A Zener diode is a voltage reference device that uses the reverse bias characteristics of a PN junction. It has a low reverse breakdown voltage and can be used as a simple voltage regulator. It can be connected to a voltage follower device like an NPN BJT emitter follower circuit to provide a regulated voltage.
A Zener diode is a voltage reference device that utilizes the reverse bias characteristics of a positive-negative (PN) doped junction, which is made of positive (P) and negative (N) type semiconductor materials. While a normal diode has a relatively high reverse breakdown voltage, a Zener diode has reverse breakdown as low as 1.2 volts of direct current (VDC). The Zener diode, like the normal diode, has a band to mark the cathode, i.e. 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 remains as an open circuit over a wide range of voltages. The normal diode may have a reverse breakdown voltage of approximately 160 volts (V), and this voltage is the common peak level of an alternating current (VAC) power line voltage of 110 volts. The Zener diode has a much lower reverse voltage. For example, a 6.8V Zener diode will fail and maintain the current allowed by its rated power. The power dissipation in the diode should be about half the rated power of the diode.
A 1 watt (W) Zener diode will allow a maximum of 0.147 amps (A). It is good practice to allow half of the rated power to be continuously dissipated in the device; therefore, the current should be halved to 0.0735 A or 73.5 milliamperes (mA). At this current, the 1W-6.8V diode will be barely warm. It should be noted that this diode would be able to supply about 70mA to an external 6.8V load. This makes this diode a simple voltage regulator.
The Zener diode can 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 to the reverse biased cathode, so the cathode will instead be connected to the base of an NPN BJT. The emitter follower will process the base voltage and use its gain to give an emitter voltage that is nearly the same as the base voltage – this makes it an emitter follower. The BJT emitter will track the diode voltage minus the silicon base-emitter voltage drop of approximately 0.7V and the output on the emitter is approximately 6.1V DC. If the forward transfer constant of the transistor’s forward current gain is 100, the diode-transistor interaction provides a regulated voltage of approximately 6.1V DC from nearly 0A to approximately 6A.
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