# Testing the Proper Functioning of DIAC to Determine the Break Over Voltage

Experiment No.: 1

Experiment Name:

Test the proper functioning of DIAC to determine the break over voltage.

Objective:

1. To test the proper functioning of DIAC.
2. To determine the break over voltage.

Theory:

A DIAC is a full-wave or bi-directional semiconductor switch that can be turned on in both forward and reverse polarities.

The name DIAC comes from the words DIode AC switch. The DIAC is an electronics component that is widely used to assist even triggering of a TRIAC when used in AC switches and as a result they are often found in light dimmers such as those used in domestic lighting. These electronic components are also widely used in starter circuits for fluorescent lamps.

Basic Operation of DIAC:

Electronic circuit designs incorporating DIACs use the fact that a DIAC only conducts current only after a certain breakdown voltage has been exceeded. This is marked as VBO on the diagram. The actual breakdown voltage will depend upon a variety of factors in its fabrication, but it will be given in the specification for the particular component type.

When the DIAC breakdown voltage occurs, the resistance of the component decreases abruptly and this leads to a sharp decrease in the voltage drop across the DIAC to VF. There is a corresponding increase in current. This can be clearly seen on the I/V characteristic for the device below.

The DIAC will remain in its conducing state until the current flow through it drops below a particular value known as the holding current. When the current falls below the holding current, the DIAC switches back to its high resistance, or non-conducting state. 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.

Most DIACs have a breakdown voltage of around 30 volts, although the exact specifications will depend upon the particular type of device.

Interestingly their behaviour is somewhat similar to that of a neon lamp, although they offer a far more precise switch on voltage and thereby provide a far better degree of switching equalisation.

DIAC Voltage Current Characteristics:

Circuit Diagram:

Observation Table:

A. Forward Bias:

B. Reverse Bias:

Graph:

Apparatus Used:

Remarks:

The forward break over voltage is 33.0 V and the reverse break over voltage is -32.8 V.

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