How Does a Thyristor Work?
The usual way an SCR is described is with an analogy to a pair of cross connected transistors – one is NPN and the other is PNP. The base of the NPN is connected to the collector of the PNP and the base of the PNP is connected to the collector of the NPN. If we connect the positive terminal of a supply to say, a light bulb, and then to the emitter of the PNP transistor and its return to the emitter of the NPN transistor, no current will flow as long as the breakdown voltage ratings of the transistor are not exceeded because there is no base current to either transistor. However, if we provide some current to the base of the NPN (IG(+).)ransistor, it will turn on and provide current to the base of the PNP transistor which will turn on providing more current to the NPN transistor. The entire structure is now in the solid on state and will stay that way even when the input to the NPN’s base is removed – until the power supply goes to zero and the load current goes below the hold value. Th
