Zener Diode As Voltage Regulator:
Zener Regulation with a Varying Input Voltage
Circuit diagram (a) illustrates that how a zener diode can be used to regulate a varying dc voltage. As the input voltage varies ( within limits ), the zener diode maintiains a nearly constant output voltage across its terminals. However, as Vin changes, Iz will changes proportionally so that the limitations on the input voltage variation are set by the minimum and maximum current values ( Izk and Izm ) with hich the zener can operate. Resistor R is the series current limiting resistor. The meters indicates the relative values and trends.
To illustrate regulation, suppose that the IN4740 10 V zener diode in circuit (c) can maintain regulation over a range of zener current values from Izk = 0.28 mA to Izm = 100 mA. Let P D(max) = 1W and Vz = 10 V ( as in above circuits )Izm = P D(max)/Vz = 1W/10V = 100 mA For the minimum zener curent, the vltage across the 220 ohm resistor is VR = IzkR = (0.25 mA)(220 ohm) = 55 mV since VR = Vin – Vz Vin (max) = VR + Vz = 55 mV + 10 V = 10.055 V For the maximum zener current , the voltage across the 220 ohm resistor is VR = IzkR = (100 mA)(220 ohm) = 22 V Therefore, Vin(max) = 22 V + 10 V = 32 V
This shows that this zener diode can regulate an input voltage from 10.055 V to 32 V and maintain an approximate 10 V output. The output will vary slightly because of the zener impedence, which has been neglected in these calculations.
Zener Regulation with Variable Load
In below circuit we can see a zener voltage regulator with a variable load resistor across the terminals. The zener diode maintains a nearly constant voltage across RL as long as the zener current is greater than Izk and less than Izm.
From No Load to Full Load
when the output terminals of the zener regulator are open (RL = ∞) the load current is zero and all of the current is through the zener, this is a no load condition. When a load resistor ( RL ) is connected, part of the total current is through the zener and and part through RL. The total current through R remains essentially constant as long as the zener is regulating. As RL is decreased, the load current, IL, increases and Iz decreases. The zener diode continues to regulate the voltage until Iz reaches is minimum value. Izk. At this point the load current is maximum, and a full load condition exists.