Transient suppression

The ac on the utility line is a sine wave with a constant rms voltage near 117 V. But there are “spikes,” known as transients, lasting microseconds or milliseconds, that attain peak values of several hundred or even several thousand volts.
Transients are caused by sudden changes in the load in a utility circuit. Lightning can also produce them. Unless they are suppressed, they can destroy the diodes in a power supply. Transients can also befuddle the operation of sensitive equipment like personal computers.
The simplest way to get rid of most transients is to place a capacitor of about 0.01 μF, rated for 600 V or more, across the transformer primary (Fig.1). Commercially made transient suppressors are also available.
In the event of a thunderstorm locally, the best way to protect equipment is to unplug it from the wall outlet. This is inconvenient, of course. But if you have a personal computer, hi-fi set, or other electronic appliance that you like a lot, it’s not a bad idea.

Fig.1: A capacitor, C, in parallel with the primary of the transformer, helps suppress transients.