Use with a resistor as a constant voltage source.

The idea

A Zener diode can provide a steady output voltage when it is reverse biased into breakdown and connected in series with a current-limiting resistor. The resistor drops the difference between the input supply voltage and the Zener voltage, so the Zener voltage appears across the load. For example, with a 9 V supply, a 5.1 V Zener and a series resistor, the resistor has about 3.9 V across it and the output across the Zener is about 5.1 V. The resistor must allow enough current for the Zener to stay in breakdown under maximum load current (when the load draws the most current), but not so much that the Zener exceeds its power rating under minimum load current (when the Zener absorbs the excess). The output is approximately constant because small changes in current cause only small changes in the Zener voltage.

What earns the marks

• The Zener is connected in reverse bias, with its cathode to the positive line and anode to the negative line, so it operates in breakdown.
• A series resistor is connected between the input supply and the Zener to limit current and drop the excess voltage.
• The output voltage is taken across the Zener and is approximately equal to its Zener voltage.
• The resistor value must ensure the Zener remains in reverse breakdown at maximum load current (when Zener current is lowest).
• The resistor value must also limit current so the Zener power dissipation does not exceed its rating at minimum load current (when Zener current is highest).
• The circuit acts as a constant voltage source because the Zener voltage changes only slightly as current through it varies.

Common mistakes and how to put them right

• Connecting the Zener in forward bias.

  Put it right: It must be reverse biased so that it operates in breakdown and regulates the voltage.

• Omitting the series resistor.

  Put it right: Without it the Zener current can become excessive and damage the diode.

• Misidentifying the worst-case condition for dropout.

  Put it right: The risk of dropping out of breakdown occurs at maximum load current, when the load diverts current away from the Zener.

• Misidentifying the worst-case condition for overheating.

  Put it right: The Zener dissipates the most power at minimum load current, when it must absorb the current not taken by the load.

Exam tips

• Draw the regulator circuit with the Zener cathode to the positive rail and label the output across the Zener.
• Show the voltage across the series resistor as the supply voltage minus the Zener voltage when explaining operation.
• State that the Zener must remain in reverse breakdown for regulation, and mention current limits when discussing resistor choice.
• Use the Zener voltage value directly in calculations rather than assuming the output equals the supply voltage.