Overview

Working on this section and the primary side of the switchmode power supply are no less dangerous than poking a screwdriver into an AC socket. If you do not understand what you are doing, you could end up on your back, or at least with some fireworks occurring in your hands. Graphic analogies aside, you must take the right precautions when probing the switching power supply.

One time after setting up the instrumentation on a bench and making sure I had all of the equipment I needed, I started my AC/DC converter. It was operating fine, until I went to view the drain voltage of the main power switch. Within a millisecond I had vaporized the ground clip of the oscilloscope voltage probe with a large flash and "pop." After a few expletives, I realized I had neglected to insert an earth ground isolator onto the AC power plug of the oscilloscope. Happily, the oscilloscope survived, having been designed for accidents or idiots such as myself. I had learned two lifelong lessons: make sure your life insurance is up to date and always keep several ground isolator plugs in your tool box. Today, my oscilloscope is always isolated from ground, no matter what type of circuit I am working on.

Several other habits I have developed over the years: don't work barefooted, know where the earth ground points are on the rest of your workbench, and know where the workbench's circuit breaker is located. It is not...