Study Characteristics of SCR

Aim

Study characteristics of SCR.

Apparatus Required:

Power Supply, SCR Kit. , Voltmeter, Ammeter, Connecting Leads.

Brief Theory:

Silicon control rectifier (SCR) is a four layer, three terminal semiconductor device, the end ‘P’ forms the anode & the end ‘N’ forms the cathode & the gate terminal ‘G’ is from the ‘P’ layer next to cathode. It is a unidirectional device. The device can exist upon either ON state or OFF state depending upon the applied voltage . When anode voltage is +ve w.r.t to cathode the SCR start Conducting. If some small gate voltage is applied, the SCR trigger at some low value of anode voltage, but it loses its all control on the SCR Current after triggering. Therefore, in order to turn the SCR to OFF position the anode voltage has to be reduced to zero.

Circuit Diagram:

Procedure:

1. Connect the circuit according to the fig.
2. First set I = 0mA , vary input voltage V gradually & measure the current I.
3. Tabulate the readings.
4. Repeat the procedure for different values of I .
5. Draw the graph between V & I .

Observation Table:

Graph:

Precaution:

1. Always connect the voltmeter in parallel & ammeter in series as shown in fig.
2. Connection should be proper & tight.
3. Switch ‘ON’ the supply after completing the circuit
4. DC supply should be increased slowly in steps
5. Reading of voltmeter & Ammeter should be accurate.

Result:The characteristics of thyristor have been plotted.

Discussion:The SCR will not conduct until it is not triggered by gate voltage.

Quiz:

Q1:Define SCR?

A1:It is a controlled rectifier constructed of a silicon material with third terminal for control purpose.

Q2:Define Forward Break over Voltage?

A2:It is voltage in forward conducting mode after which SCR starts conducting.

Q3:Define reverse Break down Voltage?

A3:If reverse voltage is increased beyond certain value, SCR will break & this voltage is Called reverse breakdown voltage.

Q4:Why it is called controlled rectifier?

A4:Because its operation as rectifier can be controlled by using gate terminal.

Q5:Define peak forward Voltage?

A5:It is limiting positive anode voltage above which SCR get damaged.

Q6:Define peak reverse Voltage?

A6:Max. reverse voltage that can be applied to SCR without conduction in reverse direction.

Q7:Define Holding Current?

A7:Min. forward current that must be maintained to keep SCR in conducting state.

Q8:Define forward current Rating?

A8:The max. Value of anode current that SCR con handle safely.

Q9:Define latching current.

A9:Min. device current which must be attained by the device before gate drive is removed.

Q10:What do you mean by commutation process?

A10:Process of turning off the SCR.