Switching PSU Theory of Operation / Repair Tips & Techniques

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 open-circuit. 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(a)uwe.ac.uk University of the West of England