Capacitor buck

Power supply buck problem. It used to be a small transformer that passed through the bridge stack and the three-terminal regulator. Now I hear the "capacitor step-down", I feel very strange, check the relevant information, and find that the principle is still very simple.

A 1uf capacitor is added to the mains with a frequency of 50Hz at both ends. The capacitance produced by the capacitor is [1/(2*pi*50)]*1000000=3184 ohms. If 220V AC voltage is directly applied to the capacitor At both ends, the current generated is about 70 mA. Although the current flowing through the capacitor is 70 mA, there is no power consumption on the capacitor, because if the capacitor is an ideal capacitor, the current flowing through the capacitor is the imaginary current (virtual The current is caused by the capacitance and inductance in the circuit, does not consume electrical energy, but it constantly affects the change of current, doing virtual work, non-stop energy storage), and its work is reactive power. According to this feature, if a resistive component is connected in series with a 1uF capacitor, the voltage obtained across the resistive component and the power dissipation it generates are completely dependent on the characteristics of the resistive component. For example, a 110V/8W bulb is connected in series with a 1uF capacitor and then connected to an AC voltage of 220V/50Hz. The bulb is illuminated and emits normal brightness without being burned. Because the 110V/8W bulb requires 8W/110V=72mA, it matches the current limiting characteristics of the 1uF capacitor. Similarly, a 5W/65V bulb and a 1uF capacitor can be connected in series to a 220V/50Hz AC, and the bulb will be illuminated without being burned. Because the 5W/65V bulb also has an operating current of about 70mA. Therefore, the capacitor buck is actually using the capacitive reactance current limit. The capacitor actually acts as a limiting current and dynamically distributing the voltage across the capacitor and the load.

Pay attention to the following points when using capacitor step-down:

1. Select the appropriate capacitor based on the current of the load and the operating frequency of the AC, not the voltage and power of the load.

2. Current-limiting capacitors must use non-polar capacitors, and electrolytic capacitors must never be used. Moreover, the withstand voltage of the capacitor must be above 400V. The most ideal capacitor is an iron-shell oil-immersed capacitor.

3. Capacitor buck cannot be used in high power conditions because it is not safe.

4. Capacitor buck is not suitable for dynamic load conditions.

5. Similarly, capacitor buck is not suitable for capacitive and inductive loads.

6. When DC operation is required, try to use half-wave rectification. Bridge rectification is not recommended. And to meet the conditions of a constant load.