How Silicon Controlled Rectifiers (SCRs) are used in Industrial Control

Brief Background on Industrial Electronics

Silicon Controlled Rectifiers (SCRs), Triacs and other high –powered transistors are used in many types of circuits to control large voltages and currents. Many of these use 480 VAC 3 Phase circuits and can control over 50 A. These devices offer control circuits for general purpose power supplies, AC and DC Variable speed motor drives, Servo motor controls, Stepper motor controls, high frequency power supplies, welding power supplies etc. SCRs, Triacs, and any other solid-state devices used for switching larger voltages and currents on and off are commonly called thyristors. Thyristors control switching in an on-off manner similar to a light switch which is different from a transistor that can vary the amount in its emitter-collector circuit by changing the bias on its base. The amount of current that flows through a thyristor must be controlled by adjusting the point in a sine wave where the device is turned on.

Silicon Controlled Rectifiers (SCRs)

The figure 1 below shows a symbol for the SCR and identifies the anode, cathode, and gate terminals. The cathode is identified by the letter C or K. The diagram also shows several types of SCRs.

Figure 1

How Silicon Controlled Rectifiers Work

The SCR acts like a solid-state switch in that the current will pass through its anode-cathode circuit to a load if a signal is received at its gate. The SCR is different from a traditional switch in that the SCR will change ac voltage to dc voltage (rectify) if ac voltage is used as the power supply. The SCR is also different from a traditional switch in that the amount of time the SCR conducts can be varied so that the amount of current provided to the load will be varied from near zero to maximum of the power supply.

Figure 2

The SCR can vary the amount of current that is allowed to flow to the resistive load by varying the point in the positive half-cycle where the gate signal is applied. If the SCR is turned on immediately, it will conduct full voltage and current for the half-cycle (180°). If the turn-on point is delayed to the 90° point in the half-cycle waveform, the SCR will conduct approximately half of the voltage and current to the load. If the turn-on point is delayed to the 175° point in the half cycle, the SCR will conduct less than 10% of the power supply voltage and current to the load, since the half-cycle will automatically turn off the SCR at the 180° point. This implies that the gate of the SCR can be used to control the amount of voltage and current the SCR will conduct from zero to maximum.

You can also read: How Test Diodes are used to Measure loop Currents 

The Operation of SCR explained by the four-layer (Two-Transistor) Model

The SCR is a four-layer thyristor made of PNPN material; in fact the proper name for the SCR is the reverse blocking triode thyristor.

Figure 3

The Figure 3 above shows the PNPN material split apart as two transistors, a PNP and a NPN. The figure (c) shows the SCR as two transistors.

The anode is at the emitter of the PNP Transistor (T2), and the cathode is at the emitter of the NPN transistor (T1). The gate is connected to the base of the NPN Transistor. Since the anode is the emitter of the PNP, it must have a positive voltage to operate and since the cathode is the emitter of the NPN transistor, it must be negative to operate.

When a positive pulse is applied to the gate, it will cause collector current Ic to flow through the NPN transistor (T1). This current will provide bias voltage to the base of the PNP transistor (T2). When the bias voltage is applied to the base of the PNP transistor, it will begin to conduct Ic which will replace the bias voltage on the base that the gate signal originally supplied. This allows the gate signal to be a pulse which is then removed since the current through SCR anode to cathode will flow and replace the base bias on transistor T1.

Methods of turning on an SCR

Normally the SCR is turned on by a pulse to its gate but we have 3 other methods you can also use to turn it on.

These methods include:

• Exceeding the forward break over voltage
• By Excessive heat that allows leakage current
• Exceeding the dv/dt level (allowable voltage change per time change) across the junction.

Methods of turning off/Commutating SCRs

Once an SCR is turned on, it will continue to conduct until it is turned off (commuted). Commutation will occur in an SCR only if the overall current gain drops below unity (1). This means that the current in the anode-cathode circuit must drop below the minimum (near zero) or a current of reverse polarity must be applied to the anode-cathode. Since the ac sine wave provides both of these conditions near the 180° point in the wave, the main method to commutate an SCR is to use ac voltage as the supply voltage. In an ac circuit, the voltage will drop to zero and across over to the reverse direction at the 180° point during each sine wave. This means that if the supply voltage is 60 Hz, this will happen every 16 msec. Each time the SCR is commutated, it can be triggered at a different point along the firing angle, which will provide the ability of the SCR to control the ac power between 0° to 180°. The main drawback with using ac voltage to commutate the SCR arises when higher-frequency voltages are used as the supply voltage. Note that, the SCR requires approximately 3-4 msec. to turn off; therefore the maximum frequency is dependent on the turn-off time.

Figure 4

Figure 4 (a) A switch is used to commutate the SCR in a dc circuit by interrupting current flow.  This type of circuit is used to provide control in alarms or emergency dc voltage lighting circuits, (b)  A series of RL resonant circuit circuit used to commutate an SCR and (c) A parallel RL resonant circuit used to commutate an SCR. 

SCRs are also used in the inverter section where the dc voltage is turned back into ac voltage. Since the devices must provide both the positive and the negative half-cycles, a diode is connected in inverse parallel to provide the hybrid ac switch. This combination of devices is not frequently used currently as larger Triacs and Power transistors can do better job in this kind of applications.

Don't miss out on key updates, join our newsletter list here.