V-i Characteristics Of Scr 1m672e

EXPERIMENT NO 1 AIM: To Study the VI characteristics of an SCR. Objective:  To produce and analyse the VI characteristics of an SCR.  To determine holding, latching current and break over voltage of given SCR.

Equipment Used: Name of the Apparatus Scientech 2712 Power Electronics Resistance 470E, ¼ W Resistance 2k2, 2W(on board) SCR TYN 616 (on board) Potentiometer 5K (10 Turn) Voltmeter, Ammeter

Maker’s Name

Quantity

Scientech

1

-

1 1

-

1

-

2

-

1,1

Circuit Diagram and Theoretical Waveform:

Figure 1(a).

Figure 1(b)

Figure 1(c) Figure 1(d)

Theory: The elementary circuit diagram for obtaining the V-I characteristics of a thyristor is shown in Fig. 1(a). The anode and cathode are connected to main source through the load. The gate and cathode are connected to main source of +15V which provides positive gate current from gate and cathode. Fig. 1(c) just depicts the junctional configuration of a thyristor. Fig 1(b) shows the V-I characteristics of a thyristor. Here V AK=Va is the anode voltage across the thyristor terminals A, K and I a is the anode current. Typical SCR V-I characteristics is shown in Fig. 1(b), reveals that a thyristor has three basic modes of operation, namely: 1. Forward Blocking Mode (Off State); 2. Forward Conduction Mode (On State); 3. Reverse Blocking Mode; The three of these modes have been discussed in details below: Forward blocking mode: When anode is positive w.r.t the cathode, with gate circuit open, thyristor is said to be forward biased i.e. when A is positive w.r.t to K (as shown in Fig. 4.3). In this situation the junctions J1 and J3 are forward biased but not junction J2. In this mode, very negligible leakage current flows called the Forward Leakage Current, and the thyristor offers very high impedance. In case the forward voltage is increased, then the reverse biased J 2 will have avalanche breakdown at a voltage called Forward Break-over Voltage, VBO.

Forward Conduction Mode: In this mode, the thyristor conducts currents from anode to cathode with a very small voltage drop across it (around 1-2V depending upon the rating of the thyristor). A thyristor can be brought to this mode by either exceeding the forward break-over voltage or by applying Gate Pulse between gate and cathode. Again there is a minimum limit to the anode current which a thyristor must attain during the turn-on and turn-off process to maintain conduction when the gate signal has been removed, known as latching current and holding current respectively. Reverse Blocking Mode: When cathode is made positive w.r.t anode with gate open, thyristor is reverse biased. Junctions J 1 and J2 are seen to be reverse biased whereas junction J2 is forward biased. The device behaves as if two diodes are connected in series with reverse voltage applied across them. A small

leakage current of order of a few milliamperes (or a few microamperes depending on the SCR rating) flows. This is the reverse blocking mode of a thyristor.

Observation table: Sl. No.

IG1=………….µA Ia(IAK)

VAK

IG2=………….µA Ia(IAK)

VAK

IG3=………….µA Ia(IAK)

VAK

Procedure: 1. Connections are made as shown in the circuit diagram. 2. Set both Potentiometers to minimum position, and then switch on the kit. 3. The value of gate current IG is set to convenient value by adjusting the knob of Potentiometer 2 (P2) slowly(as the

4.

5.

6. 7. 8. 9.

potentiometer is sensitive, and giving very high value of IG, the SCR will be triggered for small VAK, which should be avoided). By varying the setting of the Potentiometer 1 (P1), we change the anode cathode voltage V AK, and observe Ia (=IAK) in the anodic circuit. At a certain point of positive anode voltage, current I a shows sudden rise in reading and voltage falls down to almost zero. This action marks the firing of the SCR. If it does not happen, repeat the procedure from step 3 with a higher value of IG. The values of Ia and corresponding VAK values are noted. We repeat steps 3-8 for two more, different but suitable values of IG. A single graph between VAK and Ia, for three different values of IG is to be plotted.

Results and Discussion: The V-I characteristics of silicon controlled rectifier is plotted on the graph which is true according to theory.

Precautions: 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. Voltmeter & Ammeter should be accurate.

Viva Questions: 1. What are the various ways to Turn-On an SCR? 2. Define Holding current, Latching current on state resistance, Break down voltage? 3. Explain the working operation of SCR characteristics by using two transistor analogy? 4. Write an expression for anode current? 5. Mention the application of SCR? 6. Why SCR is called controlled Rectifier? 7. What do you mean by commutation process? 8. Define peak forward Voltage?