All diodes allow current to pass in one direction only. In normal use, diodes never suffer a reverse breakdown which would destroy the diode.
Zener Diodes are special because they can break down without being damaged.
They do this at a specific voltage (the zener voltage) which is accurately
determined by the manufacturing process. It is possible to purchase zener
diodes with a range of breakdown voltages between 3.3 volts to 200 volts.
- Zener diodes are connected the opposite way round to normal diodes (REVERSE
BIASED).
- 5 or more mA are required to flow through the zener diode for the zener
voltage to be reliably established. This figure is quoted in the data sheet for the diode.
- Once 5 or more mA are flowing , the current through the diode can increase without the
voltage across the diode increasing at all. In this way, the zener diode can provide a stable reference voltage.
In the circuit above, assume the load takes a maximum current of 95 mA. For relaible operation, the zener diode must take at least 5 mA. Under maximum load, the total current will be 100 mA. If the load current decreases, the zener current increases by the exact same amount. With the load disconnected, the zener current rises to 100mA. This behaviour ensures that the voltage across the zener diode remains constant.
If the zener voltage is 4V and the supply voltage is 9V, the potential difference across R will be 5V.
Using Ohm's Law ...
- R = V / I
- R = 5 / (100 x 10-3)
- R = 50Ω
The nearest suitable preferred value is 47Ω
51Ω would not be suitable because the zener current might drop below 5mA.
To calculate the resistor value ...
Resistance = (Supply_Voltage - Zener_Breakdown_Voltage) / (Maximum_Load_Current + Minimum_Zener_Current)
R = (9 - 4) / (95 x 10-3 + 5 x 10-3)
R = 50 (Nearest suitable preferred value = 47Ω)
To calculate the zener diode power rating ...
Power_Rating = (Maximum_Load_Current + Minimum_Zener_Current) x Zener_Breakdown_Voltage
P = (95 x 10-3 + 5 x 10-3) x 4
P = 400mW or 0.4W
The graph below shows the current/voltage characteristics of a zener diode. The reverse leakage current has been exagerated to make it show on the graph. The reverse breakdown voltage varies between different zener diodes. The range of values goes from a few Volts up to about 200Volts.
If the reverse bias current is less than about 5mA, the zener voltage will be lower than the expected value. When designing circuits to give a reference voltage using a zener diode, the minimum zener current should always be at least 5 mA (check the data sheet). The yellow region in the graph shows where the zener voltage is not properly established.
