SCR Full Form – Silicon Controlled Rectifier

Silicon Controlled Rectifier

A four-layer solid-state current-controlling device known as a silicon controlled rectifier or semiconductor-controlled rectifier. A specific kind of thyristor is known by General Electric under the trade name “silicon controlled rectifier.” Similar to a diode, an SCR conducts when a gate pulse is applied to it. It has four layers of semiconductors that can be either arranged to form the NPNP or PNPN structures.

Silicon Control Rectifiers, or SCRs, are used in industry because they can handle high current and voltage values.

Three terminals

Anode – P-layer

Cathode – N-layer (opposite end)

Gate – P-layer near the cathode

Three junctions – four layers

Connect power such that the anode is positive with respect to the cathode – no current will flow

A semiconductor that performs the function of an actual electronic switch is called a silicon controlled rectifier. It has the ability to modify the alternating current while also managing the power supplied to the load. SCRs combine the advantages of transistors and rectifiers.

SCR Construction

When a pn junction is added to a junction transistor the resulting three pn junction device is called a SCR. ordinary rectifier (pn) and a junction transistor (npn) combined in one unit to form pnpn device.

Three terminals are taken : one from the outer p- type material called anode a second from the outer n- type material called cathode K and the third from the base of transistor called Gate. GSCR is a solid state equivalent of thyratron. The gate anode and cathode of SCR correspond to the grid plate and cathode of thyratron SCR is called thyristor.

SCR Working Principle

Load is connected in series with anode the anode is always kept at positive potential w.r.t cathode.

SCR Operation / Working

The Silicon Control Rectifier start conduction when it is forward biased. For this purpose the cathode is kept at negative and anode at positive. When positive clock pulse is applied at the gate the SCR turns ON.

And When forward bias voltage is applied to the Silicon Control Rectifier, the junction J1 and J3 become forward bias while the junction J2 become reverse bias.

When we apply a clock pulse at the gate terminal, the junction J= become forward bias and the Silicon Control Rectifier start conduction.The Silicon Control Rectifier turn ON and OFF very quickly, At the OFF state the Silicon Control Rectifier SCR provide infinity resistance and in ON state, it offers very low resistance, which is in the range of 0.01O to 1O.

SCR Firing & Triggering

Normal operation of the Silicon Control Rectifier is below the forward break over voltage (VBO). When the Silicon Control Rectifier is turned ON, if the triggering voltage is removed, the SCR will continue to be in the ON state. To turn ON the Silicon Control Rectifier, we apply a clock pulse at the gate terminal, which is referred to as triggering of Silicon Control Rectifier. Firing voltage is the name of this voltage.

When Gate is Open

No voltage applied to the gate, j2 is reverse biased while j1 and j3 are FB . J1 and J3 is just in npn transistor with base open .no current flows through the load RL and SCR is cut off. If the applied voltage is gradually increased a stage is reached when RB junction J2 breakdown .the SCR now conducts heavily and is said to be ON state. the applied voltage at which SCR conducts heavily without gate voltage is called Break over Voltage.

When Gate is Positive with respect to Cathode

Applying a small positive potential to the gate will cause the SCR to conduct heavily at lower applied voltages. When J3 is FB and J2 is RB, an electron from an n-type material crosses J3 and begins moving leftward while p-type material moves rightward. Gate current begins to flow as a result of electrons from junction J3 being drawn across the junction. Anode current rises as soon as gate current flows. As a result of the increased anode current, J2breakdown has access to more electrons, and the SCR begins to conduct heavily. If the gate voltage is turned off, the anode current does not decrease at all, and the gate loses all control. Conduction can only be stopped by lowering the applied voltage to zero.

Break over Voltage

It is the minimum forward voltage gate being open at which SCR starts conducting heavily i.e turned on.

Peak Reverse Voltage ( PRV)

It is the maximum reverse voltage applied to an SCR without conducting in the reverse direction.

Holding Current

It is the maximum anode current gate being open at which SCR is turned off from on conditions

VI Characteristics of SCR

Forward Characteristics 

When anode is +ve w.r.t cathode the curve between V &Iis called Forward characteristics. OABC is the forward characteristics of the SCR at Ig =0. if the supplied voltage is increased from zero point A is reached .SCR starts conducting voltage across SCR suddenly drops (dotted curve AB) most of supply voltage appears across RL

Reverse Characteristics

When anode is –ve w.r.t cathode the curve b/w V&I is known as reverse characteristics reverse voltage come across SCR when it is operated with ac supply reverse voltage is increased anode current remains small avalanche breakdown occurs and SCR starts conducting heavily is known as reverse breakdown voltage.

Application

1.  As a switch
2. Half and Full wave rectifier
3.  As a static contactor
4. For power control
5.  For speed control of d.c.shunt motor
6.  Over light detector