SCR Questions & Answers
Holding Current
Minimum current to keep the SCR in the on-state even after the removal of the gate signal. Slightly less than IL.
Latching Current
Minimum current required to be built in the SCR before it can stay on even when the gate pulse is removed. Slightly gr...
SCR Questions & Answers
Holding Current - answer Minimum current to keep the SCR in the on-state even
after the removal of the gate signal. Slightly less than IL.
Latching Current - answer Minimum current required to be built in the SCR before it
can stay on even when the gate pulse is removed. Slightly greater than Ih.
Type of device - answer Current controlled switch, can block both forward and
reverse voltage
Forward breakover voltage - answer Vbo. The forward voltage which can cause
avalanche breakdown in junction J2, therefore making the SCR conduct even in the
absence of a gate signal. Immediately after the breakdown, the voltage drop across the
SCR reduces and current increases.
Reverse breakover voltage - answer Vbr. The reverse voltage which can cause
break down in junctions J1 and J3, causing SCR to conduct in reverse direction
Why is gate pulse required to turn on the SCR? - answer SCR consists of NPNP
layers. When A is + wrt K, junctions J1 and J3 are FB but J2 is reverse biased. To FB
J2, a +ve voltage must be applied to P wrt K.
Gate signal - answer Needed to forward bias J2 and turn on the SCR. Not an
impulse but remains high for a small duration to allow current to build up to latching
current.
Alpha - answer Ic/Ie <1
Leakage currents - answer From C to B in npn, and B to C in pnp. Considered a part
of the collector currents, and opposite to base currents required. They help regeneration
of the base signal and at very high temperatures, leakage of T1 may help turn on T2
and hence the whole device.
Regeneration in SCR - answer Reason why continuous gate pulse not required to
keep SCR conducting. When + at A wrt K, T2 has + at C wrt E. Now a + at G wrt K turns
on T2 and it conducts. T1 turns on, allowing currrent flow from A to K. Collector current
of T1 serves as base current of T2, so SCR on even after G signal removed. When both
T1, T2 on, a1 + a2 == 1, so Ia increases regeneratively, becoming very high and
*restricted only by load resistance*
Ways to turn on SCR (4) - answer 1. destructive turn on by applying forward Vbo; 2.
by increasing temp of device, when the leakage current of T1 acts as base current of
T2; 3. By applying light radiation to the junction J2, creating electron-hole pairs that
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