To plot V-I Characteristics of DIAC in Power Electronics Lab

Aim

To plot V-I Characteristics of DIAC.

Apparatus Required:

Theory:

A diac is an important member of the thyristor family and is usually employed for trigger¬ing triacs. A diac is a two-electrode bidirectional avalanche diode which can be switched from off-state to the on-state for either polarity of the applied voltage. This is just like a triac without gate terminal, as shown in figure. Its equivalent circuit is a pair of inverted four layer diodes. Two schematic symbols are shown in figure. Again the terminal designations are arbitrary since the diac, like triac, is also a bilateral device. The switching from off-state to on-state is achieved by simply exceeding the avalanche break down voltage in either direction

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. Connect the millimetre, DIAC, Voltmeter to the circuit.
2. Switch on the power supply.
3. Increase the supply voltage in steps; note the corresponding currents and voltages for each step.
4. Plot the graph of VI characteristics.
5. Reverse the terminal of DIAC. Increase the supply voltage in steps, note the corresponding currents and voltages for each step
6. Plot the graph of VI characteristics.

Observation:

For forward bias

For reverse bias

Results and Discussion:

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

Viva Questions

Question: What is Diac?

Answer: DIAC is a three layer; two terminal semiconductor devices.MT1 and MT2 are the two main terminals which are interchangeable .It acts as a bidirectional avalanche diode. It does not have any control terminal. It has two junctions J1 and J2 resembles a bipolar transistor, the central layer is free from any connection with the terminals. It acts as a switch in both directions.

Question: What are the applications of Diac?

Answer: DIACS are widely used in AC applications and it is found that the device is "reset" to its non-conducting state, each time the voltage on the cycle falls so that the current falls below the holding current. As the behaviour of the device is approximately equal in both directions, it can provide a method of providing equal switching for both halves of an AC cycle, e.g for triacs.

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.