To draw V-I characteristics of TRIAC for different values of Gate Currents

Aim

To draw V-I characteristics of TRIAC for different values of Gate Currents.

Apparatus Required:

Theory:

Typical V-I characteristics of a triac are shown in figure. The triac has on and off state characteristics similar to SCR but now the characteristic is applicable to both positive and negative voltages. This is expected because triac consists of two SCRs connected in paral¬lel but opposite in directions.

MT2 is positive with respect to MTX in the first quadrant and it is negative in the third quad rant. As already said in previous blog posts, the gate triggering may occur in any of the following four modes.

Quadrant I operation: VMT2, positive; VG1 positive

Quadrant II operation: VMT21 positive; VGl negative

Quadrant III operation : VMT21 negative; VGl negative

Quadrant IV operation : VMT21 negative; VG1 positive

where VMT21 and VGl are the voltages of terminal MT2 and gate with respect to terminal MT1.

The device, when starts conduction permits a very heavy amount of current to flow through it. This large inrush of current must be restricted by employing external resist ance, otherwise the device may get damaged.

The gate is the control terminal of the device. By applying proper signal to the gate, the firing angle of the device can be controlled. The circuits used in the gate for triggering the device are called the gate-triggering circuits. The gate-triggering circuits for the triac are almost same like those used for SCRs. These triggering circuits usually generate trigger pulses for firing the device. The trigger pulse should be of sufficient magnitude and dura¬tion so that firing of the device is assured. Usually, a duration of 35 us is sufficient for sus¬taining the firing of the device.

A typical triac has the following voltage/current values:

1. Instantaneous on-state voltage - 1.5 Volts
2. On-state current - 25 Amperes
3. Holding current, IH - 75 Milli Amperes
4. Average triggering current, IG - 5 Milli Amperes

Circuit Diagram:

Volt-ampere characteristic of a diac is shown in figure. It resembles the English letter Z because of the symmetrical switching characteristics for either polarity of the applied volt¬age.

Circuit Diagram:

Procedure:

1. Connections are made as shown in the circuit diagram
2. Adjust the value of to zero or some minimum value
3. By varying the voltage V _mt2mt1from 0 to 10 volts with a step of 2 volts, note down corresponding values of I ₁
4. Now apply the gate voltage gradually, until SCR fires, then note down the values of and also the values of l _gand also the value of I₁and V_{mt2 mt1}
5. Increase Vm to some value and note down I ₁and V_{mt2 mt1}
6. Reduce gate voltage to zero, observe ammeter reading by reducing VM which gives the values of Ih (holding current) at the point at which, current suddenly drops to zero
7. Repeat the steps 2, 3, 4, 5 & 6 for different values of break over voltages
8. Plot a graph of Vmt1mt1 Ii
9. Repeat the steps 1, 2, 3, 4r 5r 6 & 7 for different modes
10. Compare sensitivity of TRIAC and comment on sensitivities.
11. Refer same design procedure for selection of R _Land R_G as that of SCR.
12. Follow the same procedure as that of SCR experiment to find latching current.

Observation:

For forward bias

For reverse bias

Results and Discussion:

The V-I characteristics of TRIAC is plotted on the graph which is true according to theory.

Viva Questions

Question: What is TRIAC?

Answer: TRIAC, from Triode for Alternating Current, is a generalized trade name for an elec¬tronic component that can conduct current in either direction when it is triggered (turned on), and is formally called a bidirectional triode thyristor or bilateral triode thyristor. TRI- ACs belong to the thyristor family and are closely related to Silicon-controlled rectifiers (SCR). However, unlike SCRs, which are unidirectional devices (i.e. can conduct current only in one direction), TRIACs are bidirectional and so current can flow through them in either direction.

Question: What do you mean by gate threshold current?

Answer: A TRIAC starts conducting when a current flowing into or out of its gate is sufficient to turn on the relevant junctions in the quadrant of operation. The minimum current able to do this is called gate threshold current and is generally indicated by IGT

Question: What are the applications of TRIAC?

Answer: Low power TRIACs are used in many applications such as light dimmers, speed controls for electric fans and other electric motors, and in the modern computerized control circuits of many household small and major appliances.

Precautions:

1. Keep your hand away from main supply.
2. Do not switch on the power supply unless you have checked the circuit connections as per the circuit diagram.