Steady State Output Characteristics and Ratings of a Triac
Steady State Output Characteristics and Ratings of a Triac:
- From a functional point of view a triac is similar to two thyristors connected in anti-parallel. Therefore, it is expected that the V-I characteristics of Triac in the 1st and 3rd quadrant of the V-I plane will be similar to the forward characteristics of a thyristors.
- With no signal to the gate the triac will block both half cycle of the applied ac voltage provided its peak value is lower than the break over voltage (VBO) of the device.
- However, the turning on of the triac can be controlled by applying the gate trigger pulse at the desired instance. Mode-1 triggering is used in the first quadrant whereas Mode-3 triggering is used in the third quadrant.
- Most of the thyristor characteristics apply to the triac (ie, latching and holding current). However, in a triac the two conducting paths (from MT1 to MT2 or from MT1 to MT1) interact with each other in the structure of the triac.
- Therefore, the voltage, current and frequency ratings of triacs are considerably lower than thyristors.
- At present triacs with voltage and current ratings of 1200V and 300A (rms) are available.
- Triacs also have a larger on state voltage drop compared to a thyristor.
- Manufacturers usually specify characteristics curves relating rms device current and maximum allowable case temperature.