When VAK > 0 and I, > 0, conduction of anode current begins near gate cathode junction. If the rate of rise of anode current (di/dt) is greater than spread velocity of carriers, local hot spots will be formed near the gate due to high current density which may destroy the thyristor.
The di/dt is limited by using inductor in series with anode circuit. IES 2005
- A fuse is attached in series with SCR so that when current go above rated current, the fuse get blow off.
- To keep temperature within limit, a suitable heat sink is provided in SCR.
- We place a Zenar diode parallel to SCRAwhich maintain constant voltage across SCR and protect it from overvoltage. A resistance is also placed which limits gate current and protect SCR from overcurrent. Capacitor and a resistor combination is also placed to increase noise immunity of SCR. IES-200
- It is mounted on thyristor. In order to get best results per unit cost, heat sinks are made of aluminium. IES-2004
If the rate of rise of VAK is high then thyristor may get turn ON (explained earlier in dv/dt triggering). If we don’t want to turn ON the thyristor then we have to reduce dv/dt. So, accidental turn ON due to dv/dt can be check by using snubber circuit shown in figure.
IES 2007, IES 2001, IES 2004, IES 2002
A varistor is attached parallel to SCR. Varistor is metal oxide non- linear resistor whose v-i characteristics is nonlinear. So as voltage increases, the slope of v-i characteristics of varistor decreases or resistance decreases or it offers less resistance. So it offers high resistance at low voltage and low resistance at high voltage.