SCR Definition

SCR is a four layer 3 terminal PNPN OR NPNP device it combine the features of transistor and a rectifier figure shows the construction circuit symbol of SCR.

it consists of two main terminals anode and cathode the third terminal gate is welded to the reason in between. It is widely used as switching device in power control applications it can switch on for variable length of time and delivers selected amount of power to load.

Construction of SCR

It consists of four layers and three terminals . The four layers made of P & N layers are arranged alternately such that they form 3 junctions J₁, J₂ and J₃ . This junctions are either alloyed or diffused based on the type of construction.

The outer layers (P and N layers) are heavily doped whereas middle P and N layers are lightly doped. The gate terminal is taken at the middle P-layer. Anode is taken from outer P-layer and cathode is taken from N-layer terminal. The SCR is made of silicone because compared to germanium leakage current is silicon is very small. The junction J₁ and J₃ become forward biased while J₂ is reverse biased.

Working of SCR

In a silicon controlled rectifier, the anode is always kept at positive potential with respect to cathode. The load is connected in series with anode. When no voltage is applied to the gate the junction J₁ and J₃ are forward biased whereas junction J₂ is reverse biased. Thus, the condition of junction J₁ and J₃ is similar to NPN transistor with base open.

Hence, no current flows through the load and SCR is cut off. When the applied voltage is increased slowly, A stage reaches when junction J₂ breaks-down. Due to break-down of j₂ SCR conducts heavily and is said to be in ON state. The voltage at which breakdown occurs is known as breakdown voltage.

If a small positive voltage is applied to the gate, SCR conducts heavily at smaller applied voltage. The junction J₃ becomes forward biased and J₂ is reverse biased. The electrons present in N-type material moves across junction J₃ to left side and holes in P-types material to right side. As a result, the electrons from junction j₃ are attracted across junction J₂ and gate current start flowing. With the flow of gate current, anode current increases. The rise in anode current makes more electron available at junction J₂ .

This process continues and junction J₂ breaks-down and SCR stars conducting heavily. When the SCR start conducting the gate loses its control. Anode current does not decrease even if gate voltage is removed.

V-I Characteristics of SCR

it is the graph between anode cathode voltage and anode current at constant gate current as shown in figure. It consists of two characteristics stages forward characteristics and reverse characteristics.

Forward Characteristics

when the anode is made positive with respect to cathode the graph plotted between V and I is known as forward characteristics. Forward characteristics of SCR for different values of gate current are shown in figure. If supply voltage is increased from zero value, a stage comes when SCR start conducting.

At this point most of supply voltage appear across the load and voltage across the SCR suddenly drops. If gate signal is applied the SCR start conducting at lower value of voltage. If now the gate signal is removed, the SCR remains ON. To turn the SCR OFF, the anode current will have to be reduced to certain minimum value known as holding current .

Reverse Characteristics

The curve between V and I, when anode is made negative with respect to cathode is known as reverse characteristics. When the reverse voltage is gradually increased, the anode current is small. After sometimes, a stage is reached where avalanche breakdown occurs and the stars conducting heavily in the reverse direction. The maximum value of reverse voltage at which SCR starts conducting heavily is called reverse breakdown voltage.

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