Simple and practical LED lamp driving circuit

LED lighting is becoming increasingly popular due to its low power consumption and long life. As prices continue to fall, LEDs have begun to be used in home lighting. Recently, I have come across several commercially available 36-pin and 60-pin LED lamps that consume only about 3W of power and are as bright as a 10W incandescent lamp, which is very energy-saving.

However, after using it for a period of time, it was found that the light decay was quite serious. After disassembling and analyzing, the circuit is shown in Figure 1, which is very simple and uses a capacitor step-down circuit. Although the capacitor step-down is a constant current power supply, the capacitor is equivalent to a short circuit at the moment of power on, and the current changes greatly. When the grid voltage changes greatly, the current changes greatly.

The measured current is as follows: 13mA at 180V, 17mA at 220V, and 19mA at 250V. It is the wide range of current changes that causes the light decay of the LED. In order to find a solution, the author consulted some LED drive circuits, but professional LED drive integrated circuits are very expensive, and most of them are low-voltage DC/DC power supply methods. For this reason, the author designed a simple constant current source LED drive circuit, which has a good effect after electronic circuit simulation and actual circuit experiments. The circuit is shown in Figure 2.

C1, R1, D1, C2 form an AC/DC conversion circuit, where C=1/2πf U=(0.0000145)I, the unit is Farad. If I=10mA, then C1=0.15μF, and the actual C1 is 0.47uF. The Urm of the D1 rectifier bridge is ≥400v, or 4 1N4004 or IN4007 rectifier diodes are selected to form a rectifier bridge: the filter capacitor C2 is an electrolytic capacitor of 2.2uF~4.7uF and the withstand voltage is 400V. The constant current source circuit is composed of R2, U1, R3, R5, and Q1. Q1 is MJE13001 with a withstand voltage UceBR≥40DV. R5 is the current sampling resistor and also the current negative feedback resistor of Q1. By adjusting the value of R5, the operating current of the LED can be changed, thereby changing the brightness of the LED. Io=Uref/R5=2.5/R5. When R5=240Ω, ID=10mA.

The working principle is as follows. If the input voltage increases, the output current Io increases, and at the same time, the voltage Ur5 on R5 causes the voltage of TL431 pin ① to increase, so that the anode current of TL431 increases, Uk decreases, and the base voltage of transistor Q1 decreases. The decrease in Ube causes Ic to decrease, thereby keeping the current flowing through the light-emitting diode unchanged and achieving the purpose of constant current.

After the above circuit design is completed, Multisim 10 software is used for simulation. When the input voltage changes between 180v and 250v, the current remains unchanged at 10mA. The result is shown in Figure 2. After the simulation, the circuit is installed on the universal board for actual debugging. The result is similar to the simulation. Through continuous working experiments, the effect is good. The circuit is simple, low-cost and cost-effective.