Because R is much smaller than XC1, the voltage drop across R (VR) is much less than that across C1 (VC1). Therefore, VC1 is approximately equal to the supply voltage V, meaning VC1 ≈ V. According to electrical principles, the average DC current after rectification (Id) is related to the average AC current (I) by Id = V / XC1. If C1 is in microfarads, then Id will be in milliamperes. For a 22V, 50Hz AC input, Id = 0.62 × C1.
From this, two conclusions can be drawn:
1. When using a power transformer as a rectified power supply, once the circuit parameters are set, the output voltage remains constant, but the output current Id changes depending on the load.
2. When using a capacitor for step-down, since Id = 0.62 × C1, the output current is proportional to C1. Once C1 is fixed, the output current remains constant, while the DC output voltage varies within a certain range based on the load resistance RL. The smaller the RL, the lower the output voltage; the larger the RL, the higher the output voltage.
The value of C1 should be chosen based on the load current. For example, if the load requires a 9V operating voltage and an average current of 75 mA, then C1 = 1.2uF can be calculated. Considering losses in the Zener diode VD5, it's better to choose C1 as 1.5uF, which provides an actual current of around 93 mA.
The Zener diode’s regulation voltage must match the load’s operating voltage. Choosing the correct stable current is also important. Since the capacitor step-down power supply acts like a constant current source, it can handle short circuits without damage. However, when the load is open, all 93 mA will flow through R1 and VD5. Thus, the maximum stable current of VD5 should be at least 100 mA. Since RL is in parallel with VD5, ensuring that RL draws 75 mA, VD5 must carry at least 18 mA. Therefore, the minimum stable current should not exceed 18 mA, or the voltage regulation will fail.
The current-limiting resistor R1 should not be too large, as this increases power loss and raises the voltage rating requirement for C2. For example, if R1 = 100 ohms and the voltage drop across it is 9.3V, the power loss is 0.86W, so a 100-ohm, 1W resistor is suitable.
The filter capacitor is typically between 100uF and 1000uF, but its voltage rating must be carefully selected. As mentioned earlier, with a 9V load and a 9.3V drop across R1, the total voltage is around 18.3V. To ensure safety, a capacitor with a withstand voltage above 25V is recommended.Features
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