Constant current LED lamp driver circuit with soft start and anti-shock

1. Circuit Working Principle

1. Soft start

The circuit is composed of Q2, R3, C3, etc. When the power is just turned on, C3 is equivalent to a short circuit. The power is connected to the ground through R3, the be junction of Q2, and C3. Q2 is saturated and turned on. Point A is pulled to a low potential. Q1 is cut off due to the lack of base current, and the LED does not emit light. After a period of charging, the voltage across C3 slowly increases, the base current of Q2 gradually decreases until it is cut off, the potential of point A will slowly increase, and Q1 will gradually turn on. The LED light will slowly emit light, completing the soft start process.

2. Anti-shock

The impact voltage comes from two aspects. First, when the power is just turned on, even if there is a soft start circuit, there is still a momentary impact at both ends of the LED. This phenomenon can be found by observing the flash of the LED. After connecting C5 at both ends of the LED, this impact can be effectively absorbed.

Another impact is when the circuit is working normally and then the power is restored immediately after a sudden power outage or the power is turned on immediately after being turned off. Since the voltage across C3 cannot change suddenly, the soft start circuit will fail. Therefore, a circuit composed of Q3, R1, R2, C1, etc. is added to provide a discharge path for C3. In this way, even if the power is cut off instantly, Q3 will be saturated and turned on to discharge the voltage on C3, thereby ensuring the normal operation of the circuit.

3. Constant current

The circuit is composed of Q1, TL431, R3 and R4. When the current flowing through the LED increases or decreases due to some reason, the voltage drop across the sampling resistor R4 also increases or decreases, and the input voltage of TL431 also changes accordingly, thereby controlling the potential of the base of Q1 and keeping the current flowing through the LED constant.

2. Component Selection

C1, C2, and C5 are all capacitors with a withstand voltage of more than 400V. The leakage current of C3 should be small. If conditions permit, a non-polar capacitor with a withstand voltage of more than 16V can be selected. The resistors are all 1w, and the resistance of R4 is selected between 125Ω and 140Ω. A larger resistance value and a smaller constant current will help extend the life of the LED, but the brightness will be reduced. The LED light string is made of 80 to 90 Φ5mm diffuse white light LEDs connected in series.