Zener Diodes

The Zener Diode

In the previous Signal Diode tutorial we saw that a "reverse biased" diode passes very little current but will suffer breakdown or damage if the reverse voltage applied across it is made to high. However, Zener Diodes or "Breakdown Diodes" as they are sometimes called, are basically the same as the standard junction diode but are specially made to have a low pre-determined Reverse Breakdown Voltage, called the "Zener Voltage" (Vz). In the forward direction it behaves just like a normal signal diode passing current, but when the reverse voltage applied to it exceeds the selected reverse breakdown voltage a process called Avalanche Breakdown occurs in the depletion layer and the current through the diode increases to the maximum circuit value, which is usually limited by a series resistor. The point at which current flows can be very accurately controlled (to less than 1% tolerance) in the doping stage of the diodes construction giving it a specific Zener Breakdown voltage (Vz) ranging from a few volts up to a few hundred volts.

Zener Diode I-V Characteristics

Zener Diodes are used in the "REVERSE" bias mode, i.e. the anode connects to the negative supply, and from its I-V characteristics curve above, we can see that the Zener diode has a region in its reverse bias characteristics of almost a constant voltage regardless of the current flowing through the diode. This voltage across the diode (it's Zener Voltage, Vz) remains nearly constant even with large changes in current through the diode caused by variations in the supply voltage or load. This ability to control itself can be used to great effect to regulate or stabilise a voltage source against supply or load variations. The diode will continue to regulate until the diode current falls below the minimum Iz value in the reverse breakdown region.

The Zener Regulator

Zener Diodes can be used to produce a stabilised voltage output by passing a small current through it from a voltage source via a suitable current limiting resistor, (R_S). We remember from the previous tutorials that the DC output voltage from the half or full-wave rectifiers contains ripple superimposed onto the DC voltage and that as the load value changes so to does the average output voltage. By connecting a simple Zener stabiliser circuit as shown below across the output of the rectifier a more stable reference voltage can be produced.

Zener Diode Stabiliser

The resistor, R_S is connected in series with the Zener diode to limit the current flow with the output from the voltage source, V_S being connected across the combination while the stabilised output voltage V_outis taken from across the Zener diode. The Zener diode is connected with its Cathode terminal connected to the positive rail of the DC supply so it is reverse biased and will be operating in its breakdown condition. Resistor R_S is selected so to limit the maximum current flowing in the circuit. When no load resistance, R_L is connected to the circuit, no load current (I_L = 0), is drawn and all the circuit current passes through the Zener diode which dissipates its maximum power. Care must be taken when selecting the appropriate value of resistance that the Zener maximum power rating is not exceeded under this "no-load" condition.

There is a minimum Zener current for which the stabilization of the voltage is effective and the Zener current must stay above this value operating within its breakdown region at all times. The upper limit of current is of course dependant upon the power rating of the device.

Example No1.

A 5.0v stabilised power supply is required from a 12v d.c. input source. The maximum power rating of the Zener diode is 2W. Using the circuit above calculate:

a) The maximum current flowing in the Zener Diode.

b) The value of the series resistor, RS

c) The load current IL if a load resistor of 1kΩ is connected across the Zener diode.

d) The total supply current IS

Zener Values

As well as producing a single stabilised voltage output, Zener diodes can also be joined together in series along with normal silicon signal diodes to produce a variety of different reference voltage values as shown below.

Zener Diodes Connected in Series

The values of the individual Zener diodes can be chosen to suit the application while the silicon diode drops about 0.7v in the forward bias condition. The supply voltage, Vin must of course be higher than the largest output reference voltage and in our example above this is 19v.

A typical Zener diode is the 500mW BZX55 series or the larger 1.3W BZX85 series were the Zener voltage is given as for example, C7V5 for a 7.5V device giving a diode reference of BZX55C7V5. The individual voltage values for these small but very useful diodes are given in the table below.