Harold Hall
Whilst a thyristor is no doubt considered to be an electronic device they do not fit easily into the normal understanding of electronics as they can be used up to hundreds of Amps and at main's potentials. It is here that they find one of their main uses, that is speed control of DC motors. By delaying or advancing the point at which the thyristor conducts the speed of the motor can be controlled. For this to work a diode is also connected across the motor so that current continues to flow, due to the motors back EMF, during the time that the thyristor is switched off. This is a normal diode, power rated to suit of course, but in this application is called a fly wheel diode Sk 10.
Metalworking
Workshop Data
The gate pulses have to be accurately timed and because of this the firing circuits have to be synchronised to the supply waveform and then advanced and retarded as required.
Thyristors are also commonly known as SCR's, silicon controlled rectifiers.
Thyristors
These are yet another form of diode and like others, conduct in one direction only. Their difference being the time at which they start to conduct is controlled using a third connection, called a gate. Once conducting though, the diode cannot be turned off by this means. This only happening when the current through the device ceases to flow for some other reason.
As thyristors are mostly, though not always, used in an AC circuit, current will typically cease when it passes zero as the supply polarity reverses. Current will not then flow again until it is instructed to by the signal to the gate, usually every cycle of the AC waveform.
The essential characteristic of this device is that the trigger pulse can be arranged at the start of a positive going cycle when it will act much like a normal diode or delayed to any point later in the cycle when it will conduct for only a portion, see Sk 9.
Note how on the left it conducts for almost a full half cycle but on the right it is only a fragment of the half cycle. See my pages on electric motor theory and control.