Capacitor Typical Application Circuit Principles (I)

Capacitor step-down circuit:

The most common way to reduce the 220V AC voltage to low voltage is to use a power transformer, and one way is a capacitor step-down circuit. The advantages are small size, low cost, high efficiency, and the disadvantage is that there is no power transformer safety.

The power supply indicates a capacitor step-down circuit. In the circuit, C1 is a step-down capacitor, VD1 and VD2 are light-emitting diodes, R2 is a current-limiting protection resistor, and R1 is a bleeder resistor.

working principle:

Since the capacitive reactance of C1 is relatively large, the current in the loop is limited, so that the current flowing through the LED is appropriately sized to cause it to emit light. The positive half cycle of the alternating current VD1 emits light, and during the turn-on of VD1, VD2 is turned off. The negative half cycle of the alternating current causes VD2 to conduct, at which point VD1 is turned off. Although VD1 and VD2 are alternately turned on, it is perceived that VD1 and VD2 are always illuminated due to the high conduction frequency and the persistence of vision of the human eye.

Capacitor step-down circuit: Capacitor has capacitive reactance in the AC circuit, so there is a voltage drop across the capacitor. The AC mains frequency is 50Hz lower frequency, so the capacitive reactance is large and the voltage drop across the capacitor is large. Greatly reduce the AC output voltage.

R1 is used to bleed the charge on C1 as quickly as possible. After the AC power is disconnected, the internal charge of C1 is discharged through the loop of R1 to release the internal charge, so that there is no voltage across C1. Only then can the safety of this circuit be guaranteed.

Capacitor step-down half-wave rectifier circuit:

Capacitor step-down half-wave rectification circuit, C1 is a step-down capacitor, which reduces the 220V AC voltage to an appropriate degree u, through VD1 half-wave rectification, and then C2 filter to obtain DC voltage. R1 is the bleeder resistor.

Capacitor buck bridge rectifier circuit:

Since the internal resistance of the half-wave rectifying circuit is relatively large, in order to provide a larger power supply current, a capacitor buck bridge rectifier circuit having a smaller internal resistance can be used.

C1 is a step-down capacitor, R1 is a bleeder resistor, VD1-VD4 is a bridge rectifier diode, and C2 is a filter capacitor.

Step-down capacitor selection method

The size of the step-down capacitor determines the current in the buck circuit, and the capacity of the buck capacitor can be selected according to the load current limit.

The relationship between current magnitude and capacitance in a 220V/50Hz capacitor step-down circuit. The current in the table is the maximum current value for a particular buck capacitor capacity.

How to choose the bleeder resistor:

In the capacitor step-down circuit, a bleeder resistor needs to be connected in parallel across the step-down capacitor. A large bleeder resistor can reduce power consumption, but the bleeder effect is poor, and the step-down capacitor has a long discharge time. The bleeder resistance is typically 500 kΩ to 1 megaohm. Some fine-tuning is required depending on the size of the buck capacitor to achieve better bleed.

important hint:

1. Capacitor buck power supply is a non-isolated power supply, which actually introduces 220V AC into the load circuit. When applying, pay special attention to isolation to prevent electric shock. Use 1:1 isolation transformer for debugging to ensure safety.

2. Compared with the resistor step-down circuit, the step-down capacitor is superior to the resistor step-down circuit because the loss of the alternating current is small.

3. The capacitance of the buck capacitor in the capacitor step-down circuit is the internal resistance of the power supply. Its capacitive reactance is large, the internal resistance of the power supply is large, and the current supplied by the power supply is small. Due to the large internal resistance of the power supply and the change of the load current, the DC working voltage also has a corresponding change. To ensure the stable DC operating voltage, a Zener diode can be set to stabilize the output DC operating voltage.

4. The voltage resistance of the step-down capacitor is preferably selected above 400V. The non-polar capacitor is used, and the ideal one is the iron-shell oil-immersed capacitor.

5. Capacitor step-down circuits cannot be used for high power conditions, are not suitable for dynamic load conditions, and are not suitable for capacitive and inductive loads.