Diode Zener and DC Voltage Regulation - Electrical Circuits

Diode Zener and DC Voltage Regulation

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.

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.

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.