Zener Diodes and Regulation

Zener diodes are often found as voltage regulators in dc power supplies. Zeners can also be used as simple voltage limiters or clippers. In the course SO4204-5A Semiconductor Components, you learned about the fundamentals and operating principle of these components.


Input or Line Regulation:

Zener diodes can be used to regulate a varying dc voltage. Called input or line regulation, this technique was introduced earlier on the page titled "Basic Transistor Series Regulator". As the input voltage varies within some limits, the Zener diode maintains a nearly constant voltage across the output terminals. As the input voltage changes, however, the input current changes proportionally so that the limitations on the input variation are set by the minimum and maximum current values (Imin and Imax) at which the Zener can operate. R is a series current-limiting resistor.

fixed-10 v-source-zener-diode-circuit.jpg

For example, if a Zener diode can maintain regulation over a range of current values from 4 mA to 40 mA, we can calculate the input voltage's limits as follows:

If the resistor's value is 1 kΩ,


The calculations above show that the Zener diode can regulate an input voltage from 14 V to 50 V and maintain an approximate 10 V output, which will vary very slightly, just because of the Zener resistance.

Load Regulation:  

The next circuit shows a Zener 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 Imin and less than Imax. This is the load regulation technique introduced on the page titled "Basic Transistor Series Regulator". When the load resistor RL is very high, the load current is zero and all of the current flows through the Zener. As RL is decreased, the load current, IL increases and the Zener current IZ decreases. The Zener diode continues to regulate until IZ reaches its minimum value. At this point, the load current is maximized. For simplified calculations, the Zener's internal resistance is usually considered to be zero.


Zener Limiting:

Zener diodes can be used in ac applications to limit voltage swings to desired levels; the diagrams below show how this is done. You might want to compare the output with the clippers introduced in the section on Voltage Multipliers.


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