 Welcome myself Giridhar Jain, Assistant Professor in Electronics and Telecommunication Engineering Walsh and Institute of Technology Solop. Now today I am going to explain AC power control using DIAC and TRIAC. Now learning outcomes of this session. At the end of this session students will be able to explain circuit and waveforms of AC power control using DIAC and TRIAC and the second outcome is students will be able to explain various applications of the circuit. Now this is the circuit diagram of AC power control using DIAC and TRIAC and this is the waveform for load voltage. Now let us understand what is the circuit and how the circuit operates. Now if we look at the circuit for the circuit this is the input 230 volt AC 50 hertz, lamp is used as a load, this is a TRIAC, TRIAC is a bi-directional device which conducts in forward bias as well as reverse bias and TRIAC is a power device. Now this is a DIAC, DIAC is also bilateral device but it is a low power device and DIAC is used to trigger the TRIAC. Now in series with the lamp fixed resistance R1 in series with a potentiometer VR1 variable resistance and capacitor C and this is the waveform for voltage across the load that is lamp. Now in positive half cycle of the input capacitor C charges and the charging path for the capacitor C is from this positive of the lamp, resistance R1, VR1 and this capacitor. Now when voltage across capacitor C exceeds the breaker voltage of DIAC, DIAC becomes on or DIAC conducts. When DIAC conducts, capacitor C discharges through conducting DIAC gate and empty one of the TRIAC therefore a positive gate pulse is applied with respect to the empty one and the TRIAC is forward bias. Therefore TRIAC becomes on, when TRIAC becomes on it is equivalent to a closed switch therefore the current flows from positive of the lamp and the TRIAC and the power is applied to the lamp. Now when TRIAC is conducting, voltage drop across the conducting TRIAC is about one volt which is very very small as compared to the input 230 volt. Therefore this one volt is neglected in comparison with the input while drawing the waveform. So during this period T1 the TRIAC is in off state therefore current flowing through the TRIAC is leakage current and therefore voltage across load is 0. Now after this T1 at this point the DIAC becomes on and it triggers the TRIAC therefore TRIAC becomes on. Then TRIAC becomes on, voltage across the load is equal to part of input voltage. Now once TRIAC conducts it will continue to conduct till current through TRIAC becomes less than holding current as shown in figure. So this is the holding current. So TRIAC will continue to conduct till current falls below the level of holding current. Now as the load is lamp which is a resistive load so waveform for the load current and the load voltage or applied input voltage are in phase as shown in figure. So at this point of the time the current through TRIAC becomes less than holding current therefore TRIAC becomes off and the after 0 cross of the line negative half cycle of the input starts. So in negative half cycle of input again capacitor C charges through this lamp R1 and VR1 to negative voltage. Now when voltage across capacitor C exceeds break over voltage of the DIAC conducts and it will apply a negative gate pulse to the TRIAC. Now how this capacitor C discharges through MT1 gate and DIAC? So in this way a negative gate pulse is applied to the gate with respect to MT1 of the TRIAC and TRIAC becomes on when it is reverse bias. Therefore neglecting potential drop across the TRIAC in comparison with applied input voltage voltage across the load is equal to part of input voltage as shown in figure. So during this period TRIAC is in off state therefore load voltage is 0 and after TRIAC becomes on the part of negative half cycle of input will appear across the load. And TRIAC will continue to conduct till current through TRIAC becomes less than holding current as shown in figure. So at this point again TRIAC becomes off and the next cycle starts. So this is from this point up to this point. So this is one cycle of the output that is load voltage. Now for this one cycle of the output this is positive half cycle and this is negative half cycle. By observing the waveforms of the load voltage here firing angle is varied this is a firing angle. So firing angle is varied by varying this what we are one and by varying the firing angle we can control the load voltage thereby we can control the load power. Since in this circuit or using this circuit AC power is controlled by controlling the firing angle and firing angle is controlled by varying this part VR1. By varying this VR1 we can change the firing angle and we can control or we can change the load power. Now pause this video and think on the following question. If above circuit is driven by 230 volt AC and load is 100 watt lamp then how much percentage of the load power is dissipated across the TRIAC. Now to answer this question load will get a maximum power at a firing angle of 0 degree that is first. Now under this condition halted drop across the TRIAC is about 1 volt conducting TRIAC. Now TRIAC and load are connected in series therefore current flowing through TRIAC and the load is same. Therefore the power dissipated across TRIAC depends on the voltage across the TRIAC. Now halted drop across a conducting TRIAC is about 1 volt and the applied input voltage is 230 volt RMS. So power loss across the TRIAC is equal to halted drop across the TRIAC divided by that 230 volts. So here we are calculating percentage power dissipated across the TRIAC. So 1 divided by 230 and if we multiply it by 100 so it comes to 0.435 percent. So this is less than 0.5 percent or less than half percent means power dissipated across the TRIAC is very very small less than 0.5 percent. Now coming to the applications of this circuit what are the applications of this circuit? Now this circuit is a AC power control using DAC and TRIAC therefore basically whatever loads which are operating from AC can be controlled by this circuit. Now first application is lamp dimmer. So intensity of lamp is controlled by varying the firing angle. So in the circuit the lamp is connected as a load so circuit becomes a lamp dimmer. Now second application is a fan regulator. Now for the fan regulator the load is replaced by a fan and by changing pot by varying vr1 we can change the firing angle and we can control the voltage applied to the fan and we can control the speed of fan. Hence this is fan regulator and the third application is a heater control. Now in the heater control we replace the load by heater and heater is used to heat the water. So by fairing the firing angle by using the potentiometer vr1 we can change the voltage applied to the heater and thereby we can control or we can change the temperature of the water to be heated in this application. Now these are references power electronics by M.D. Singh and K.B.Khan Chandani McGrahill education and this is the web link. Thank you for watching this video.