The relationship between capacitive reactance (X), capacitance (C) and output current (I) is expressed as:
As a safety measure, R2 (470-kilo-ohm) is placed in parallel to C1 as a bleeder resistor. It removes the stored current from C1 when the power supply is disconnected from the mains. This prevents shock hazards, since the capacitor stores more than 400 volts when fully charged. This current may be present in the capacitor for many days even after the circuit is switched off.
Since the circuit is directly connected to high-voltage AC, power rating of the components is important. Capacitor C1 must be X-rated type with a voltage rating of 400 volts or more. The power rating of resistors R1 and R2 must be high enough to handle AC.
Current through R1 is full-wave current, so it is equivalent to the line voltage divided by the impedance of C1. Therefore R1 and R2 must be rated at 1 watt.
Diodes D1 and D2 are rectifiers. Maximum RMS current passes through D1 and D2. So the peak inverse voltage of the diodes must be sufficient to handle this high current. Diode 1N4007 is a good choice as it can handle 1000 volts and 1A current.
Capacitor C2 smooths rectified DC and acts as a buffer to release current to the load. The value of C2 should be double the voltage rating of the zener diode. A 25V electrolytic capacitor will work smoothly.
Zener diode ZD is used to regulate the output to 9.1 volts. It is subjected to most of the current in no-load condition and will get most of the full-wave current once C2 is charged. So the power rating of ZD must be sufficient to handle this high current. Maximum power dissipation that can be allowed in the zener is the zener voltage multiplied by the current flowing through it. If the 9.1V zener passes 35mA current, its power dissipation will be around 318 mV. So a zener rated at 400 milliwatts or more can be safely used.
Resistive power supply
In contrast to the capacitive type, a resistive power supply uses the property of resistance to limit the current. As with the capacitive type, the output voltage will remain stable as long as the output current is less or equal to the input current. Resistive power supply is cost-effective and requires less space for its accommodation. But severe power loss through heat is an important drawback. Like capacitive type, it also has no isolation from mains. The output supply is less energy-efficient compared to a capacitive power supply.
Fig. 3 shows the design of a simple resistive power supply. Before making a resistive power supply, power rating of the components needs critical consideration. All the components should have sufficient current capabilities. Since the resistor is used to drop voltage and current from the mains, power rating of the resistor is important to avoid burnout. Table II shows the current-handling capacity of various types of resistors.