Phil Allison wrote:
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

Hi Phil,

I was forgetting about the pulsing action. I guess I was thinking about a
standard AC transformer operating in the 60 Hz range. I imagine it would
hold true that whatever the transformer pulse/sinusoidal frequency is, the
choke belongs at the end of the line.

Thanks for your reply.
--
David Farber
Los Osos, CA