Selection and precautions for capacitors in capacitor step-down power supplies

Among commonly used low-voltage power supplies, compared with using a transformer to reduce voltage using a capacitor (actually capacitor current limiting), the power supply using a capacitor to step down the voltage is small, economical, reliable, and highly efficient. The disadvantage is that it is not as safe as a power supply transformed by a transformer. AC power is introduced into the load through the capacitor, and there is a voltage of 220V to the ground. People are prone to electric shock. However, if it is used in the internal circuit power supply that does not require human contact, this weakness can be overcome. Power supplies such as refrigerator electronic thermostats or remote control power switches are made of capacitors to reduce voltage.

Compared with resistor voltage reduction, for the lower frequency 50Hz AC, the heat energy loss generated on the capacitor is very small, so capacitor voltage reduction is better than resistor voltage reduction.
The size of the current passing through the capacitor is affected by the capacitive reactance Xc=1u (2πfC). The unit of Xc is ohms; the unit of AC frequency f is Hertz; the unit of capacitor C is Farad.
   
   

When capacitors C (as shown in Figure 1) with different capacities are connected to an AC 220V 50Hz AC circuit, the capacitive reactance of C and the current it can pass are as listed in the attached table. This current is the maximum current value that capacitor C can provide.

When using capacitors to step down the voltage to make a power supply, you must pay attention to the following points:
(1) After the capacitor steps down the voltage, it must be rectified, filtered and stabilized by a Zener diode as shown in Figure 2 before a stable voltage power supply can be obtained (Note: The rectifier circuit can also use half-wave rectification).
(2) The withstand voltage of the capacitor is preferably above 630V, and non-polar capacitors should be used. Polarized capacitors cannot be used.
(3) Connect a 500K-1M bleeder resistor in parallel to both ends of the capacitor.
(4) If you need to add a power switch, in order to prevent the surge current from flowing in parallel to the load RL, as shown in Figure 3.
(5) During the assembly and debugging process, a 1:1 isolation transformer must be used to connect to the AC220V circuit to prevent electric shock.