SCR Circuit Design

- details of how to design circuits using the SCR, silicon controlled rectifier, or thyristor including design concepts and equations

The silicon controlled rectifier or SCR is a very useful component and can be used to produce many different circuits. In view of its properties, the SCR is used in many power switching circuits as the device is able to handle large current levels.

As it is a component that is different to many other types, SCR circuit design is not as widely described.

However SCR circuit design is not difficult and provided there is an understanding of the basic mode of operation of the SCR, it follows the same basic circuit design concepts that can be used with any other form of component.

SCR basics

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

The silicon controlled rectifier 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 SCR.

SCR Circuit Symbols
SCR Circuit Symbol

Note on the SCR, Silicon Controller Rectifier:

The Silicon Controller Rectifier, SCR is used in many power switching applications. Here is provides many advantages over mechanical systems that were previously available. The SCR is also used in many electronics orientated applications, where power levels may not be so high. However its switching action is still used whatever the power level needed

Click on the link for further information about the SCR, Silicon Controlled Rectifier

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.

SCR Equivalent Circuit
SCR Equivalent Circuit

As can be gathered, the 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 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.

SCR gate circuit design

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

SCR gate circuit
Gate resistors in SCR circuit

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.

By Ian Poole

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