What are Thyristors?

A thyristor is one of the several controllable semi conductor devices that can act either like a switch, rectifier or as a voltage regulator.
The thyristor is a solid-state analogue of the thyratron vacuum tube. The name ‘thyristor’ is a combination of two words – the thyratron and the transistor. A thyristor functions a little like a transistor. It consists of three electrodes: the gate, the anode and the cathode. The gate acts as the controlling electrode. When a small current flows into the gate, it allows a larger current to flow from the anode to the cathode. A thyristor can be switched from a blocking state (high voltage, low current) to a conducting state (low voltage, high current) by a suitable gate pulse. Forward conduction is blocked until an external positive pulse is applied to the gate terminal. A thyristor cannot be turned off from the gate.

It is common to have two or more thyristor assembled into a thyristor module. The base of this type of pack is not electrically active, so it can be mounted directly onto the heat-sink of a converter. Large thyristor units are usually of the disc type for better cooling.

Thyristor – The Basics

A thyristor is a four-layer semiconductor device, consisting of alternating P type and N type materials (PNPN). The four layers act as bistable switches. As long as the voltage across the device has not reversed (that is, they are forward biased), thyristors continue to conduct electric current.
The most common type of thyristor is the silicon-controlled rectifier (SCR). When the cathode is negatively charged relative to the anode, no current flows until a pulse is applied to the gate. Then the SCR begins to conduct, and continues to conduct until the voltage between the cathode and anode is reversed or reduced below a certain
threshold value. Using this type of thyristor, large amounts of power can be switched or controlled using a small triggering current or voltage.

Uses of Thyristors

The most common use of thyristors is in AC circuits. In an AC circuit the forward current drops to zero during every cycle so there will always be a turn off function. This does, however, mean that the gate needs to be triggered every cycle just to turn it back on again. It is in the relative timing of these two functions that the thyristor has it’s most important role, i.e. Power Control.
Thyristors are also used in motor speed controls, light dimmers, pressure-control systems, and liquid-level regulators.

Today, thyristors are manufactured and sold as modules right upto 570 Amps.Also discrete forms as Stud and disc types are available ,till 570 Amps modules dominate the industry.