I am designing switch circuit that switches on and off a relay that switches on and off a 115AC light.So the switching speed is not a concern as it is a once in a while on or off.
The relay for the circuit is not accessible to me and therefore I can't add a flyback diode across it.
My question is will the shown snubber circuit be sufficient to protect the Mosfet Drain-source? Do I still need a TVS? Thank you
Best Answer
In these circumstances I'd use a zener diode across Q1 - then there is no doubt that the maximum voltage that is imposed by the relay coil (when it is open circuited) is limited to the zener voltage thus protecting the transistor. If the transistor AMR (absolute maximum rating) was 60 volts, then choose a zener that's rated at two-thirds of 60V i.e. 40 volts (a 39 volt zener will do just fine).
The problem with a snubber is that when the MOSFET open-circuits, 100mA will flow thru the 100 ohm resistor and charge the capacitor up rapidly to some voltage that is hard to determine - if you knew the inductance of the relay coil (and hence the stored energy) this could be calculated but I guess you don't or you would have said.