Selection and precautions of capacitors in capacitor step-down power supply

　　In commonly used low-voltage power supplies, capacitors are used to reduce voltage (actually capacitors are used to limit current) compared to transformers. Capacitors are smaller, more economical, more reliable, and more efficient. However, they are not as safe as transformers. When AC is introduced into the load through capacitors, there is a 220V voltage to the ground, which can easily cause electric shock. However, this weakness can be overcome if it is used in the power supply of the internal circuit of the circuit that does not require human contact. For example, the power supply of the refrigerator electronic thermostat or the switch of the remote control power supply is made by reducing the voltage with capacitors.

　　Compared with the voltage reduction by resistors, the heat energy loss generated on the capacitor is very small for the lower frequency 50Hz AC, so the voltage reduction by capacitors is better than the voltage reduction by resistors.

　　The magnitude of the current passing through the capacitor is affected by the capacitive reactance Xc=1╱(2πfC), where the unit of Xc is ohm; the unit of the AC frequency f is hertz; and the unit of the capacitor C is farad.

　　When capacitors C of different capacities (as shown in Figure 1) are connected to an AC220V 50Hz AC circuit, the capacitive reactance of C and the current that can pass through it are 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, the following points must be noted:

　　(1) After the voltage is stepped down by the capacitor , it must be rectified, filtered and stabilized by the voltage regulator diode as shown in Figure 2 to obtain a stable voltage power supply (Note: the rectifier circuit can also use half-wave rectification).

　　2) The capacitor's withstand voltage should be above 630V , and non-polar capacitors should be used. Polarized capacitors cannot be used.

　　(3) Connect a 500K-1M discharge resistor in parallel across the capacitor.

　　(4) If a power switch is required, it should be connected in parallel to the load RL to prevent surge current, as shown in Figure 3.

　　(5) During the assembly and debugging process, use a 1:1 isolation transformer to connect to the AC220V circuit to prevent electric shock.