Transient suppression

Chapter Transient suppression

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • Transient suppressionThe ac on the utility line is a sine wave with a constant rms voltage near 117 V. But thereare “spikes,” known as transients, lasting microseconds or milliseconds, that attainpeak values of several hundred or even several thousand volts.Transients are caused by sudden changes in the load in a utility circuit. Lightningcan also produce them. Unless they are suppressed, they can destroy the diodes in apower supply. Transients can also befuddle the operation of sensitive equipment likepersonal computers.The simplest way to get rid of most transients is to place a capacitor of about 0.01µF, rated for 600 V or more, across the transformer primary (Fig. 21-13). Commerciallymade transient suppressors are also available.In the event of a thunderstorm locally, the best way to protect equipment is to un-plug it from the wall outlet. This is inconvenient, of course. But if you have a personalcomputer, hi-fi set, or other electronic appliance that you like a lot, it’s not a bad idea.394 Power supplies21-13A capacitor, C, in parallelwith the primary of thetransformer, helpssuppress transients.Fuses and breakersA fuse is a piece of soft wire that melts, breaking a circuit if the current exceeds a cer-tain level. Fuses are placed in series with the transformer primary (Fig. 21-14). Anycomponent failure, short circuit, or overload that might cause catastrophic damage (orfire!) will burn the fuse out. Fuses are easy to replace, although it’s aggravating if a fuseblows and you don’t have replacements on hand.21-14A fuse, F, in series withthe ac input protects thetransformer and diode,in case of overload.If a fuse blows, it must be replaced with another of the same rating. If the replace-ment fuse is rated too low in current, it will probably blow out right away, or soon after