Page 71 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

In Fig 2c, when S1 is opened, gate current becomes zero. This will have no effect on the current through the SCR
and the heavy load current continues to flow through the SCR in the DC gate supply.

In Fig 2d, if a shorting wire is placed across the anode and the cathode terminals, the current though the SCR
gets by-passed and all the current starts flowing through the shorted wire instead through the SCR. This means
the current through the SCR is reduced below the rated holding current(minimum current required through SCR
to keep it latched). This turns-OFF the SCR. Even when the shorting wire is removed the SCR remains to be in
OFF state.
Fig 2e shows an alternative method of turning-OFF the SCR. In this instead of shorting the anode and cathode
terminals of the SCR, the load current is cut-off by opening the Switch S2. This reduced current through the SCR
below the holding current and thus turns- OFF the SCR. Once the SCR is turned_OFF, the SCR does not turn-ON
even if the switch S2 is closed. To make the SCR fire again, with the switch S2 closed, the gate current should be
made to flow by closing the switch S1.
Since the SCR does not conduct in the reverse direction, the anode of the SCR should always be positive with
respect to cathode for conduction.

Fig 2

Important features of SCR,

• A very small gate current will control the switching OFF a large load current.
SCR operation with AC supply
Operation of SCR with AC circuit is similar to SCR operation . Fig 3 illustrates working of SCR in AC control
circuits.

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CITS : Power - Electrician & Wireman - Lesson 60-69