1. The local hot spot formation in the cross-section of the SCR is avoided by
a) reducing the junction temperature
b) applying gate current nearer to the maximum gate current
c) using only R loads
d) proper mounting of the SCR on heat sink
Explanation: Applying the higher gate current spreads the ions quickly and avoids hotspot formation.
2. The dv/dt protection is provided in order to
a) limit the power loss
b) reduce the junction temperature
c) avoid accidental turn-on of the device
d) avoiding sudden large voltage across the load
Explanation: Accidentally some voltage spike or noise may occur in the vicinity of the device, if the magnitude is large enough it may turn on the SCR.
3. dv/dt protection is provided to the SCR by
a) connecting a capacitor in parallel with the load
b) connecting an inductor in series with the load
c) connecting a capacitor & resister in parallel with the device
d) connecting an inductor & resister in parallel with the device
Explanation: Snubber circuit R-C in parallel with SCR is connected for dv/dt protection.
4. The effect of over-voltages on SCR are minimized by using
a) RL circuits
b) Circuit breakers
c) Varistors
d) di/dt inductor
Explanation: Varistors are non-linear voltage clamping devices, RC circuits across the loads can also be used.
5. Over-current protection in SCRs is achieved through the use of
a) Varistors
b) Snubber Circuits
c) F.A.C.L.F & C.B.
d) Zener diodes
Explanation: FACLF stands for Fast Acting Current Limiting Fuse.
6. False triggering of the SCRs by varying flux & noise is avoided by using
a) F.A.C.L.F & C.B
b) Shielded cables & twisted gate leads
c) Snubber circuits
d) di/dt inductor in series with the gate terminal
Explanation: Shielded cables provide isolation from outside noise.
7. Thyristors are used in electronic crowbar protection circuits because it possesses
a) high surge current capabilities
b) high amp 2-sec rating
c) less switching losses
d) voltage clamping properties
Explanation: Crowbar protection circuits have high surge current capabilities.
8. SCRs are connected in parallel to fulfill the ___________ demand
a) high voltage
b) high current
c) size
d) efficiency
Explanation: Number of devices connected in parallel can carry huge amounts of current.
9. The term used to measure the degree of utilization of SCRs connected in series & parallel is
a) tuf
b) string efficiency
c) voltage/current utilization ratio
d) rectification efficiency
Explanation: String Efficiency = Rating of the whole string/(rating of one SCR x number of SCRs)
10. To have maximum possible string efficiency
a) SCRs of same rating must be used
b) SCRs with similar V-I characteristics must be used
c) SCRs with the same dimensions must be used
d) SCRs with similar thermal characteristics must be used
Explanation: Having similar ratings does not mean they have similar charc.