"Phil Allison" wrote in message
...
David Farber wrote:


I was wondering what would happen if the choke were placed before the
capacitor and zener diode.

** A choke would present a high impedance in series with each current
pulse from the switching tranny - causing a big drop in the voltage
appearing on the electro cap.


Wouldn't the choke filter out the spikes and then
make the zener diode unnecessary or at least less likely to short?

** The zener (probably 6.2 V) conducted heavily and failed short when the
electro went high ESR and caused the peak output voltage to go high.

An electro that has developed high ESR cannot smooth the current pulses
being delivered by the switching tranny and diode, so the output wave has
continuous high peaks with a low *average* value. The control loop
responds to the low average and tries to correct it by making each current
pulse stronger, which only makes things worse.

In short, the output electros in a SMPS are critical to it operation and
in many cases there in nothing to prevent the output voltage going high
when they wear out.

I have seen serious damage done to 5V logic when this happens.



... Phil


David - see Phil's answer regarding the choke being first. And +1 on his
observations about serious damage occurring when a cap goes high ESR and the
control loop 'lets go' in its efforts to correct for what it sees as the low
output voltage. Notable among these cases are cheap Chinese DVD players and
set-top boxes, but I have also seen it happen on equipment from what you
would normally consider to be 'reputable' manufacturers ...

Arfa