the pcb i took it from had a small flat square of aluminum bolted
to it 1.5cm x 1.5cm x 1 mm

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?

no


This is *very* important, to prevent the regulator bursting
into vicious ultrasonic or higher oscillation.

uh-oh,

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.

i'll try the test again with those mods

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 ...


it is 115v plugged into 115v supply
i suppose if i could address the issue of the seemingly unrelated
supply rails i could probably figure it out

if this info helps ....

the machine *was working* when i replaced a VFD display driver
chip. That was easy problem to fix as it was very specific. I
started with closest components, checked pin signals with oscope
and i had a datasheet for guidance. Everything was working i just
needed to bolt it together.

*BUT* i found a yucky ring in peaks of the clock signal and
thought i should do something to repair that (find the source of
ring) again more very easy specific tasks, just follow traces and
check for expected behavior what goes in and what comes out

well i forgot to put the oscope probe tip guard on (to prevent
shorting pins) and i bridged the 32v VFD supply line to the 5V
rail (a little snap) and all sorts fo jitters came out of the
connected devices, i disconnected all those devices and i found
one lone fuse between a bridge and the transformer was blown i
think the 8V or 16V line that supplied the voltage regulators and
servo controler chips {L387,L298}

i replaced fuse , powered on and all three fuses blew

now i am here with very nebulous problem with no specific ideas
about how to repair

looking for help

thanks again for your help arfa,
robb