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RC downgrade secret, I don't know how many people will be fooled


I guess a lot of people have a better understanding of what RC step-down is, but they don't show much interest in RC step-down. This is because:

  • Design parameters, as a rule, are far from real test (because idea is wrong, we will talk about this in more detail below);

  • Lowering resistance-capacitance is more dangerous because input and output are not isolated (in fact, isolation and ground are also very safe).

  • But advantages in terms of low power are very noticeable: simple design, low cost, small size and wide application in small household appliances.

    Resistance-capacitance reduction can actually be calculated very accurately precisely because its mains input voltage will vary (220V ± 10%), so an accurate calculation is required in order for circuit to work at best design parameters to ensure reliable quality.

    As shown in Picture 1, this is a 3W LE fixture, its circuit driven by an RC step-down chain. Let's use this real object to explain correct calculation method:

    Fig. 1. LED lighting

    Resistance-capacitance pulldown uses capacitance generated by capacitor at AC frequency to limit maximum current to achieve pulldown goal. For example, Figure 2 is an RC buck circuit for LED lighting.

    If R2, R3 and C1 in Figure 2 are removed, then circuit is a simple rectifier circuit. Among them, function of R2 is to discharge C1 after external power supply is turned off, so as to prevent electric shock and harm to people; this resistance is necessary.

    Figure 2. RC (Full Wave Rectification) LED Lighting Driver Step-Down Circuit Diagram

    Calculate capacitive reactance of capacitor C1 (C1 is capacitor CBB) according to parameters given in circuit diagram Fig. 2 so that calculation is shown in Fig. 3. strong>.

    f: AC mains frequency 50 Hz

    C: 824 means 0.82uF

    Figure 3: Calculation of capacitance C1

    And here is important point, scheme Fig. 2 Udc is rectified and filtered DC voltage (also peak AC voltage), so Udc needs to be divided by root #2 can only be calculated by converting it to effective communication value;

    The half cycle of rectifier bridge goes through two diodes, so subtract 2Ud=1.4V voltage drop across two diodes;

    According to design requirements, voltage Udc is about 200 V to power 4 LED lamps, and voltage drop across resistor R3 is not taken into account, so calculation is shown in fig. 4.

    Figure 4: Capacitor voltage drop calculation

    Using above two formulas, current flowing through capacitor can be calculated as shown in Fig. 5:

    Figure 5: Calculating current flowing through a capacitor

    Connect LED lighting to test network as shown in Fig. 6, measured mains AC voltage is about 220V.

    Figure 6: Measured AC mains voltage

    As shown in Fig. 7, connect a multimeter in series with live wire input terminal (L) and measure current flowing through the step-down capacitor C1. strong> should be 20.2 mA.

    Figure 7: Measured current

    As shown in Figure 8, voltage (Udc) of filter capacitor C2 measured with a multimeter is about 192V, and calculated parameter is 200V, and two values ​​are very close.

    Figure 8: Measured rectified voltage


    The measured values ​​are very close to calculated parameters. This calculation method cannot mix AC and DC and requires an RMS conversion process. In other articles, this clause is ignored. I'm afraid that even author himself Nobody will know.


    1. Blow Capacitor Output Current <100mA;

    2. The load is a resistive load and dynamic load change should be small;

    3. You can add a zener diode if you need a stable rectified voltage (Udc) and adjustable value of zener diode should be slightly lower than rectified voltage value;

    4. The step-down capacitor is a non-polar capacitor, usually a CBB capacitor, with a withstand voltage of 400V or more;

    5. Resistance and capacitance reduction uses half-wave rectification, and output current is half that of full-wave rectification.