Thyristor Circuit Design: SCR circuits

Thyristor or SCR circuits are used for many power control uses from lighting control to AC power motors, and other switching applications.

Thyristor Circuit Design Includes:
Thyristor circuit design primer     Circuit operation     Triggering / firing circuit design     Overvoltage crowbar     Triac circuits    

The thyristor or silicon controlled rectifier, SCR is a particularly useful component and it finds many uses in areas like power control where these components can be used to switch high voltages and currents. Thyristors have taken over most of the power switching applications that were once handled by relays, although very high voltage contactors are still used.

Thyristor or silicon controlled rectifier, SCR design can be undertaken in a straightforward manner. The devices, although a little unusual follow the same basic circuit design rules that govern other components as well.

The main issue is to make sure that all components are adequately rated, as often thyristor circuits are used in high power applications.

Thyristor, SCR circuit basics

The thyristor or silicon controlled rectifier operates in a different way to that of a standard bipolar transistor or FET.

The thyristor has two electrodes that are connected to the main circuit to be controlled. These two electrodes are called the anode and cathode.

A third electrode called the gate is used to control the thyristor within the circuit.

Thyristor or SCR circuit symbol
Thyristor or SCR circuit symbol

Note on Thyristor Technology:

Thyristors or SCRs are based around a unique structure of a PNPN structure and have three electrodes: anode, cathode and the gate. When the gate receives a triggering current, it fires the thyristor enabling current to flow until the voltage between anode and cathode is removed. This enables the thyristor to switch high voltages and currents, although it only over one half of the cycle. Two can be used to cover both halves of the cycle.

Read more about Thyristor Technology

To understand how the SCR operates within a circuit, it is best to look at its equivalent circuit. From this it can be seen that the SCR can be considered to consist of two interconnected transistors.

Under initial conditions there is no conduction between the anode and cathode. However, if current is applied to the gate in a sense that makes TR2 conduct, the SCR will turn on, but in one direction only. This conduction will be maintained even if the gate current is removed. In this way the gate current can be considered as a trigger impulse.

In order to stop conduction, the voltage between anode and cathode needs to be reduced to below the drop out level. This occurs when one or both of the transistors reach their cut-off mode. At this point conduction of the whole device will stop and the gate will need to be re-triggered.

Equivalent circuit of a thyristor / SCR
Equivalent circuit of a thyristor

As can be gathered, the thyristor, SCR only conducts in one direction. When used with an AC signal it needs to be re-triggered for each conduction half cycle.

Once the thyristor, SCR is in its fully conducting state, the voltage drop across the device is generally around 1 V for all values of anode current up to its rated value.

The SCR then continues to conduct while the anode current remain above the holding current for the device which is normally denoted as IH. Below this value the SCR stops conducting. Therefore in DC and some highly inductive AC circuits there has to be a means of turning the device off as the SCR will continue conducting.

Thyristor gate circuit design

In order to prevent overloading the gate and also false triggering, some resistors are often placed in the gate circuit.

Thyristor showing gate resistors to prevent false triggering
Thyristor circuit showing additional gate resistors

When designing an SCR circuit, two gate resistors are often included.

In the diagram R1 is included to limit the gate current to an acceptable level. This resistor is chosen to provide sufficient current to trigger the SCR while not providing so much that the gate junction is placed under stress.

The second resistor, R2 is the gate cathode resistor, sometimes denoted as RGK included to prevent spurious triggering. It effectively reduces the sensitivity of the gate.

Sometimes this resistor may be included within the SCR package itself and no external resistor may be required. It is necessary to check the manufacturers datasheet to determine what is needed.

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