Characteristics of Triac
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 char acteristic is applicable to both positive and negative voltages. This is expected because triac consists of two SCRs connected in parallel but opposite in direc tions.
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 duration so that firing of the device is assured. Usually, a duration of 35 us is sufficient for sustaining the firing of the device.
A typical triac has the following voltage/current values:
- Instantaneous on-state voltage – 1.5 Volts
- On-state current – 25 Amperes
- Holding current, IH – 75 Milli Amperes
- Average triggering current, IG – 5 Milli Amperes