For an SCR which of the following increases when a resistor is connected across its gate and cathode?

For an SCR which of the following increases when a resistor is connected across its gate and cathode? Correct Answer Noise immunity of SCR

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Gate Protection:

Gate circuit should protect against overvoltages, over currents and noise signals.

Overvoltages across the gate circuit can cause false triggering of the SCR. Protection against overvoltages is achieved by connecting Zener diode ZD across the gate circuit.

Overcurrent may raise junction temperature beyond the specified limit leading to its damage. A resistor R₂ connected in series with the gate circuit provides protection against overcurrents.

Noise immunity:

• A common problem in thyristor circuits is that they suffer from spurious, or noise, firing.
• Turning-on or turning-off of an SCR may induce trigger pulses in a nearby SCR.
• Sometimes transients in a power circuit may also cause an unwanted signal to appear across the gate of a neighboring SCR.
• These undesirable trigger pulses may turn on the SCR leading to false operation of the main SCR. Gate protection against such spurious firing is obtained by using shielded cables or twisted gate leads.
• A varying flux caused by nearby transients cannot pass through twisted gate leads or shielded cables.
• As such no emf is induced in these cables and spurious firing of thyristors is thus minimized.
• A capacitor and a resistor are also connected across the gate to cathode to bypass the noise signals. The capacitor should be less than 0.1 μF and must not deteriorate the waveshape of the gate pulse.