Example of LED AC drive circuit

As shown in the figure, the modulation signal in Figure (a) is input to the base of VT through C. The capacitor C determines the lower limit of the modulation signal frequency, and the bandwidth is determined by the response time of the LED and the response time of VT. In general, the two are made to correspond, the typical response time is between 50ns and 200ns, and the bandwidth is (5~50)MHz. In the figure, the selection of the variable resistor R2 must ensure that the LED has a certain forward current when there is no signal input, and when the signal input can give the maximum current amplitude. As for R3, it plays the same current limiting role as R in the DC drive, especially when V1 is saturated and turned on, the forward current flowing through the LED will not exceed the maximum allowed value. The circuit shown in Figure (b) is a pulse light generator in the car ignition device, which uses the light emitted by the LED to control a light-controlled thyristor. VT1 and VT2 are turned on and off in turn. When VT1 is turned on, the LED emits light, and the light-emitting time is 0.7Rb1C2; the cut-off time is 0.7Rb1C1. The 15Ω resistor R in series with the LED plays a current limiting protection role.