For example, if a 0.22µF capacitor is used, it gives about 14.4-ohm reactance at 50Hz, 230V.
Suppose the current passing through the capacitor is ‘I’ and mains voltage is 230 volts, then current I through the capacitor is V/X. That is, 230V/14.4 = 15.9 mA.
Thus a 0.22µF capacitor can give only 15mA current for the circuit. So it is important to fix the current requirement of the circuit before selecting the dropping capacitor.
Capacitive power supply
The simplest form of a capacitive type power supply uses an ‘X’ rated capacitor along with rectifier and smoothing components (Fig. 2).
Here the voltage through the load remains constant as long as the output current is less or equal to the input current. This current is limited by resistor R1 and capacitor C1. R1 is necessary to limit the inrush current at power-’on.’ The high-voltage capacitor C1 acts as a current and voltage limiter which drops 230V AC to a low-voltage AC.
When current flows through D2, capacitor C2 charges to its full voltage capacity and 9.1 volts appears across zener diode ZD. The maximum DC current available from the power supply depends on the value of C1. A low-value capacitor gives lesser current and a high-value capacitor gives more current. Maximum output current (IMax) available from the circuit is:
IMax = 2 π × 50C × 230
where ‘C’ is the value of the dropping capacitor in µF and 50 is the mains frequency at 230V AC. The current available from the circuit shown in Fig. 2 is around 35 milliamps.
To get the desired output current, it is important to fix the value of the dropping capacitor. The easiest method is to divide the required current by 75, i.e., output current (mA)/75. So the value of the capacitor in µF can be obtained. Table I shows some common ‘X’ rated AC capacitors and the current available from them.
The actual value of current given in Table I will not be available at the output since it may vary slightly depending on the input voltage and current. Voltage drop in the mains severely affects the output current in a transformerless power supply. The components of the power supply will also consume some current. It is better to use a high-value capacitor like 2.2µF if the load requires more than 50mA current.