Lasted edited by Andrew Munsey, updated on June 14, 2016 at 10:10 pm.
Zener Diodes can be called Breakdown Voltage Diodes or Voltage Breakdown Diodes or Avalanche Diodes. The Zener diode has been an important component in both Voltage and Current Regulators. Prior to solid state components - a gas tube was used as a voltage regulator. Examples of gas tubes are OB2 and OB3.
There are two categories of Zener Diode. These are Voltage Breakdown Diodes and Avalanche Diodes.
The difference between the two depends on the level of doping. Voltage Breakdown Diodes operate within the region of 0 - 6 VDC. Avalanche Diodes operate within the region 5 - greater than 200 VDC. See manufacturers for the largest Zener Diode that they manufacture. Note the overlap of 1 Volt for both diodes. This overlap has been noted because the difference in action between Voltage Breakdown and Avalanche is different for every manufacturer and diode built that works in this region. If a Zener Diode is between 5 and 6 VDC then if can be described as either type.
Zener diodes are constructed to be voltage regulators, which means that they will operate at an approximate but pre-determined voltage.
They operate as a voltage regulator because they maintain a pre-determined voltage despite changes in zener current. This is only true if the current remains within the specifications engineered into the Zener.
The external circuit components must be within specifications if the Zener diode is to function properly. This is usually a single resistor.
This resistance has a minimum value, which permits a maximum zener current, thus a maximum zener voltage. If this resistance were to reduce below this value then the zener current would burn out the diode.
This resistance also has a maximum value, however, increasing the resistance only decreases the current, and no damage will occur to the Zener. What does happen is that the Zener no longer regulates voltage but will experience large voltage changes for minimum current changes. These voltages will be less than the minimum zener voltage and normally should not damage any associated circuitry.
A zener diode is normally connected in parallel with the load.
If the input voltage changes, and in this example, increases, then the circuit, acting as voltage divider will see an increase in voltage across the load. This increase is also in parallel with the Zener, which because of the increase in voltage will increase Zener current. This increase in current is felt through the series resistance, dropping more voltage, thus returning the load voltage to near original value.
Obviously, decreasing the input voltage will reverse the previous action.
If the load increases, then the output voltage will increase, which in turn increases the voltage across the zener diode. This will increase zener current, which will be felt through the series resistor. Increased current here will drop more voltage returning the output voltage to near original value.
Obviously, decreasing the load will reverse the previous action.
Consequently, within specifications, a zener diode will maintain a constant output voltage despite changes in input voltage and changes in load current demand.