Construction and Operation of SCR

Thyristors are a broad range of semiconductor components used for electronically controlled switches.

They are semiconductor devices with a bistable action that depends on a PNPN regenerative feedback to stay turned on or off. Bistable action refers to locking onto one of two stable states. Regenerative feedback is a method of obtaining an increased output by feeding part of the output back to the input.

Thyristors are widely used for applications where DC and AC power must be controlled. They are used to apply power to a load or remove power from a load.

In addition, they can also be used to regulate power or adjust the amount of power applied to a load—for example, a dimmer control for a light or a motor speed control.

Construction and Operation of SCR

Silicon-controlled rectifiers are the best known of the thyristors and are generally referred to as SCRs. They have three terminals (anode, cathode, and gate) and are used primarily as switches.

An SCR is basically a rectifier because it controls current in only one direction. The advantage of the SCR over a power transistor is that it can control a large current, dependent on an external circuit, with a small trigger signal.

An SCR requires a current flowing to stay turned on after the gate signal is removed. If the current flow drops to zero, the SCR shuts off and a gate signal must be reapplied to turn the SCR back on.

A power transistor would require ten times the trigger signal of an SCR to control the same amount of current.

Construction of SCR

An SCR is a solid-state device consisting of four alternately doped semiconductor layers. It is made from silicon by the diffusion or diffusion-alloy method.

Figure 1 shows a simplified diagram of an SCR. The four layers are sandwiched together to form three junctions. Leads are attached to only three of the layers to form anode, cathode, and gate.

construction and operation of scr
Fig. 1. Simplified SCR.
construction of scr
Fig. 2. Equivalent SCR.

Figure 2 shows the four layers divided into two three-layer devices. They are a PNP and an NPN transistor interconnected to form a regenerative feedback pair. Figure 3 shows the schematic symbols for these transistors. The figure shows that the anode is positive with respect to the cathode, and the gate is open.

Schematic representation of an equivalent SCR.
Fig. 3. Schematic representation of an equivalent SCR.

Operation of SCR

The NPN transistor does not conduct because its emitter junction is not subject to a forward-bias voltage (provided by the PNP transistor’s collector or gate signal). Because the NPN transistor’s collector is not conducting, the PNP transistor is not conducting (the NPN transistor’s collector provides the base drive for the PNP transistor). The circuit does not allow current to flow from the cathode to the anode under these conditions.

If the gate is made positive with respect to the cathode, the emitter junction of the NPN transistor becomes forward biased and the NPN transistor conducts. This causes base current to flow through the PNP transistor, which in turn allows the PNP transistor to conduct.

The collector current flowing through the PNP transistor causes the base current to flow through the NPN transistor. The two transistors hold each other in the conducting state, allowing current to flow continuously from the cathode to the anode.

This action takes place even though the gate voltage is applied only momentarily. The momentary gate voltage causes the circuit to switch to the conducting state and the circuit to continue conducting even though the gate voltage is removed. The anode current is limited only by the external circuit.

To switch the SCR off, it is necessary to reduce the anode-to-cathode voltage below the hold-on value, typically close to zero. This causes both transistors to turn off and remain off until a gate voltage is again applied.

The SCR is turned on by a positive input gate voltage and turned off by reducing the anode-to-cathode voltage to zero.

When the SCR is turned on and is conducting a high cathode-to-anode current, it is conducting in the forward direction. If the polarity of the cathode-to-anode bias voltage is reversed, only a small leakage current flows in the reverse direction.

Fig. 4

Figure 4(a) shows the schematic symbol for an SCR. This is a diode symbol with a gate lead attached. The leads are typically identified by the letters K (cathode), A (anode), and G (gate).

A properly biased SCR is shown in Figure 5. The switch is used to apply and remove gate voltage. The resistor RG is used to limit current to the specified gate current. The anode-to-cathode voltage is provided by the AC voltage source.

testing scr with ohmmeter
Fig. 5. Properly biased SCR.

The series resistor (RL) is used to limit the anode-to-cathode current to the specified gate current when the device is turned on. Without resistor RL the SCR would conduct an anode-to-cathode current high enough to damage the SCR.

SCRs are used primarily to control the application of DC and AC power to various types of loads. They can be used as switches to open or close circuits. They can also be used to vary the amount of power applied to a load.

In using an SCR, a small gate current can control a large load current. When an SCR is used in a DC circuit, there is no inexpensive method of turning off the SCR without removing power from the load. This problem can be solved by connecting a switch across the SCR (Figure 6).

Fig. 6. Removing power in a DC circuit.

When switch S2 is closed, it shorts out the SCR. This reduces the anode-to-cathode voltage to zero, reducing the forward current to below the holding value, and turning off the SCR.

When an SCR is used in an AC circuit it is capable of conducting only one of the alternations of each AC input cycle, the alternation that makes the anode positive with respect to the cathode.

When the gate current is applied continuously, the SCR conducts continuously. When the gate current is removed, the SCR turns off within one-half of the AC signal and remains off until the gate current is reapplied.

It should be noted that this means only half of the available power is applied to the load. It is possible to use the SCR to control current during both alternations of each cycle.

This is accomplished by rectifying the AC signal so that both alternations of each cycle are made to flow in the same direction before being applied to the SCR.

Fig. 7

Figure 7 shows a simple variable half-wave circuit. The circuit provides a phase shift from 0 to 90 electrical degrees of the anode voltage signal. Diode D1 blocks the reverse gate voltage on the negative half-cycle of the anode supply voltage.

Testing SCR with Ohmmeter

  1. Determine the polarity of the ohmmeter leads. The red lead is positive and the black lead is negative.
  2. Connect the ohmmeter leads, positive to the cathode and negative to the anode. The resistance should exceed 1 meg-ohm.
  3. Reverse the leads, negative to the cathode and positive to the anode. The resistance should again exceed 1 meg-ohm.
  4. With the ohmmeter leads connected as in step 3, short the gate to the anode (touch the gate lead to the anode lead). The resistance should drop to less than 1 meg-ohm.
  5. Remove the short between the gate and the anode. If a low-resistance range of the ohmmeter is used, the resistance should stay low. If a high-resistance range is used, the resistance should return to above 1 meg-ohm. In the higher resistance ranges, the ohmmeter does not supply enough current to keep the gate latched (turned on) when the short is removed.
  6. Remove the ohmmeter leads from the SCR and repeat the test. Because some ohmmeters do not give significant results on step 5, step 4 is sufficient.

Summary

  • An SCR controls current in one direction by a positive gate signal.
  • An SCR is turned off by reducing the anode-to-cathode voltage to zero.
  • SCRs can be used to control current in both AC and DC circuits.
  • SCRs can handle up to 1400 A compared to 25 A for TRIACs.
  • SCRs have voltage ratings up to 2600 V compared to 500 V for TRIACs.
  • SCRs can handle frequencies of up to 30,000 hertz compared to 400 hertz for TRIACs.

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