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There are three modes of opearation for an SCR depending upon the biasing given to it:
Forward blocking mode:
In this mode of operation anode is given an positive potential while cathode is given negative voltage keeping gate at zero potential i.e. disconnected. In this case junction J1 and J3 are forward biased while J2 is reversed biased due to which only a small leakage current flows from anode to cathode till applied voltage reaches it breakover value at which J2 undergoes avalanche breakdown and at this breakover voltage it stars conducting but below breakover voltage it offers very high resistance to the flow of current through to it and said to be in off state .
Forward conduction mode:
SCR can be brought from blocking mode to conduction mode in two way either by increasing the voltage across anode to cathode beyond breakover voltage or by application of positive pulse at gate. Once it stars conducting no more gate voltage is required to maintain it in on state. Now there are two way to turn it off i.e.
Reverse blocking mode:
In this mode anode it supplied with negative and cahtode with positive voltage which reverse bias J1 and J3 and no current passes through it.