PCB Fabrication

Do I have to use a transformer for step-down?

by:A-TECH      2021-03-17
Do I have to use a transformer for step-down? In fact, this is not the case. In addition to transformers, capacitors can also step down. Today, I will lead you to learn about capacitor step-down! The working principle of the capacitor step-down utilizes 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 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. Knock on the blackboard, let me first talk about what is reactive power. What is reactive power? Reactive power means that in an AC circuit with reactance, the electric field or magnetic field absorbs energy from the power supply during a part of a cycle, and releases energy during the other part of the cycle. The average power during the entire cycle is zero, but the energy is in the power supply and the reactance. The components (capacitors, inductors) are exchanged continuously. The maximum value of the exchange rate is the 'reactive power'. In a single-phase AC circuit, its value is equal to the product of the RMS value of the voltage, the RMS value of the current, and the sine of the phase angle between the voltage and the current. The unit is Var, KVar. Note: Remember to understand by the way: active power and apparent 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 its The resulting power consumption depends entirely on the characteristics of this 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/110Vu003d72mA, 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 and a 1uF capacitor in series to a 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. 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 (note that for safety, this resistance must be present, and safety is always the first place), D1 is half-wave Rectifier diode, D2 provides a discharge circuit for C1 in the negative half cycle 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 higher current applications, full-wave rectification can be used. As shown in the figure below: In the case of low-voltage full-wave rectification output, the maximum output current is: Capacitive reactance: Xcu003d1/(2πfC) Current: Ic u003d U/Xcu003d2πfCU When using capacitors to step down, the following points should be noted: according to the load Choose an appropriate capacitor for the current size and AC operating frequency, rather than based on the voltage and power of the load. 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. Capacitor step-down cannot be used in high-power conditions because it is not safe. Capacitor step-down is not suitable for dynamic load conditions. Similarly, capacitor step-down is not suitable for capacitive and inductive loads. 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. Here to enter the pricing page https://www./
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