Silicon-Controlled Rectifier (SCR) Characteristics

Aim: To draw the V-I Characteristics of Silicon controlled rectifier.

Apparatus:       

• SCR (TYN616)
• Regulated Power Supply (0-30V)
• Resistors 10kΩ, 1kΩ
• Ammeter (0-50)mA
• Voltmeter (0-20V)
• Breadboard
• Connecting Wires.

Circuit Diagram:

Theory:

It is a four layer semiconductor device being alternate of  P-type and N-type silicon. It consists os 3 junctions J1, J2, J3 the J1 and J3 operate in forward direction and J2 operates in reverse direction and three terminals called anode A, cathode K , and a gate G. The operation of SCR can be studied when the gate is open and when the gate is positive with respect to cathode.

When gate is open, no voltage is applied at the gate due to reverse bias of the junction J₂ no current flows through R₂ and hence SCR is at cutt off. When anode voltage is increased J₂ tends to breakdown.

When the gate positive,with respect to cathode J₃ junction is forward biased and J₂ is reverse biased .Electrons from N-type material move across junction J₃ towards gate while holes from P-type material moves across junction J₃ towards cathode. So gate current starts flowing ,anode current increaase is in extremely small current junction J₂ break down and SCR conducts heavily.

When gate is open thee breakover voltage is determined on the minimum forward voltage at which SCR conducts heavily.Now most of the supply voltage appears across the load resistance.The holfing current is the  maximum anode current gate being open , when break over occurs.

Procedure:

1. Connections are made as per circuit diagram.
2. Keep the gate supply voltage at some constant value
3. Vary the anode to cathode supply voltage and note down the readings of voltmeter and ammeter. Keep the gate voltage at standard value.
4. A graph is drawn between V_AK and I_AK.

Observation:

Model Wave form:

Result: SCR Characteristics are observed.

Viva questions:

1. What is an SCR?

Ans: Silicon-controlled rectifier (or semiconductor-controlled rectifier) is a four-layer solid state current controlling device. The name "silicon controlled rectifier" or SCR is General Electric's trade name for a type of thyristor

2. What is the difference between SCR and TRIAC?

Ans: SCRs are unidirectional devices (i.e. can conduct current only in one direction) as opposed to TRIACs which are bidirectional (i.e. current can flow through them in either direction). SCRs can be triggered normally only by currents going into the gate as opposed to TRIACs which can be triggered normally by either a positive or a negative current applied to its gate electrode.

3. What are the applications of SCR?

Ans: SCRs are mainly used in devices where the control of high power, possibly coupled with high voltage, is demanded. Their operation makes them suitable for use in medium to high-voltage AC power control applications, such as lamp dimming, regulators and motor control.

SCRs and similar devices are used for rectification of high power AC in high-voltage direct current power transmission. They are also used in the control of welding machines, mainly MTAW and GTAW processes.

4. Why is Peak Reverse Voltage Important?

Ans: When an SCR is used for rectification, during the negative half cycle of given ac supply, reverse voltage is applied across the SCR. If Peak Reverse Voltage is exceeded, there may be an avalanche breakdown and the SCR will be damaged (unless the external circuit limits the current).Commercial SCRs have a PRV up to 2.5kV.

5. What is asymmetrical SCR?

Ans: SCR incapable of blocking reverse voltage are known as asymmetrical SCR, abbreviated ASCR. They typically have a reverse breakdown rating in the 10's of volts. ASCR are used where either a reverse conducting diode is applied in parallel (for example, in voltage source inverters) or where reverse voltage would never occur (for example, in switching power supplies or DC traction choppers).

Asymmetrical SCR can be fabricated with a reverse conducting diode in the same package. These are known as RCT, for reverse conducting thyristor.

6.What is the equivalent circuit of SCR?

Ans:

7.What are the methods to trigger SCR.

Ans:1.Thermal triggering, 2.Radiation triggering, 3.Voltge triggering, 4.dV/dt triggering, 5.Gate triggering.