 Hello, everyone. Welcome to this session. I am Dr. Asha Thalange and today we are going to learn different turn on methods of SCR. At the end of this session, you will be able to describe different turn on methods of SCR. These are the contents we will be covering in this session. Triggering, also known as firing, is one of the major aspects related to SCR. Triggering of SCR means turning the device on from off-state, that is, from forward blocking state to forward conduction state. Different methods are used to turn on SCR. Forward voltage triggering is simplest method used to turn on SCR. In this, forward, that is, positive voltage is applied between anode to cathode, keeping gate terminal open. This forward voltage makes junction J1 and J3 forward biased and junction J2 reverse biased. Due to this reverse biased junction J2, no current flows through the device except leakage current. At this forward voltage is increased at a specific voltage, that is, forward break-over voltage VBO, reverse biased junction J2 undergoes avalanche breakdown, which causes a large amount of anode to cathode current to flow through the device, suddenly turning the device on. This simplest method of turning on SCR is known as forward voltage triggering. Another method uses thermal triggering. This method of triggering is also known as temperature triggering. Like other semiconductors, the width of depletion layer of reverse junction of a SCR can be decreased by increasing its junction's temperature. In this, when the anode to cathode voltage is slightly less than the forward break-over voltage VBO, the device is turned on by increasing junction J2's temperature. At a certain temperature, this reverse junction collapses and large amount of current immediately flows turning on the SCR. This method of triggering by heating the junction is known as thermal triggering. Another triggering method uses radiation. This method is also known as light triggering method. In this, energy is imparted by radiation or light. To use this method, SCRs are made having special terminal or slot inside the inner P layer instead of gate terminal. Through this slot, light particles such as high energy photons and neutrons are bombarded inside the SCR. This generates more free electron hole pairs in the device increasing the charge carrier in the device. As the intensity of light becomes more than a normal value, it makes instantaneous current to flow through the device triggering it on. These SCRs are called light-activated silicon-controlled rectifier and light-activated silicon-controlled switch. Next is DV by DT triggering method. It is a technique in which SCR is turned on by sudden change in forward bias voltage across the device. DV by DT itself means rate of change of voltage with respect to time. We know that when device is applied with forward voltage, J1 and J3 are forward biased and J2 is reverse biased. This reverse junction J2 behaves like a capacitor due to the charge existing across the junction. If the forward voltage applied across the device is V, C is the capacitance of the junction J2, then the charge Q across it is given by Q is equals to CV. In order for the current to flow through it, there should be a flow of charges as I is equals to DQ by DT. Substituting Q equal to CV, we get I is equals to DCV by DT, which on solving gives I is equals to CDV by DT plus VDC by DT. Thus current I is equal to CDV by DT as rate of change of capacitance DC by DT is negligible, that is nearly zero as the junction capacitance is almost constant. This shows that the current flowing through the device is directly proportional to the rate of change of voltage across the junction or in turn across the device. When this rate of change of voltage across the device becomes large, sufficient charging current flows through the device turning it on even if the voltage across it is small. Let us see gate triggering. Gate triggering is the most commonly used simple, reliable and efficient method for triggering the SCR. In this positive gate current is passed in forward biased SCR to turn it on at much lower forward voltage than VBO. For gate triggering a signal is applied between the gate and cathode of the device. Depending on the signal used gate triggering are classified into three types DC, AC and pulse gate triggering. When DC gate triggering as shown in the figure DC supply is connected between gate and cathode. This gate current forces the electrons from the cathode N layer to be injected into the gate P layer. These injected electrons in gate P layer reach junction J2 and reduce the width of the depletion region. As forward voltage is already applied between anode to cathode of SCR, the moment gate current becomes sufficient enough to break the depletion layer, large current flows through the device turning on the SCR even at a lower forward voltage less than VBO. More is the value of gate current, more is the reduction in forward voltage required to turn on the SCR. The major drawback of this method is that both power and control circuits are DC and there is no isolation between the two. Once the SCR starts conducting junction J2 no longer exist. Thus no gate current is required for SCR to maintain it in on state provided current through the SCR is greater than latching current. This gives another drawback. As a continuous DC signal is applied at the gate even after turn on it causes more gate loss. In many power control circuits that use AC inputs AC gate signal is used. This method provides proper isolation between power and control circuits. Also firing angle control is obtained very conveniently by changing the phase angle of the control signal which provides variable on time for SCR. However the gate drive is maintained for one half cycle after the device is turned on. And the reverse voltage is applied between gate and cathode during negative half cycle. The drawback of this method is that a separate transformer is required to step down AC supply required for the gate circuit which in turn increases its cost. Pulse gate triggering is the most popular method used to trigger the SCR. In this gate drive consists of a single pulse appearing periodically or a sequence of high frequency pulses. This is known as carrier frequency gating. Here pulse transformer is used for isolation. Electric isolation is provided between main device supply and its gating signals. The main advantage is that there is no need of applying the gate signal continuously hence reduces the gate loss. Now after studying these different turn on methods pause the video and answer the following question. An SCR is dash triggered device. Choose the correct option from the given alternatives. Well the answer is current. Triggering on and off state of SCR is controlled by its gate current, anode current, latching and holding current. Thus in this session we saw different turn on methods of SCR. These are the references you can refer. Thank you.