STUDY OF V-I CHARACTERISTICS OF A SCR

EXP.NO: 01

DATE:

AIM:

To study the V-I characteristics of S.C.R. and determine the Break over Voltage, Holding current & Latching current APPARATUS REQUIRED:

Sl.No

1

23

4

56

NAME OF THE COMPONENTS SCR - TYN604

Power Supplies

Wattage Resistors Ammeter

Voltmeter Bread Board & Connecting Wires

RANGE

-

(0-30)V 100ohm

(0-30)mA

(0-30)V -

QUANTITY

1

21

1

1As required

PRECAUTION: 1. Initially keep all the switches in the OFF position 2. Initially keep duty cycle POT in minimum position THEORY:

Silicon Controlled Rectifier (SCR): Thyristor (generally known as SCR) is a four layer, three junction, pnpn semiconductor switching device. It has three terminals; anode, cathode and gate. Basically, a thyristor consists of four layers of alternate p-type and n-type silicon semiconductors forming three junctions J1,J2 and J3. A gate terminal is usually kept near the cathode terminal. The terminal

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CIRCUIT DIAGRAM: CHARACTERISTICS OF SCR

BASE DIAGRAM OF TY604

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connected to outer p region is called anode (A), the terminated connected to outer n region is called cathode and that connected to inner p region is called the gate (G). The symbol of SCR is shown in figure.

A circuit diagram for obtaining static V-I characteristics of a thyristor is shown in Fig . The anode and cathode are connected to main source through the load. The gate and cathode are fed from a source Vs which gives positive gate current from gate to cathode. Fig , shows static V-I characteristics of a thyristor. Va is the anode voltage across thyristor terminals A, K and Ia is the anode current. Fig reveals that a thyristor has three basic modes of operation; namely, reverse blocking mode, forward blocking (off-state) mode and forward conduction (on-state) mode. These three modes of operation are now discussed below. (A). When cathode is made positive with respect to anode with gate open, thyristor is reverse biased. Junctions J1, J3 are reverse biased whereas junction J2 is forward biased.

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OBSERVATION TABLE

Anode to cathode voltage(Vak) Anode current (Ia) Sl.No. (Volt) (Ampere)

Static V-I Characteristics of SCR For Ig2 = ----- = Constant For Ig1 = ----- = Constant

MODEL CALCULATION

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The device behaves like two diodes connected in series with reverse voltage appearing across them. A small leakage current of the order of a few mill amperes or few microamperes flows depending upon the SCR rating.. This is reverse blocking mode, called the off state of the SCR. If the reverse voltage is increase, then at a critical breakdown level, called reverse breakdown voltage VBR, an avalanche occurs at J1 and J3 and the reverse current increases rapidly. A large current associated with VBB gives rise to more losses in the thyristor. This may lead to thyristor damage as the junction temperature may exceed its permissible temperature rise. It should, therefore, be ensured that maximum working reverse voltage across a SCR does not exceed VBR.

(b). forward blocking mode: When anode is positive with respect to the cathode with gate circuit open, SCR is said to be forward biased. During this mode, junctions J1, J3 are forward biased but junction J2 is reverse biased. In this mode, a small current, called forward leakage current, flows.

In case the forward voltage is increased, ten the reverse biased junction J2 will have an avalanche breakdown at a voltage called forward break over voltage VBO. When forward voltage is less than VBO, thyristor offers high impedance. Therefore, a SCR can be treated as an open switch even in the forward blocking mode.

(c). Forward conduction mode: In this mode, SCR conducts currents from anode to cathode with a very small voltage drop across it. A SCR is brought from forward blocking mode to forward conduction mode by turning it on by exceeding the forward break over voltage or by applying a gate pulse between gate and cathode. In this mode, SCR is on state and behaves like a closed switch. Voltage drop across thyristor in the on state is of the order of 1 to 2V depending on the rating of thyristor. This voltage drop increases slightly with an increase in anode current.

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In conduction mode, anode current is limited by load impedance alone as voltage drop across thyristor is quite small. This small voltage drop VT across the device is due ohmic drop in the four layers.

SPECIFIC TERMINOLOGY

Break over Current (IBO) Principal current at the break over point Break over Voltage (VBO) Principal voltage at the break over point Gate Trigger Current (IGT) Minimum gate current required to maintain the SCR in the on state Holding Current (IH) Minimum principal current required to maintain the SCR in the on state Latching Current (IL) Minimum principal current required to maintain the SCR in the on state immediately after the switching from off state to on state has occurred and the triggering signal has been removed On-state Voltage (VT) Principal voltage when the SCR is in the on state Gate Trigger Voltage (VGT) Gate voltage required to produce the gate trigger current On-state Current (IT) Principal current when the SCR is in the on state

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PROCEDURE:

Static V-I Characteristics of SCR

1. Connections as made as per the circuit diagram 2. Connect millimeter across G-K, across the thyristor (anode and cathode), Across the supply terminals Vs to measure gate voltage Vg, Va and Vs. (All in dc mode). An ammeter of the range (0-50) mA is connected to Measure the load current Il.

3. Keep initially the gate potential Vg at very low value say around 0.4 Volts. Vary the supply voltage Vs in steps and note whether ammeter shows any Reading. For every step of Vs note the ammeter reading. Also note corresponding readings of Va respectively. 4. If the ammeter doesn't indicate any reading, increase the gate potential Vg to Some higher value says around 0.6 Volts & follows the procedure given in step No. (3). 5. Further increase the gate potential to some higher values and repeat the Procedure followed in step no. (3). 6. Tabulate the readings in the observation column. 7. Finally a graph is drawn between anode current (Ia = Il = load current) And the device voltage Va respectively.

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MODEL GRAPH

RESULT:

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