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Circuits / Power

Zener regulator

Reversed, the Zener holds its voltage; R1 takes the rest of the supply

Zener regulator, simulated on a breadboard

Tap a part to see its voltages and currents. It is a model, not a measurement.

Predict, measure, explain

  1. Predict If the supply goes from 9 V to 12 V, how much does the voltage on R2 change?
  2. Measure Tap R2. Then change the supply to 12 V and tap it again.
  3. Explain
    Show the answerHide the answer

    Very little: from 4.79 V to 4.89 V. The Zener takes the extra current (R1 goes from 9 to 15 mA) and holds the voltage.

What do you need?

How does a Zener diode regulate voltage?

Connected in reverse, a Zener diode conducts only once the voltage reaches its breakdown value, and then it holds that voltage over a wide range of currents. The series resistor takes the rest of the supply. Here R1 (470 Ω) drops 4.2 V of the 9 V, and the 1N4733A holds the load, R2 (1 kΩ), at about 4.8 V. Of the 9 mA through R1, 4.8 mA goes to the load and the Zener soaks up the other 4.2 mA. Change the supply to 7 V or 12 V and the load still sees 4.6 V or 4.9 V: the Zener's current changes, not the output. The 1N4733A is sold as 5.1 V, but that figure is at its 49 mA test current; at a few milliamps it sits a little lower. It is simple, but it wastes current, so it suits small loads and references.

Values from the Protoboard ISO simulation: a model, not a bench measurement.