Zener Diode as Voltage regulator

 A Zener Diode is a special purpose diode which acts as simple PN diode in forward biased but acts as a voltage regulator in reverse biased. In this article zenor diode 's practical use as voltage regulator, it's testing and it's calculations will be discussed.

Testing of Zener Diode:-  

To test Zener Diode  put multimeter in diode mode and check as per ordinary diode, connect red lead to anode and black lead to cathode if it shows 0.4 to 0.7 V and on connecting red probe to cathode and black probe to anode if it shows open circuit then zener diode is OK. For more detail you can check previous article 'How to test electronics components using multimeter'.

 Zener Regulator:- 

To use Zener Diode as voltage regulator, the source voltage must be greater than the zenor breakdown voltage. A series resistor is always used to limit the zener current to less than its maximum current rating for safe and durable operation. 

The Zener Diode operates in the breakdown region keeps load voltage constant, even if the source voltage changes or load resistance varies. 

Fig.1. Zener Regulator circuit diagram 

To check whether zener diode is operating in breakdown region or not, disconnect the zener diode from the circuit and measure the voltage that exists at zener connection point. That voltage must be greater than zener breakdown voltage otherwise zener will not work in breakdown region.

Mathematically this voltage which appears after disconnecting the zener diode can be calculated using thevenin theorem. This voltage Vth must be greater than Zener breakdown voltage Vz.

Vth= RL×Vs/(Rs + RL)

Vth will be voltage across zener point after disconnecting it.

Zener Diode calculations:-

 Let we have 1N961B zener diode, on reading it's data sheet we can get it's details. It is a 10 Volt, 0.5 Watt Zener Diode with maximum zener current 32 mA (continuous). Maximum current rating of Zener Diode can be find out by diving power rating with zener voltage, which will come out 50 mA(instantaneous). But for long life and proper working of Zener Diode we use it 60 to 70 % of maximum current rating that is why data sheet showing maximum zener current 32 mA ( Izm) which can pass through zener diode without affecting its proper working. So to ensure that zener current does not exceed 32 mA , a series resistor is used. 

Fig.2. 10 Volt Zener Regulator 

The value of series resistor is find out by following formula 

Rs= (Vs - Vz ) / Izm

Rs = ( 20 - 10 ) / 50 mA = 200 ohm

Wattage of resistor = I^2 × Rs  = .05 ×.05 × 200

Wattage of resistor = 0.5 watt

So a series resistor of 200 ohm, 1/2 watt is used for zener over current protection. If current exceeds this value it will burn out and zener got protected against over current.

Author:- Amarjeet Singh Jamwal authored articles on Basic Electronics EngineeringElectric Traction, and Electronics Practical for INFO4EEE Website.

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