Zener diodes are primarily designed to regulate the output voltage of a power supply. As a voltage regulator, a stable maximum voltage is allowed to pass through the diode. This maximum voltage is determined by the voltage rating of the zener diode. Regulation of a specific voltage output ensures electronic equipment does not receive electrical power surges. These surges often destroy electronic components in computers, televisions and other electronic equipment.
Voltage regulation occurs two ways with a zener diode. Normal voltage regulation occurs when the diode's anode is connected to a power supply in the circuit's electronic ground. This is called reverse bias and results in voltage flow through the diode at the maximum voltage rating of the diode--for example, 5.1 volts, which is less than 12 applied volts in this case. The second method is when the anode of a diode is connected to a circuit's positive voltage; this is called forward biased. This forward bias does not allow voltage to flow through the diode, limiting the output voltage to the zener diode's operating voltage drop--for example, 0.7 volts, which is less than 12 applied volts in this case.
Isolating the positive voltage from the circuit's electronic ground is an important feature of zener diodes. This isolation protects electronic components in equipment from electrical power surges and destruction of these components. Isolating different output voltages from each in a regulated power supply is another function of zener diodes. In these situations, a diode cathode is connected to another diode cathode. This arrangement ensures that a higher voltage on one output does not leak over to a lower output voltage--for example, a 20-volt output leaking over to a 10-volt output.
Besides regulating the amount of voltage provided by power supplies, zener diodes may also be used to regulate the size of a voltage output by clipping the output of an alternating current (AC) power supply. Clipping an output voltage to a desired level is accomplished by connecting the anode of a zener diode to the main power source and the cathode to a secondary power source. The voltage output is clipped to the size of the secondary power source--for example, positive 10 AC volts and negative 20 AC volts. The main input AC input was positive and negative 20 volts, and the second power source was positive 10 AC volts.
Zener diodes are available in a number of power and voltage ratings. These ratings are based on the two typical power output ratings of 0.5 and 1.3 watts. Diodes with a power rating of 0.5 watts have typical voltage ratings of 2.7 to 20 volts. Diodes with a power rating of 1.3 watts have typical voltage ratings of 4.7 to 200 volts.
Excessive heat caused by disconnection of a regulated power supply from a load can cause zener diodes to break down and fail. This is eliminated by placing an appropriately sized load resistor across the output of the diode to act as the load when the power supply is not connected to an electronic circuit--for example, not connected to a load such as a radio.