Construction of SCR
Q. Draw construction of SCR. Explain its operating principle using two transistor analogy.
Q. Draw construction of SCR and explain its operation with V-I characteristics.
Q. Describe the construction of SCR with neat sketch. Also draw SCR’s symbol.
Q. Draw the symbols of SCR
- The basic structure of SCR is as shown in Figure (b). It is a four layer PNPN device, with three terminals brought out for the user, namely Anode, Cathode and Gate. The “gate” terminal is the controlling terminal that can turn on the device whenever required.
- The symbol for SCR is as shown in Figure (a). It is the symbol of a rectifying diode with a third additional control terminal i.e. gate.
- The direction of forward anode current, voltage across the thyristor and direction of conventional gate current are as shown in Figure (a).
- Number of junctions : As seen in the Figure (b), there are three junctions, J1, J2 and J3. In order to turn on SCR, the anode must be at a higher positive potential than cathode. That means SCR should be forward biased.
- Current directions : The directions of the anode and gate current in Figure (a) are conventional current directions. It clearly indicates that SCR is a unidirectional device and that the gate current can be only positive. The gate current can flow only in one direction i.e. into the gate terminal.
Figure (a) 
Figure (b)