switching power supply Q - test-drb@ccmp.vtda.org

4 Sep 2009

...
 >> If this is acting up, for testing purposes it
would seem to be safe
>> to remove it to see if the voltage changes (jumps back to around
>> 12V) ? 
...
 > No, that would remove all regulation from the 12V
rail. 
...
  If wired per the reference in the PDF... for testing
with no devices
 attached (will prob need a dummy load for 5V I'd imagine) to see if
 the part is what is causing the voltage to drop to 8.5V. Not to
 actually use. 
I still wouldn't do it, some SMPSs can self destruct without feedback.

...
  This is also a small part... wouldn't it be
dissipating a large amount
 of current clamping the rail from 12+V to 8.5V? 
No, it's not used as a clamp like that. The 12V will be divided down by
some R and the pot so that normally 2.5V would be on the REF pin. The
K pin then sinks current that is the feedback to regulate the voltage.
This is often the LED side of an optoisolator.

...
  I have an ESR meter. 
So do I. I wish I'd got one years earlier with the effort it has saved
since.

...
  Have you ever seen a cap 'leak' oil? 
I've seen all kinds of strange substances leak out of caps including
oil, wax and what looked and felt like oily, grey cotton wool.

...
  but I've never seen a semiconductor device leak
'oil'? 
Some RF power modules are mineral oil filled but I've not seen it in
a discrete device.

...
  Guess I'll have to plan on pulling the caps to see
if they could
 be the source of the leak.... 
I'd measure ESR first then only pull random caps if nothing showed
up.

...
  but if so... I'd imagine they should have a film
on them too, no? 
It depends, it may have evaporated from around a hot capacitor. If
that's where it came from.

...
  And of course they have that white 'glue'
between them (more like
 a hard caulk than glue I guess ?).... what is that for anyway ? 
Mostly to annoy me I think.

Lee.