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Have you ever encountered these 6 common pits of capacitor step-down?

by:Shenmao     2021-04-30
1. The principle of capacitor step-down The working principle of capacitor step-down is to use the capacitive reactance generated by the capacitor at a certain AC signal frequency to limit the maximum operating current. For example, under 50Hz power frequency conditions, the capacitive reactance generated 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, because 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 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 figure below can be used instead of the figure above. Z1 forward and reverse characteristics are used here. Its reverse characteristics (that is, its voltage stabilization 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 in the figure below: When the output is small-voltage full-wave rectification, the maximum output current is: capacitive reactance: Xcu003d1/(2πfC) current: Ic u003d U/Xcu003d 2πfCU should pay attention to the following points when using capacitors to step down: ①Select appropriate capacitors 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. ②Current-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. ③Capacitor step-down cannot be used in high-power conditions because it is not safe. ④The 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. 2. Device selection 1. When designing the circuit, first determine the accurate value of the load current, and then refer to the example to select the capacity of the step-down capacitor. 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, and 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 will easily cause the Zener tube to burn. 2. In order to ensure the reliable operation of C1, its withstand voltage selection should be greater than twice the power supply voltage. 3. The selection of bleeder resistor R1 must ensure that the charge on C1 is discharged within the required time. 3. Design example 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: Xcu003d1 /(2 πf C) u003d 1/(2*3.14*50*0.33*10-6) u003d 9.65K The charging current (Ic) flowing through the capacitor C1 is: Ic u003d U / Xc u003d 220 / 9.65 u003d 22mA Generally, the relationship between the capacity C of the step-down capacitor C1 and the load current Io can be approximated as: Cu003d14.5 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.
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