A voltage reference diode is a device that is able to maintain an accurate voltage level given a suitably wide range of reverse current through it. With this characteristic, a voltage reference diode is very useful in providing precision voltage levels, such as in regulated power supplies and voltage calibration. A voltage reference diode is usually found inside circuits and integrated circuits inside electronic equipment and appliances.
Regular diodes are two-terminal devices that act like one-way valves. The diode has an anode, which is the positive part, and a cathode, which is the negative part. Diodes conduct electricity, and when the anode is positive while the cathode is negative, it is referred to as forward biased or conducting state. A voltage reference diode is used in the reverse bias mode when the diode is not supposed to conduct.
Unlike ordinary diodes, the voltage reference diode, which is usually referred to as a Zener diode, conducts in the reverse bias mode but with a fixed reverse voltage across it. The Zener diode is specially designed and manufactured so that the chemical doping of the semiconductor used makes the Zener diode exhibit a fixed reverse voltage. Zener diodes are made with voltage ratings of about 1.2 to 200 volts (V).
The most common use for the voltage reference diode is the voltage regulator, which could be either a diode or linear regulator. Diode regulators have a limited range of load currents that can be supported, which is why linear regulators are more popular. The linear regulator is usually a three-terminal integrated circuit that has a ground terminal, an input terminal, and an output terminal. The input terminal is usually the unregulated voltage, while the output terminal is usually the regulated voltage.
Without a voltage regulator, a piece of electronic equipment will not work properly. For instance, a 6 V amplitude modulation (AM) radio cannot be connected to a 12 V car battery because the radio will get damaged. To make use of the 12 V battery, a 6 V linear regulator should be added, which will appear between the main terminals of the regulator. All electronic circuits are designed with a fixed voltage level, such as 6 V, 12 V, 24 V, or 48 V. Voltage levels of 50 V and above can cause significant currents through the skin, so extra care should be taken and regulator specification sheets that provide details of connections should be followed.