What is Silicon Controlled Rectifier (SCR) – it’s Features, Working & Application

SCRs or silicon controlled rectifiers are members of the thyristor family. Commonly known as a thyristor. 3-terminal 4-layer configuration solid state control unidirectional switch. Made of silicon material that controls high power and converts (rectifies) high level AC power to DC power. Hence it is called a silicon controlled rectifier.

It has three terminals: anode (A), cathode (C), and gate (G). The anode and cathode are the main terminals used for power conduction and the gate terminal is the control terminal used to trigger or start the SCR.

Features of SCR

  • It has a high operating voltage.
  • It has a very high power rating.
  • It has higher power losses.
  • It can block current in forward bias.
  • It starts conduction when the gate pulse is provided.
  • It is larger in size.

Symbol of SCR

Construction of SCR

SCRs have four layers of outer semiconductor material. These four layers form three PN junctions named J1, J2, and J3. Layers can be either NPNP or PNPN. The anode and cathode terminals are placed on the last layer and the gate terminal is placed on the third layer. The outer layer is heavily doped and the inner two layers are lightly doped. The structure of SCR is shown below.

VI Characteristics of SCR

The VI characteristic curve of an SCR is obtained by varying the voltage across the SCR and noting the current through the SCR. The V-I characteristics of SCR is divided into three regions.

  • Forward blocking region
  • Reverse blocking region
  • Forward conduction region

Forward blocking region

When connected in forward bias, the SCR does not allow any current. This range is called forward bocking mode and the range is below the VBO point. Part of the VI characteristics is in the first quadrant. In this range the current is below the holding current.

Reverse blocking region

When connected in reverse polarity, the SCR conducts a small current, leakage current up to breakdown voltage (VBR). After this point the SCR fails and starts acting like a short circuit. The range below the breakdown point is called reverse blocking mode.

Forward conduction region

When the voltage reaches the VBO point, the SCR starts to flow current. Alternatively, the SCR can be placed in forward conduction mode by energizing the gate. This increases the anode current beyond the latch current. As shown in the figure Ig3 > Ig2 > Ig1, the higher the gate current, the faster the SCR will enter forward conduction mode. When the anode current exceeds the holding current, the SCR remains in forward conduction mode.

Working or Modes of Operation of SCR

SCR operation depends on battery polarity and gate input. SCR can operate in three different modes.

  • Forward Blocking Mode
  • Forward Conduction Mode
  • Reverse Blocking Mode

Forward Blocking Mode

If the anode of the SCR is connected to the positive terminal and the cathode of the SCR is connected to the negative terminal of the battery terminal. Also, no pulse is applied to the gate terminal. SCR operates in forward blocking mode. This means that even if the SCR polarity is forward biased, the SCR will not conduct.
In forward blocking mode, his PN junctions of J1 and J3 are forward biased. However, since the middle junction J2 is reverse biased, the SCR does not conduct in forward blocking mode.

Forward Conduction Mode

Forward conduction mode is the only SCR mode for conduction. The SCR can be put into forward conduction mode in two ways. First, provide a gate pulse to forward bias the J2 junction. Second, he destroys the J2 junction by increasing the voltage from anode to cathode. The gated pulse method is preferred and suitable for many applications. Decomposition methods shorten the life of the SCR.

The SCR remains in conduction mode even after removing the gate pulse or reducing the applied voltage. When the SCR anode current drops below the holding current, the SCR stops reverting to forward blocking mode.

Reverse Blocking Mode

When the SCR anode terminal is connected to the negative terminal and the SCR cathode terminal is connected to the positive terminal of the battery terminal. SCR is in reverse lock mode. Junctions J1 and J3 are reverse biased in this mode. When middle junction J2 is forward biased. Since the two junctions are reverse-biased, no current flows, only a small leakage current due to drift carriers.

S. NoAdvantages of SCRDisdvantages of SCR
1It can handle large voltages, currents, and powers.It can only lead in one direction. Therefore, the power can be controlled only during one half cycle of the AC power supply.
2The voltage drop across a conductive SCR is small.SCR cannot be used at high frequencies. The maximum operating frequency is 400 Hz.
3It can be protected with a fuse.Gate current cannot be negative.

Applications of SCR

  • AC Voltage stabilizers
  • Electronic Chopper circuit
  •  Inverters
  •  DC Circuit breaker
  • Control Motors speed
  • Adjust Light dimmer
  • Battery Charger circuits

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