Capacitor step-down LED driver circuit

The capacitor step-down circuit is a common low-current power supply circuit. It is also often used in LED drive circuits due to its advantages such as small size, low cost, and relatively constant current.

Figure 1 is an actual LED drive circuit using capacitor voltage reduction: Please note that most application circuits do not have varistors or transient voltage suppression transistors connected. It is recommended to connect them because varistors or transient voltage suppression transistors can effectively discharge sudden currents at the moment of voltage mutation (such as lightning, starting of large electrical equipment, etc.), thereby protecting the secondary switch and other transistors. Their response time is generally in micromilliseconds.

Circuit working principle:

The function of capacitor C1 is to reduce voltage and limit current: As we all know, the characteristics of capacitors are to pass AC and block DC. When a capacitor is connected to an AC circuit, the formula for calculating its capacitive reactance is:

XC = 1/2πf C

In the formula, XC represents the capacitive reactance of the capacitor, f represents the frequency of the input AC power supply, and C represents the capacity of the step-down capacitor.

The current calculation formula flowing through the capacitor voltage step-down circuit is:

I = U/XC

Where I represents the current flowing through the capacitor, U represents the power supply voltage, and XC represents the capacitive reactance of the capacitor.

In a 220V, 50Hz AC circuit, when the load voltage is much less than 220V, the relationship between current and capacitance is:

I = 69C The unit of capacitance is uF and the unit of current is mA

The following table compares the theoretical current and the actual measured current in a 220V, 50Hz AC circuit.

Resistor R1 is a discharge resistor. Its function is: when the sine wave is cut off at the maximum peak, the residual charge on capacitor C1 cannot be released and will exist for a long time. If the human body touches the metal part of C1 during maintenance, there is a strong possibility of electric shock. The existence of resistor R1 can discharge the residual charge, thereby ensuring the safety of people and machines. The resistance value of the discharge resistor is related to the size of the capacitor. Generally, the larger the capacity of the capacitor, the more residual charge there is, and the resistance value of the discharge resistor should be smaller. The empirical data is shown in the following table for reference during design:

The function of D1 ~ D4 is rectification, which is to rectify the alternating current into pulsating direct current voltage.

The function of C2 and C3 is filtering, which is to filter the rectified pulsating DC voltage into a stable DC voltage.

The function of the varistor (or transient voltage suppression transistor) is to discharge the instantaneous pulse high voltage in the input power supply to the ground, thereby protecting the LED from being broken down by the instantaneous high voltage.

The number of LEDs connected in series depends on their forward voltage (Vf). In a 220V AC circuit, the maximum number can be around 80.

Component selection: The withstand voltage of the capacitor is generally required to be greater than the peak value of the input power supply voltage. In a 220V, 50Hz AC circuit, you can choose a polyester capacitor or paper dielectric capacitor with a withstand voltage of more than 400 volts.

D1 ~D4 can choose IN4007.

The withstand voltage of filter capacitors C2 and C3 depends on the load voltage, which is generally 1.2 times the load voltage. The capacitance depends on the load current.

The following circuit diagram is another form of capacitor step-down drive circuit for reference in design:

In the circuit of Figure 2, the thyristor SCR and R3 form a protection circuit. When the current flowing through the LED is greater than the set value, the SCR conducts at a certain angle, thereby shunting the circuit current and making the LED work in a constant current state, thereby preventing the LED from being damaged by instantaneous high voltage.

In the circuit of Figure 3, C1, R1, varistor, L1, and R2 form the primary filter circuit of the power supply, which can filter out the input instantaneous high voltage. C2 and R2 form the step-down circuit, and C3, C4, L2, and the varistor form the filter circuit after rectification. This circuit uses a double filter circuit, which can effectively protect the LED from being damaged by instantaneous high voltage breakdown.

Figure 4 is the simplest capacitor step-down application circuit. In the circuit, two anti-parallel LEDs are used to rectify the stepped-down AC voltage. It can be widely used in night lights, button indicator lights, position indicator lights with low requirements, and other occasions.