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# What is the function of the capacitor inside the transmitter?

Published:2019/10/19 12:25:25 Visits:

How capacitors reduce voltage：

For example, at a power frequency of 50 Hz, a 1 uF capacitor produces a capacitive reactance of approximately 3180 ohms. When an AC voltage of 220V is applied across the capacitor, the maximum current flowing through the capacitor is approximately 70 mA. Although the current flowing through the capacitor is 70 mA, there is no power consumption on the capacitor. If the capacitor is an ideal capacitor, the current flowing through the capacitor is the imaginary current, and the work done is reactive power.

According to this feature, if we connect a resistive component in series with a 1uF capacitor, the voltage obtained across the resistive component and the power dissipation it generates depends entirely on the characteristics of the resistive component. For example, we connect a 110V/8W bulb in series with a 1uF capacitor. When connected to an AC voltage of 220V/50Hz, the bulb is illuminated and emits normal brightness without being burned. Because the 110V/8W bulb requires 8W/110V=72mA, it matches the current limiting characteristics of the 1uF capacitor.

Similarly, we can also connect a 5W/65V bulb and a 1uF capacitor to a 220V/50Hz AC. The bulb will also be lit without being burned. Because the 5W/65V bulb also has an operating current of about 70mA. Therefore, the capacitor buck is actually using the capacitive reactance current limit. The capacitor actually acts as a limiting current and dynamically distributing the voltage across the capacitor and the load.

Typical application of RC

The following figure shows the typical application of RC and RBI. C1 is the step-down capacitor. R1 is the bleeder resistance of C1 when the power is off. D1 is the half-wave rectification diode. D2 provides the discharge circuit for C1 in the negative half of the mains. Otherwise Capacitor C1 is not fully charged, Z1 is a Zener diode and C2 is a filter capacitor. The output is a stable voltage value of Zener diode Z1. In practical applications, the following figure can be used instead of the above figure. Here, the Z1 forward characteristic and the reverse characteristic are used, and the reverse characteristic (that is, its voltage regulation characteristic) is used to stabilize the voltage, and the forward characteristic is used in the commercial power. A negative half cycle provides a discharge loop for C1. In larger current applications, full wave rectification can be used. As shown below: In the case of a small voltage full-wave rectified output, the maximum output current is:

Capacitance: Xc=1/(2πfC) Current: Ic = U/Xc=2πfCU

Pay attention to the following points when using capacitor step-down

1. Select the appropriate capacitor based on the current of the load and the operating frequency of the AC, not the voltage and power of the load.

2. Current-limiting capacitors must use non-polar capacitors, and electrolytic capacitors must never be used. Moreover, the withstand voltage of the capacitor must be above 400V. The most ideal capacitor is an iron-shell oil-immersed capacitor.

3. Capacitor buck cannot be used for high power conditions because it will be destroyed.

4. Capacitor buck is not suitable for dynamic load conditions.

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

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

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