Construction and operating principle of Triac
Construction of TRIAC:
- Triac is a three terminal, four layer bilateral semiconductor device. It incorporates two SCRs connected in inverse parallel with a common gate terminal in a single chip device. The arrangement of the triac is shown in figure.
- As seen, it has six doped regions. The gate terminal G makes ohmic contacts with both the N and P materials. This permits trigger pulse of either polarity to start conduction.
- Since the triac is a bilateral device, the term “anode” and ”cathode” has no meaning, and therefore, terminals are designated as main terminal 1. (MT1), main terminal 2 (MT2) and gate G.
- To avoid confusion, it has become common practice to specify all voltages and currents usingMT1 as the reference.
Operation and working of a Triac:
- Since a Triac is a bidirectional device and can have its terminals at various combinations of positive and negative voltages, there are four possible electrode potential combinations as given below
1. MT2 positive with respect to MT1, G positive with respect to MT1
2. MT2 positive with respect to MT1, G negative with respect to MT1
3. MT2 negative with respect to MT1, G negative with respect to MT1
4. MT2 negative with respect to MT1, G positive with respect to MT1
- The triggering sensitivity is highest with the combinations 1 and 3 are generally used. However, for bidirectional control and uniforms gate trigger mode sometimes trigger modes 2 and 3 are used.
- Trigger mode 4 is usually averted.
- In trigger mode-1 the gate current flows mainly through the P2 N2 junction like an ordinary thyristor. When the gate current has injected sufficient charge into P2 layer the triac starts conducting through the P1 N1 P2 N2 layers like an ordinary thyristor.
- In the trigger mode-3 the gate current Ig forward biases the P2 P3 junction and a large number of electrons are introduced in the P2 region by N3. Finally the structure P2 N1 P1 N4 turns on completely.