What is a Thyristor, SCR?
Thyristors or silicon controlled rectifiers (SCR) as they are sometimes known may appear to be unusual electronics components in many ways, but they are particularly useful for controlling power circuits. As such these electronics components are often used for applications such as light dimmers, and there may be thyristor circuits used in many power supply applications. Thyristors are simple to use and cheap to buy and often thyristor circuits are easy to build and use. All these reasons make thyristors ideal components to consider for many applications.
The idea for the thyristor is not new. The idea for the device was first put forward in 1950 by William Shockley, one of the inventors of the transistor. Although some later investigation of the device was undertaken by others a couple of years later, it was not until the early 1960s when they became available. After the introduction of the thyristor, they soon became popular for power supply circuits.
What is a thyristor?
The thyristor may be considered a rather an unusual form of electronics component because it consists of four layers of differently doped silicon rather than the three layers of the conventional bipolar transistors. Whereas conventional transistors may have a p-n-p or n-p-n structure with the electrodes named collector, base and emitter, the thyristor has a p-n-p-n structure with the outer layers with their electrodes referred to as the anode (n-type) and the cathode (p-type). The control terminal of the SCR is named the gate and it is connected to the p-type layer that adjoins the cathode layer.
Structure of a thyristor or silicon controlled rectifier (SCR)
Thyristors are usually manufactured from silicon, although, in theory other types of semiconductor could be used. The first reason for using silicon for thyistors is that silicon is the ideal choice because of its overall properties. It is able to handle the voltage and currents required for high power applications. Additionally it has good thermal properties. The second major reason is that silicon technology is well established and it is widely used for a variety of semiconductor electronics components. As a result it is very cheap and easy for semiconductor manufacturers to use.
How does a thyristor work?
The way in which a thyristor operates is different to other devices. Normally no current flows across the device. However if a supply is connected across the device, and a small amount of current is injected into the gate, then the device will "fire" and conduct. It will remain in the conducting state until the supply is removed.
To see how the thyristor operates, it is worth looking at a thyristor equivalent circuit. For the sake of an explanation, the thyristor circuit can be considered as two back to back transistors. The first transistor with its emitter connected to the cathode of the thyristor is an n-p-n device, whereas a second transistor with its emitter connected to the anode of the thyristor, SCR is a p-n-p variety. The gate is connected to the base of the n-p-n transistor as shown below.
Thyristor equivalent circuit
When a voltage is applied across a thyristor no current flows because neither transistor is conducting. As a result there is no complete path across the device. If a small current is passed through the gate electrode, this will turn "on" the transistor TR2. When this occurs it will cause the collector of TR2 to fall towards the voltage on the emitter, i.e. the cathode of the whole device. When this occurs it will cause current to flow through the base of TR1 and turn this transistor "on". Again this will now try to pull the voltage on the collector of TR1 towards its emitter voltage. This will cause current to flow in the emitter of TR2, causing its "on" state to be maintained. In this way it only requires a small trigger pulse on the gate to turn the thyristor on. Once switched on, the thyristor can only be turned off by removing the supply voltage.
It can be seen that current will only flow in one direction through the thyristor. If a reverse voltage is applied, then no current will flow, even if some gate current is applied. In this way for thyristor circuits used for AC, operation only occurs over one half of the AC waveform. For the other half of the cycle the device remains inoperative and no current can flow.
Thyristor symbol
The thyristor symbol is easy to recognise. Like the circuit symbols for most electronic components, the symbols may vary slightly dependent upon who has generated them, but in general it is as shown below. The thyristor symbol effectively shows a diode rectifier symbol with a control gate.
Thyristor symbol used in circuit diagrams
Thyristor circuit
There are many thyristor circuits that are in common use. They can be sued in many applications from AC control as in the case of motor or light dimmers to other circuits including power supply crowbar circuits.
The circuit below shows a power supply crowbar circuit. It can be used to protect circuitry within the main equipment from the effects of the failure of the series regulator in a power supply. If the series regulator fails short circuit, then high voltages can be paled on the power rail inside the equipment and this could result in serious damage to the overall equipment.
Thyristor overvoltage crowbar circuit
The SCR over voltage crowbar or protection circuit is connected between the output of the power supply and ground. The zener diode voltage is chosen to be slightly above that of the output rail. Typically a 5 volt rail may run with a 6.2 volt zener diode. When the zener diode voltage is reached, current will flow through the zener and trigger the silicon controlled rectifier or thyristor. This will then provide a short circuit to ground, thereby protecting the circuitry that is being supplied form any damage.
