A Zener diode is often used in circuits that regulate (or stabilize) a voltage. Starting from a moderately or strongly fluctuating input voltage, an almost constant output voltage is obtained with a voltage regulator (Figure 2.39).
Figure 2.39 A voltage regulator ensures that the voltage across a certain load (RL) remains stable. The input voltage should always be large enough to ensure that the Zener diode breaks down.
The regulator only works if the input voltage is large enough to put the Zener diode in breakdown mode. In that case there will be an almost constant voltage across the Zener (the breakdown voltage) and since the load is in parallel with the Zener, the voltage across the load will be stable. Example waveforms are shown in Figure 2.40.
Figure 2.40 Example waveforms for a voltage regulator. The difference between the input voltage and the output voltage is taken by the resistor R.
The resistor R is responsible for ‘handling’ the voltage difference between the input voltage and the desired output voltage. This will result in a varying current (if the input voltage changes). In case the load does not change, it will require a constant current. Thus, the Zener diode will take care of the current fluctuations (the varying resistor current diminished by the current drawn by the load). A fluctuating current has very limited influence on the voltage of a Zener in breakdown, given the steep characteristic at breakdown.
The resistor value should not be too large. The load needs a current VBD / RL, while also the Zener diode needs a minimal current at breakdown. If the resistor is too large, the current will be too small compared to the current that the load ‘needs’. The worst case scenario occurs at minimal input voltage, resulting in a small resistor current, while the load needs a very high current.
The resistor should not be too small either. In that case the resistor current will be large. The part of the current that is not drawn by the load, will flow through the Zener diode, but care should be taken that the maximum allowed power dissipation is not exceeded. The Zener diode’s power is maximal when the input voltage is maximal, and the load current is minimal.