Though the TRIACs principle of operation is often compared to two thyristors working in antiparallel, the construction of TRIAC devices (with a mix of N-type and P-type areas) means they can perform a switching function over both parts of the AC waveform. This means, unlike standard thyristors, TRIACs can work with the current flowing in either direction so only one device needs to be used for many applications.
It also means the device can perform conduction whether the polarity sent through the terminals is positive or negative. However, the sensitivity of the current required to trigger the device is highest when both terminals are the same type of polarity.
As such, the four triggering modes of operation are defined as follows:
- I+ Mode: Terminal 2 current is +ve, gate current is +ve
- I- Mode: Terminal 2 current is +ve, gate current is -ve
- III+ Mode: Terminal 2 current is -ve, gate current is +ve
- III- Mode: Terminal 2 current is -ve, gate current is -ve