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In the commonly used low-voltage power supply, the capacitor is stepped down (actually the capacitor current limit). Compared with the transformer, the capacitor step-down power supply is small, economical, reliable, and efficient. The disadvantage is that the power supply is not as good as the transformer transformer. The AC power is introduced into the load through a capacitor, and the voltage is 220V to the ground, and the person is easily exposed to electricity. However, if used in a circuit internal power supply that does not require human contact, this weakness can be overcome. For example, the power supply of the refrigerator electronic temperature controller or the remote control power supply is made by stepping down the capacitor.
Compared with the resistor step-down, for the lower frequency 50Hz AC, the thermal energy loss generated on the capacitor is small, so the capacitor step-down is better than the resistor step-down.
When using a capacitor to step down the power supply, you must pay attention to the following points:
(1) After the capacitor is stepped down, it must be rectified, filtered, and stabilized by a Zener diode to obtain a voltage-stabilized power supply (Note: The rectifier circuit can also be half-wave rectified).
2) The capacitor withstand voltage is preferably above 630V, and a non-polar capacitor is used, and a polar capacitor cannot be used.
(3) Connect 500K-1M bleeder resistors across the capacitor.
(4) If it is necessary to add a power switch, in order to prevent the inrush current from flowing in parallel with the load RL.
(5) In the assembly and debugging process, use a 1:1 isolation transformer to connect to the AC220V circuit to prevent electric shock.
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