Switching PSU Theory of Operation / Repair Tips & Techniques

From: John Honniball <John.Honniball_at_uwe.ac.uk>
Date: Tue Jul 25 07:24:10 2000

I have both helpful hints and a cautionary tale about
switchmode PSUs...

It's possible to get a clue about the nature of the fault
by a simple voltage measurement. First switch on, and
measure the voltage across the smooting capacitor on the
high-voltage DC side. Should be a few hundred volts, but
if it's zero, then the rectifier or input filter is

Now switch off, with the voltmeter connected. If the
voltage drops quite rapidly, this indicates that the
chopper transistor is probably OK -- the capacitor is
dicharging throught it and the transformer primary. If the
voltage remains high for some time (see below), then the
chopper transistor is suspect, since nothing is draining
the cap. (unless there's a bleeder resistor).

Good, modern electrolytic capacitors can hold a charge of a
few hundred volts for some time. Days, in fact. I once
picked up a dead switch-mode PSU which had a failed chopper
transistor, and hence a well charged-up capacitor. The
middle finger of my left hand made contact with the
terminals of said capacitor, causing me to exclaim loudly.
Well, run around the office yelling and cursing, in fact.
I ended up with two neat little burn marks on my finger,
clearly labelled "+" and "-" (from the capacitor terminals).

John Honniball
Email: John.Honniball_at_uwe.ac.uk
University of the West of England
Received on Tue Jul 25 2000 - 07:24:10 BST

This archive was generated by hypermail 2.3.0 : Fri Oct 10 2014 - 23:32:58 BST