Zener Diode Basics, Operation, Characteristics
What is Zener Diode?
The characteristics of a regular junction diode will show that it is designed primarily for operation in the forward direction. Forward biasing will cause a large IF with a rather small value of VF. Reverse biasing will generally not cause current conduction until higher values of reverse voltage are reached. If VR is great enough, however, breakdown will occur and cause a reverse current flow. Junctions diodes are usually damaged when this occurs.
Special diodes like zener diodes are designed, manufactured to operate in the reverse direction without being damaged.
- In the forward bias direction, the zener diode behaves like an ordinary silicon diode.
- In the reverse bias direction, there is practically no reverse current flow until the breakdown voltage is reached. When this occurs there is a sharp increase in reverse current.
- Varying amount of reverse current can pass through the diode without damaging it. The breakdown voltage or zener voltage (VZ) across the diode remains relatively constant.
- The maximum reverse current is limited, however, by the wattage rating of the diode.
- Manufacturers rate zener diodes according to their Vz value and the maximum Power Dissipation (PD) at 25ºC. This gives an indication of the maximum reverse current(IR)that a diode an safely conduct.
- For example, a 1W, 15V zener diode can conduct an IR of 0.066A(P=VI).
- Zener diodes are available in a wide range of breakdown voltages (VZ = 1.4V to 200V).
- Each VZ value is generally specified at minimum zener current(IZ).
- In the reverse direction, once this value has been reached, the VZ remains constant over a wide range of IZ.
- So the power dissipation values are used to indicate the safe operating range. Typical Power Dissipation ratings are from 150mW to 50W.
- While purchasing zener diode we have to ask like 1W zener diode at 8.2VZ with 10% accuracy.
Symbol of Zener Diode:
We can identify the terminals of zener diode by observing a black color ring at cathode terminal(refer the first figure in this post). If it is a SMD component, then a band will be available for cathode terminal.
By Identifying the Zener Diode Code marked over the device, we can know the value it.
Operation of Zener Diode:
- Zener diodes are normally used only in the reverse bias direction.
- It means that the anode must be connected to the negative side of the voltage source and the cathode must be connected to the positive side.
- A main difference between zener diodes and regular silicon diodes is the way they are used in the circuits.
- It is primarily used to regulate the circuit voltage as it has constant VZ.
- A large change in IR will cause only a small change in VZ. It means that a zener diode can be used as an alternate current path. The constant VZ developed across the diode can then be applied to a load.
- Thus the load voltage remains at constant by altering the current flow through the zener diode.