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What issues should be paid attention to when the capacitor is stepped down?

new.qq.com | 16/04/2021

Everyone will encounter various problems in circuit design. Capacitor step-down is also one of the common problems. In fact, the working principle of capacitor step-down is not complicated: that is, the capacitance generated by the capacitor at a certain AC signal frequency is used. Resistance to limit the maximum operating current.

For example, under 50Hz power frequency conditions, the capacitive reactance produced by a 1uF capacitor is about 3180 ohms. When 220V AC voltage 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 in the capacitor. If the capacitor is an ideal capacitor, the current flowing through the capacitor is the imaginary 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 generates completely depend on the characteristics of the resistive element.

For example, we connect a 110V/8W light bulb with a 1uF capacitor in series, and when it is connected to an AC voltage of 220V/50Hz, the light bulb is lit and emits normal brightness without being burnt. Because the current required by the 110V/8W bulb is 8W/110V=72mA, it is consistent with the current-limiting characteristics produced by the 1uF capacitor.

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

Therefore, the capacitor step-down is actually the use of capacitive reactance to limit 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 a typical application of RC step-down, C1 is a step-down capacitor, R1 is the bleeder resistance of C1 when the power is disconnected, D1 is a half-wave rectifier diode, D2 provides a discharge circuit for C1 in the negative half of the mains, otherwise The capacitor C1 will not work when it is fully charged, Z1 is a Zener diode, and C2 is a filter capacitor. The output is the stable voltage value of the Zener diode Z1.

 

In practical applications, the EDA365 Electronics Forum believes that the following figure can be used instead of the above figure. Here, the forward and reverse characteristics of Z1 are used. Come to provide a discharge circuit for C1 in the negative half cycle of the mains.

 

In higher current applications, full-wave rectification can be used. As shown below:

 

In the case of low-voltage full-wave rectification output, the maximum output current is:

Capacitive reactance: Xc=1/(2πfC)

Current: Ic = U/Xc=2πfCU

Pay attention to the following points when using capacitors to step down

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

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

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

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

5. Similarly, capacitor step-down is not suitable for capacitive and inductive loads.

6. When DC work is required, half-wave rectification should be used as much as possible. Bridge rectification is not recommended. And to meet the condition of constant load.

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