table of Contents
1. Overview
The working principle of the resistance-capacitance 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, 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. 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 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.
2. Schematic
The basic circuit of the capacitor step-down simple power supply is shown in the figure. C140 is a step-down capacitor, MB6F rectifier bridge stack, T1 and T2 are Zener diodes, and R153 is the charge discharge resistance of C1 after the power is turned off. R142 is a PTC resistor, and R143 is a varistor. C103 is the filter capacitor.
3. Calculation formula
1μF, when the AC input is 220V/50Hz, the capacitive reactance Xc is:
Xc=1 /（2 πf C）
= 1/（2*3.14*50*1*10-6）
= 3184.7Ω
The charging current (Ic) flowing through the capacitor C1 is:
Ic = U / Xc
= 220 / 3184.7 = 69mA 。
4. Device selection
When designing the circuit, you should 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 provided to the load through the step-down capacitor is actually the charge and discharge current that flows. The larger the capacity of the step-down capacitor and the smaller the capacitive reactance Xc, the greater the charge and discharge current flowing through the step-down capacitor. When the load current is less than the charging and discharging current of the step-down capacitor, the excess current will flow through the zener tube. If the maximum allowable current of the zener tube is less than the difference between the current of the step-down capacitor and the load current, it will easily cause voltage regulation. The pipe burned. The selection of the bleeder resistor must ensure that the charge on the step-down capacitor is discharged within the required time. In order to ensure reliable operation of the step-down capacitor, its withstand voltage should be greater than twice the power supply voltage.
5. Use attention
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 capacitor with an iron shell.
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 it should meet the conditions of constant load.
7. This circuit is not isolated from 220V AC high voltage, please pay attention to safety and prevent electric shock!
8. The current-limiting capacitor must be connected to the live wire, the withstand voltage must be large enough (greater than 400V), and a series of anti-surge impact and insurance resistance and parallel discharge resistance should be added.
9. Pay attention to the power consumption of the Zener tube. It is strictly forbidden to disconnect the Zener tube.
6. Voltage regulator tube
Zener diode (also known as Zener diode or reverse breakdown diode)
Zener diode has the unidirectional conductivity characteristics of ordinary diodes, but it works in the reverse breakdown zone. When the reverse voltage reaches a certain value, the reverse current suddenly increases and the Zener diode enters the breakdown zone, but it does not damage but enters the normal working state. This is the biggest difference from ordinary diodes. After the diode enters this working state, even when the reverse current changes within a large range, the reverse voltage across the Zener diode can remain basically unchanged, which is the voltage regulator characteristic of the Zener diode. If the reverse current continues to increase to a certain value, the Zener diode will be completely broken down and damaged.