**Capacitor Step-Down Power Supply Features**
**I. Overview**
Electronic engineers are constantly working to reduce the size of equipment and optimize designs in order to lower costs. One of the most challenging tasks is reducing the size of the DC stabilized power supply circuit, which often serves as an auxiliary power source. Traditional linear DC power supplies have low efficiency, and their power transformers are bulky, heavy, and expensive.
Switching power supplies, on the other hand, have a more complex structure, higher cost, and tend to produce more ripple. They also suffer from issues with RFI (Radio Frequency Interference) and EMI (Electromagnetic Interference), which are difficult to manage.
To address these challenges, a novel capacitor step-down DC regulated power supply circuit has been developed. This circuit eliminates the need for a power transformer, making it simple in structure, compact, lightweight, and cost-effective. It offers fast dynamic response, high stability, and efficiency that can reach up to 90% or more.
**II. Principle of Capacitor Step-Down**
When a sinusoidal AC voltage, such as 220V at 50Hz, is applied across a capacitor, the charge on its plates and the electric field between them vary over time. The effective value and amplitude of the voltage and current through the capacitor follow Ohm’s Law. That is, when the voltage amplitude is constant and the frequency is fixed, a stable alternating current flows through the capacitor. The smaller the capacitive reactance (the larger the capacitance), the greater the current flowing through the capacitor.
By connecting a suitable load in series with the capacitor, a reduced voltage source can be obtained, which can then be rectified, filtered, and regulated. Since capacitors do not consume energy but only store and release it, the efficiency of a capacitor step-down circuit is very high.
**III. Principle Block Diagram**
[Image: Circuit diagram showing the components of a capacitor step-down power supply.]
The circuit typically includes a buck capacitor, current limiting resistor, rectification and filtering stage, and a shunt voltage regulation section.
1. **Buck Capacitor**: Acts like a step-down transformer, directly connected to the AC input and bearing most of the voltage. It should be a non-polar metal film capacitor.
2. **Current Limiting Circuit**: Prevents large inrush currents when the power is first turned on. A resistor is added in series to protect the circuit.
3. **Rectification and Filtering**: Can use either half-wave or full-wave rectification, similar to traditional DC power supplies.
4. **Voltage Regulation Shunt**: Ensures stable output by adjusting the current flow based on the load.
**IV. Design Examples**
**1. Full-Wave Rectifier Circuit**
Specifications: Output DC voltage 12V, current 300mA; Input: 220V AC / 50Hz.
- **Step-down Capacitor C1**: Calculated as 5μF, 250V AC.
- **Current Limiting Resistor R1**: 47Ω, 7.5W.
- **Voltage Regulator Circuit**: Uses a Zener diode and transistor for regulation.
**2. Half-Wave Rectifier Circuit**
Specifications: Output 24V DC and 5V DC, current 60mA; Input: 220V AC / 50Hz.
- **Step-down Capacitor C1**: 2μF, 250V AC.
- **Current Limiting Resistor R1**: 100Ω, 3W.
- **Regulation Circuits**: Zener diodes and transistors regulate both 24V and 5V outputs.
**Usage Precautions**
- The output DC voltage is not isolated from the AC input, so this circuit is suitable for non-isolated applications like control systems, sensors, and small household appliances.
- It is not recommended for devices requiring electrical isolation.
- Due to the limited capacitance of the metal film capacitor, it is best suited for low-power DC power supplies.
**Advantages**
- Simple structure, compact size, and light weight, ideal for miniaturization.
- No power transformer needed, leading to low component requirements and reduced cost.
- Acts as a current source, allowing for flexible adjustment of output voltage by changing reference components.
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