The working principle of the capacitor step-down is to use the capacitive reactance generated by the capacitor at a certain frequency of the AC signal to limit the maximum working current.

A capacitor step-down principle

The working principle of the capacitor step-down is to use the capacitive reactance generated by the capacitor at a certain frequency of the AC signal to limit the maximum operating current.

For example, at a power frequency of 50Hz, the capacitive reactance produced by a 1uF capacitor is about 3180 ohms. When the AC voltage of 220V is applied to both ends of the capacitor, the maximum current flowing through the capacitor is about 70mA. Although the current flowing through the capacitor is 70mA, there is no power consumption on the capacitor. If the capacitor is an ideal capacitor, the current flowing through the capacitor is the imaginary part current, and the work it does is reactive power.

According to this feature, if we connect a resistive element in series with a 1uF capacitor, the voltage obtained at both ends of the resistive element and the power consumption it produces depend entirely on the characteristics of the resistive element.

For example, we connect a 110V/8W bulb in series with a 1uF capacitor, and when connected to an AC voltage of 220V/50Hz, the bulb is lit and emits normal brightness without burning out. Since the current required for a 110V/8W bulb is 72mA, it matches the current-limiting characteristic produced by a 1uF capacitor.

In the same way, we can also connect a 5W/65V light bulb to a 220V/50Hz AC power in series with a 1uF capacitor, and the light bulb will also be lit without being burned. Because the working current of the 5W/65V bulb is also about 70mA.

Therefore, the capacitor buck actually uses the capacitive reactance to limit the current. The capacitor actually plays a role in limiting the current and dynamically distributing the voltage across the capacitor and the load.

The following figure shows the typical application of RC step-down, C1 is the step-down capacitor, R1 is the bleeder resistor of C1 when the power is disconnected, D1 is the half-wave rectifier diode, and D2 provides a discharge circuit for C1 in the negative half cycle of the mains, otherwise Capacitor C1 will not work when fully charged, Z1 is a Zener diode, and C2 is a filter capacitor. The output is the regulated voltage value of the Zener diode Z1. In practical applications, the following figure can be used instead of the above figure. Here, the forward and reverse characteristics of Z1 are used, and its reverse characteristics (that is, its voltage regulation characteristics) are used to stabilize the voltage, and its forward characteristics are used in the mains. The negative half cycle provides a discharge circuit for C1. In larger current applications, full-wave rectification can be used, as shown below: In the case of small voltage full-wave rectification output, the maximum output current is:

Capacitance: Xc=1/(2πfC)

Current: Ic = U/Xc=2πfCU

The following points should be noted when using capacitors to reduce voltage:

①Select the appropriate capacitor according to the current size of the load and the operating frequency of the alternating current, rather than the voltage and power of the load.

②The current limiting capacitor must use non-polar capacitors, and electrolytic capacitors must not be used. And the withstand voltage of the capacitor must be above 400V. The most ideal capacitors are iron-case oil-immersed capacitors.

③Capacitor step-down cannot be used for high-power conditions, because it is not safe.

④Capacitor step-down is not suitable for dynamic load conditions.

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

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

Two device options

1. When designing the circuit, the accurate value of the load current should be measured first, and then the capacity of the step-down capacitor should be selected with reference to the example. Because the current Io provided to the load through the step-down capacitor C1 is actually the charge and discharge current Ic flowing through C1. The larger the capacity of C1, the smaller the capacitive reactance Xc, the greater the charge and discharge current flowing through C1. When the load current Io is less than the charge and discharge current of C1, the excess current will flow through the Zener tube. If the maximum allowable current Idmax of the Zener tube is less than Ic-Io, it is easy to cause the Zener tube to burn.

2. In order to ensure reliable operation of C1, its withstand voltage selection should be greater than twice the power supply voltage.

3. The selection of the discharge resistor R1 must ensure that the charge on C1 is discharged within the required time.

Three design examples

Knowing that C1 is 0.33μF and the AC input is 220V/50Hz, find the maximum current that the circuit can supply to the load.

The capacitive reactance Xc of C1 in the circuit is:

Xc=1/(2πfC)=1/(2*3.14*50*0.33*10-6)=9.65K

The charging current (Ic) flowing through capacitor C1 is:

Ic = U / Xc = 220 / 9.65 = 22mA

Usually, the relationship between the capacity C of the step-down capacitor C1 and the load current Io can be approximated as: C=14.5 I, where the capacity unit of C is μF, and the unit of Io is A. Capacitor step-down power supply is a non-isolated power supply, special attention should be paid to isolation in application to prevent electric shock.