Diode Zener as DC Voltage Regulation
As previously noted, the dynamic resistance of a reverse-biased Zener diode will diminish as the current increases. This relationship begins at the point where breakdown in the diode begins at
its Zener voltage and is approximately linear over a limited range. The unique behavior of the Zener makes it usable as a very simple voltage controller when placed in series with a resistor as shown in
Figure 26-19. It is helpful to imagine the diode and the resistor as forming a kind of voltage divider, with power being taken out at point A in the schematic. If a supply fluctuation increases the input voltage, this will tend to increase the current flowing through the Zener, and its dynamic resistance will diminish accordingly. A lower resistance in its position in the voltage divider will reduce the output voltage at point A, thus tending to compensate for the surge in input voltage.
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| Figure 26-19. diode Zener as DC Voltage Regulation basic circuit |
Conversely, if the load in the circuit increases, and tends to pull down the input voltage, the current flowing through the Zener will diminish, and the voltage at point A will tend to increase, once again compensating for the fluctuation in the circuit.
As the series resistor would be a source of heat, a transistor could be added to drive the load, as shown in Figure 26-20.
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| Figure 26-20. diode Zener and transistor |
A manufacturer’s datasheet may provide guidance regarding the dynamic resistance of a Zener diode in response to current, as previously shown in Figure 26-6. In practice, a packaged voltage regulator such as the LM7805 would most likely be used instead of discrete components, since it includes self-calibrating features, requires no series resistor, and is relatively unaffected by temperature. However, the LM7805 contains its own Zener diode, and the principle of operation is still the same.
