1
Chapter 2
ZENER DIODE
Chapter 2 ZENER DIODE
1
Analysis of networks employing Zener diodes is quite similar to that applied to the analysis
of semiconductor diodes in previous sections.
Here, first the state of the diode must be determined followed by a substitution of the
appropriate model.
Then, the other unknown quantities of the network will be determined.
Chapter 2 ZENER DIODE
2
Equivalent circuit
2
(i) Vi and R Fixed
Basic zener regulator
Analysis procedure
Step (1) Determine the state of the zener diode by removing it from the network and
calculating the voltage across the resulting open circuit.
Chapter 2 ZENER DIODE
3
Step (2) Substitute the appropriate equivalent circuit and solve for the desired unknowns.
Ex: In case of “on” state.
Voltages across the parallel elements must be the
same.
When the system is turned on, the Zener diode will turn “on” as soon as the voltage across
The Zener diode is V volts It will then “lock in” at this level and never reach the higher
Chapter 2 ZENER DIODE
4
z
.
Level of V volts.
Zener diodes are most frequently used in regulator networks or as a reference voltage.
Regulator is designed to maintain a fixed voltage across the load RL. For values of applied
voltage greater than required to turn the Zener diode “on”, the voltage across the load will
be maintained at Vz volts.
3
Example (1): (a) determine VL, VR, Iz and Pz.
(b) change RL to 3 kOhm.
Solution (a)
Step (1) Removing the diode
V < Vz
Chapter 2 ZENER DIODE
5
The diode is
“off”
Step (2) Substitute open-circuit equivalent.
Solution (b)
Step (1) Removing the diode
V > Vz , the diode is “on”
Step (2) Substitute the equivalent circuit.
Chapter 2 ZENER DIODE
6
4
(ii) Fixed Vi, Variable RL
Too small a load resistance RL will result in a voltage VL
across the load resistor less than Vz and the Zener device
will be in the “off” state.
To determine the minimum load resistance that will turn
the Zener diode on, simply calculate the value of RL that
will result in a load voltage VL = Vz.
Any