A Zener diode is a voltage reference device that makes use of the reverse bias characteristics of a doped positive-negative (P-N) junction, which is made of positive (P)-type and negative (N)-type semiconductor materials. While a normal diode has a relatively high reverse-breakdown voltage, a Zener diode has a reverse breakdown as low as 1.2 volts direct current (VDC). The Zener diode, like the normal diode, has a band to mark the cathode, or the negative electrode. In forward bias, where the anode is positive and the cathode is negative, the Zener diode works like a normal diode.
In reverse bias operation, the normal diode stays as an open circuit for a wide range of voltages. The normal diode may have a reverse-breakdown voltage of around 160 volts (V), and this voltage is the common peak level of a 110 volts alternating current (VAC) power line voltage. The Zener diode has a much lower reverse voltage. For instance, a 6.8 V Zener diode will reach breakdown and hold current that its power rating allows. The power dissipation in the diode has to be about half of the rated power for the diode.
A 1-watt (W) Zener diode will allow a maximum of 0.147 ampere (A). It is a good practice to allow half of the rated power to continuously be dissipated in the device; therefore, the current should be halved to 0.0735 A or 73.5 milliamperes (mA). At this current, the 1 W-6.8 V diode will just be warm. It should be noted that this diode would be able to provide about 70 mA to an external load at 6.8 V. This makes this diode a simple voltage regulator.
The Zener diode may be connected to a voltage follower device such as a negative-positive-negative (NPN) bipolar junction transistor (BJT) emitter follower circuit. Previously, the positive output was at the reverse-biased cathode, so the cathode will instead be connected to the base of an NPN BJT. The emitter follower will tract the base voltage and use its gain to deliver an emitter voltage that is almost the same as the base voltage — this makes it an emitter follower. The BJT emitter will follow the voltage of the diode less the roughly 0.7 V silicon base-to-emitter voltage drop, and the output at the emitter is about 6.1 VDC. If the forward transfer constant of the transistor forward current gain is 100, the diode and transistor interaction deliver a regulated voltage of about 6.1 VDC from near 0 A to about 6 A.