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.