However, now-a-days, this device is often referred by Thyristor. Silicon controlled rectifiers are used in power control applications such as power delivered to electric motors, relay controls or induction heating elements where the power delivered has to be controlled. Silicon Controlled Rectifier Symbol. The schematic symbol of a silicon controlled rectifier is shown in the below figure.
The diode arrow represents the direction of conventional current. Construction of Silicon Controlled Rectifier. It has three P-N junctions namely J 1 , J 2, J 3 with three terminals attached to the semiconductors materials namely anode A , cathode K , and gate G.
Anode is a positively charged electrode through which the conventional current enters into an electrical device, cathode is a negatively charged electrode through which the conventional current leaves an electrical device, gate is a terminal that controls the flow of current between anode and cathode. The gate terminal is also sometimes referred to as control terminal.
The anode terminal of SCR diode is connected to the first p-type material of a PNPN structure, cathode terminal is connected to the last n-type material, and gate terminal is connected to the second p-type material of a PNPN structure which is nearest to the cathode. In silicon controlled rectifier, silicon is used as an intrinsic semiconductor. When pentavalent impurities are added to this intrinsic semiconductor, an N-type semiconductor is formed. When trivalent impurities are added to an intrinsic semiconductor, a p-type semiconductor is formed.
When 4 semiconductor layers of alternating P and N type materials are placed one over another, three junctions are formed in PNPN structure. Modes of Operation in SCR. In this case, the junction J 1 and junction J 3 are forward biased whereas the junction J 2 becomes reverse biased. Due to the reverse bias voltage, the width of depletion region increases at junction J 2.
This depletion region at junction J 2 acts as a wall or obstacle between the junction J 1 and junction J 3. It blocks the current flowing between junction J 1 and junction J 3. Therefore, the majority of the current does not flow between junction J 1 and junction J 3. However, a small amount of leakage current flows between junction J 1 and junction J 3. When the voltage applied to the SCR reaches a breakdown value, the high energy minority carriers causes avalanche breakdown.
At this breakdown voltage, current starts flowing through the SCR. Thus, by providing the triggering pulse to the gate terminal, we can control the operation ON or OFF of thyristor. Hence, the thyristor is also called as controlled rectifier or silicon controlled rectifier. The silicon controlled rectifier or thyristor is represented by the symbol as shown in the figure. Silicon controlled rectifier is also a unidirectional device as it conducts only in one direction.
By triggering appropriately, the thyristor can be used as an open circuit switch and also as a rectifying diode. However, thyristor can not be used as an amplifier and it can be used only for switching operation controlled with triggering pulse of gate terminal.
Thyristor can be manufactured using a variety of materials such as silicon, silicon carbide, gallium arsenide, gallium nitride, and so on.
But, the good thermal conductivity, high current capability, high voltage capability, economical processing of silicon has made it to prefer compared to other materials for making thyristors, hence, they are also called as silicon controlled rectifiers.
The thyristor working can be understood by considering the three states modes of operation of silicon controlled rectifier. The three modes of operation of thyristor are as follows:. If we reverse the anode and cathode connections of the thyristors, then the lower and upper diodes are reverse biased. SCRs are unidirectional devices, i. SCRs are triggered only by currents going into the gate.
SCRs are generally used in power switching applications. In the normal OFF state, the device restricts current flow to the leakage current. When the gate-to-cathode current exceeds a certain threshold, the device turns ON and conducts current. The SCR will remain in the ON state even after gate current is removed so long as the current through the device exceeds the holding current.
Once the current falls below the holding current for a period of time, the device will switch OFF. If the gate is pulsed and the current through the device is below the latching current, the device will remain in the OFF state. Looking at figure 1 a , the four layer structure of the SCR, we see the three terminals, one from the outer p-type layer called the anode A, the second from the outer n-type layer called the cathode K and the third from the base of the lower NPN transistor section and is called gate G.
The SCR, as shown in figure 1 b , can be visualized as separated into two transistors. We see that the collector of each transistor is connected to the base of the other, forming a positive feedback loop. The SCR has two stable states. The first is the non-conducting OFF state. With the Gate terminal open let us first assume that no current is flowing into the base terminal of NPN transistor Q 2.
Given zero base current the collector current of Q 2 will also be zero. Given zero collector for Q 2 we infer that there should be zero current flowing out of the base of PNP transistor Q 1. Given zero base current in Q 1 we infer that there should be zero collector current in Q 1. This is consistent with our original assumption of zero current in the base of Q 2.
This results in varying the average power delivered at the load , by varying the ON periods of the SCR. It can handle several thousands of voltages and currents. SCR symbol and its terminals are shown in figure. Back to top. The SCR is a four layer and three terminal device. The four layers made of P and N layers, are arranged alternately such that they form three junctions J1, J2 and J3.
These junctions are either alloyed or diffused based on the type of construction. The outer layers P and N-layers are heavily doped whereas middle P and N-layers are lightly doped. The gate terminal is taken at the middle P-layer, anode is from outer P- layer and cathode is from N- layer terminals.
The SCR is made of silicon because compared to germanium leakage current in silicon is very small. To manufacture the SCR, three types of constructions are used, namely the planar type, Mesa type and Press pack type. In mesa type construction, junction J2 is formed by diffusion method and thereby outer layers are alloyed to it.
This construction is mainly used for high power Silicon Controlled Rectifiers. To provide high mechanical strength, the SCR is braced with plates made up of either molybdenum or tungsten. And one of these plates is soldered to a copper stud which is further threaded to connect the heat sink.
They are. In this mode of operation, the Silicon Controlled Rectifier is connected such that the anode terminal is made positive with respect to cathode while the gate terminal kept open.
In this state junctions J1 and J3 are forward biased and the junction J2 reverse biased.
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