6 Dec
2007
6 Dec
'07
1:19 p.m.
On Wed, 2007-12-05 at 23:25 +0000, Tony Duell wrote:
Indeed it is. Another example is the startup resistor in many small
SMPSUs. Typically a few hunderd k in value, and may be 2 or more
resistors in series, it goes from the +ve side of the rectified mains
smoothing caapacitor to a point in the chopper control circuit, and
provides current to get the thing started when you turn it on. It often
fails, resulting in a supply that runs fine, but when you turn it off (or
the mains fails), it won't re-start. I've fixed _dozens_ of such supplies
by lookking for a reissotr matching the description and replaicnig it if
open-circuit.
-tony
The fualt I was thinking of was actually a leaky
capacitor, part of the
RC filter network on the A1 supply. It was pulling the supply down, of
course. The series reisstor was fine in this case, and yes I have had
high-value resistorrs go even hihgher, or open.
It seems particularly common with highish value resistors (more than a
few hundred k) used at high voltages, even if the current through them
is fairly tiny.