Transient Protection

Author: Leonard Krugman

In addition to its power handling limitations, the transistor is susceptible to damage by excessive values of current and voltage. It is particularly important to protect the transistor from those transient surges which may be caused by switching or sudden signal shifts. Transient effects are particularly predominant in oscillator, i-f, and high frequency amplifiers due to the storage capacity of the reactive components. Limiting devices are usually incorporated into the circuit. The series resistors in the emitter and collector arms of the base-controlled negative-resistance oscillator are typical examples. In more complicated circuits, transient limiting elements are usually selected on the basis of tests made on experimental breadboard models. If a scarce or expensive transistor is involved, the equivalent passive "T", made up of standard carbon resistors, can be substituted for this measurement. When connecting a transistor into a live circuit, the base lead must always be connected first. In disconnecting the transistor from a live circuit, the base lead must be removed last.

It is an easy matter to mistakenly reverse the polarities of bias supplies, particularly in complementary-symmetrical circuits. Reverse polarities will not impair the transistor as long as the maximum ratings are not exceeded. It is always a good plan to check for proper capacitor polarity, since almost all of the circuits require polarized types.