6.1: Theory Overview

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When forward biased, the Zener diode behaves similarly to an ordinary switching diode, that is, it incurs a 0.7 volt drop for silicon devices. Unlike a switching diode, the Zener is normally placed in reverse bias. If the circuit potential is high enough, the Zener will exhibit a fixed voltage drop. This is called the Zener potential or \(V_Z\). Manufacturer’s specify this voltage with respect to the Zener test current, or \(I_{ZT}\); a point past the knee of the voltage-current curve. That is, if the Zener’s current is at least equal to \(I_{ZT}\), then its voltage is approximately equal to the rated \(V_Z\). Above this current, even very large increases in current will produce only very modest changes in voltage. Therefore, for basic circuit analysis, the Zener can be replaced mathematically by a fixed voltage source equal to \(V_Z\). In practice, some series resistance is usually required to limit the current to a value below the Zener’s maximum in order to prevent damage.