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

2023-09-05【Archive】

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

**Browse:**

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.

**Highlights:**

Blow Capacitor Output Current <100mA;

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

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;

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

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

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