“The working principle of capacitor buck is not complicated. Its working principle is to use the capacitive reactance generated by the capacitor at a certain frequency of the AC signal to limit the maximum operating current.

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The working principle of capacitor buck is not complicated. Its working principle is to use the capacitive reactance generated by the capacitor at a certain frequency of the AC signal to limit the maximum operating current.

**Example 1**

Under the power frequency condition 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, because 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.

**Example 2**

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 being burned. Because the current required by a 110V/8W bulb is 8W/110V=72mA, it is consistent with 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.

Figure 1 shows the typical application of RC step-down, C1 is the step-down capacitor, R1 is the bleeder resistor of C1 when the power supply 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.

figure 1

In practical applications, Figure 2 can be used instead of Figure 1. 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.

figure 2

In higher current applications, full-wave rectification can be used, as shown in Figure 3.

image 3

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:**

• Select an appropriate capacitor according to the load’s current and AC operating frequency, not the load’s voltage and power.

• Current-limiting capacitors must be non-polar capacitors, and electrolytic capacitors must not be used; and the withstand voltage of the capacitors must be above 400V; the most ideal capacitors are iron-shell oil-immersed capacitors.

• Capacitor bucks cannot be used in high power conditions because they are unsafe.

• Capacitive bucks are not suitable for dynamic load conditions.

• Likewise, capacitive bucks are not suitable for capacitive and inductive loads.

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

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