 Hello, myself Sunil Kalshatti, Assistant Professor, Department of Electronics Engineering, Valchin Institute of Technology, Solapur. Today, I am going to explain the bi-directional power devices, DIAC and TRIAC. Learning outcome. At the end of this session, students can describe the construction, working and characteristics of DIAC and TRIAC, no DIAC, bi-directional diode thyristor, it is also called as a DIAC. DIAC is an important member of the thyristor family. It is a two electrode, four layer bi-directional avalanche diode. The DIAC is basically two terminal, parallel, inverse combination of semiconductor layers that permits triggering in either direction. That is why generally, the DIAC is used to trigger the bi-directional power devices. It is a bi-directional power semiconductor switch that can be conducted in both forward and reverse polarities. The DIAC gains its name from contraction of the word diode alternating current. That's why name is the DIAC. It is mainly used to trigger the bi-directional power device like TRIAC and SBS. This device works when avalanche breakdown occurs. The advantage of using this device is that it can be turned on or off simply by reducing voltage level below its avalanche breakdown voltage. It can also be called as a transistor without base. These are the schematic symbol of the DIAC. So first one represents that anti-parallel connection of two diodes similar with and second transistor without base. MT1 main terminal 1, MT2 main terminal 2. These are the two terminals of the DIAC. Basic structure of DIAC. DIAC is a three terminal four layer PNPN device. MT1 is the main terminal 1, MT2 main terminal 2. This is the VI characteristics of the DIAC. It operates in first hand, third quadrant. When MT2 is more positive with respect to MT1, then the current does not flow through the corresponding N layer but flows through the P2, N2, P1, N1. Initially, small amount of leakage current flows as shown in the characteristics curve. When the input crosses the forward breaker voltage, DIAC fires. The DIAC acts as an open circuit when the voltage is less than its avalanche breakdown voltage. As long as input is less than avalanche breakdown, DIAC remains open and when the input crosses avalanche breakdown, DIAC conducts, DIAC conducts. For the third quadrant operation, when MT1 is more positive than MT2, then the current flows through the P1, N1, P2, N3, P2, P1, N2 forward bias, P2, N3 forward bias and N2, P2 reverse bias. When the input crosses the breaker voltage, then N2, P2 breaks down and the conduction takes place and the current flows through the P1, N2, P2, N3. The construction resembles the diode connected in series. From the diagram, DIAC, unlike a diode, resembles a VJT but with following specification. There is no terminal is attached to the middle layer. Three regions are nearly identical size. The doping level at the two-layer is same. That is why DIAC gives the symmetrical switching characteristics for either polarity of the applied voltage. At the two, at the two P-end layer, the doping level is same. That is why the DIAC gives the symmetrical switching characteristics for either polarity of the applied voltage. It is also called as a bi-directional triode thyristor. It is the three terminal five-layer device and the number of terminals are three, MT1 terminal one, MT2, main terminal two and G is the gate. It is a symbol of triad. The triad gains its name from contraction of the word triode for alternating current. Triad is five-layer, three-terminal, power semiconductor device. It is a bi-directional device. It is the pair of phase-controlled SCRs connected in inverse parallel manner on the same chip. It is equivalent to the anti-parallel connection of two thyristors. The triac of maximum rating 16kW is available in the market. It is the electrical equivalent circuit of triac. When MT2 is positive and MT1 is negative, the current flows through the P1N1, P2N2 And when MT2 negative, then the current flows through the P2N1, P1N4. It is a schematic construction of triac. It is the three-terminal, five-layer device. Via characteristics of triac, for mode one and mode two operation triac operates in first quadrant. For mode three and mode four operation triac operates in third quadrant. Triggering modes of triac. mode 1, MT2 is positive, gate current is positive, mode 2, MT2 positive, gate current negative, mode 3, MT2 is negative, gate current is positive, mode 4, MT2 is negative, gate current is negative, mode 1, MT2 is positive and gate current is positive. When MT2 is positive, P1N1 is forward bias and P2N2 is also forward bias and P2N1 reverse pass. The gate injects the number of charge carriers into the P2. So, effect of this finally P2N1 breaks down and current flows through the P1N1 P2N2. In this mode track is more sensitive for the positive IG. That's why this mode is recommended for the operation. Mode 2. Empty to positive gate current is negative. In Empty to positive P1N1 forward bias, gate current is negative. P2N3 is forward bias and P2N1 reverse bias. In this mode the gate injects the number of charge carriers into the P2 region. So, effect of this P2N1 becomes more forward bias. So, initially the current flows through the P1N1 P2N3. So, it acts as a pilot thyristor. When the device conducts the drop across devices releases. The potential of P2 with respect to MT2 is more than with respect to MT1. That's why the current is diverted from the P1N1 P2N3 to P1N1 P2N2 and finally the conduction starts in the direction P1N1 P2N3. In this mode track is less sensitive for the gate current. That's why this mode is not recommended. In this mode the track acts as a pilot thyristor. Mode 3. MT2 is negative gate current is positive. When MT2 is negative and gate current is positive. In this mode P2N1 forward bias, P1N4 forward bias and N1P1 reverse bias. The gate injects the large number of charge carriers. So, effect of this the P1N1 becomes forward bias. And finally the device conducts and the current flows through the P2N1 P1N4. In this mode track is less sensitive for the positive edge. That's why this mode is not recommended for the operation. In this mode P2N1 forward bias, P1N4 forward bias, P1N1 reverse bias. The N3 injects the large number of charge carriers into the P2. So, effect of this the P1N1 becomes more forward bias and current flows through the P2N1 P1N4. In this mode track is more sensitive for the gate current. That's why this mode is recommended for this operation. Why? Track is called as a AC switch and SCR is called as a DC switch. Track conducts in both half cycle, but SCR conducts only in positive half cycle. That's why SCR is called as a DC switch and track is called as a AC switch. These are references. Thank you.