Power rating of the dropping resistor depends on the current flowing through it. Before selecting a resistor, it is important to assess how much current is required for the load. The output voltage also depends on the value of the resistor and the zener diode. Table III helps to select the resistor and zener diode to get the desired output voltage.
A high-wattage type resistor is necessary to reduce the heat generation and power loss. A 10-watt resistor can be used safely without much heat generation. Instead of using a single resistor, it is better to use two resistors in series to get the total value of the resistance required. For example, instead of using a single 10-kilo-ohm resistor, it is better to use two 5-kilo-ohm resistors in series. This will prevent high-voltage transients appearing in the circuit and also lower the potential across the resistor, so the chance of arcing can be reduced.
Power rating of the zener diode is also important as it handles large current in no-load condition. It is better to use a 1W zener diode for regulation. Electrolytic capacitor C1 should be rated twice the value of the zener diode. For fine smoothing, it is better to use a 1000µF, 25V capacitor.
To increase the power supply efficiency, the circuits shown in Figs 4 and 5 can be used.
In both the circuits, current is supplied from the source during both the halves of the AC cycle. So the output voltage remains more stable than with half-wave rectification. Since the output voltage is not referred to phase or neutral, these power supplies cannot be used to control thyristors.