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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

http://imagizer.imageshack.us/v2/700x500q90/661/HzLU7u.jpg

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  • \$\begingroup\$ You don't have access to the +24 V rail? \$\endgroup\$ – Biduleohm Oct 9 '14 at 19:11
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    \$\begingroup\$ A zener diode, say 30V or 36V, across the MOSFET would clamp the inductive spike, if the MOSFET avalanche ratings are in doubt. TVS at similar rating would do too. \$\endgroup\$ – Brian Drummond Oct 9 '14 at 19:23
  • \$\begingroup\$ @Biduleohm, no access to the 24V rail. \$\endgroup\$ – Rocky79 Oct 9 '14 at 19:27
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    \$\begingroup\$ @Brian Drummond, Are you suggesting a Zener or TVS instead of the RC snubber? I think you mean in conjunction as the zeners are slow to react. \$\endgroup\$ – Rocky79 Oct 9 '14 at 19:29
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    \$\begingroup\$ I like the zener... who said they were slow? \$\endgroup\$ – George Herold Oct 9 '14 at 19:35
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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.

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  • \$\begingroup\$ Thanks for the reply. Once the switch is open The RC will eventually charge up to some voltage close to 24v since this is a toggle switch ( I didn't mention it before) absorbing any fast rising spikes. Once the switch is closed the RC will discharge through the mosfet. Why would this be a problem? \$\endgroup\$ – Rocky79 Oct 9 '14 at 23:11
  • \$\begingroup\$ @Rocky79 the capacitor won't just charge to 24V, the relay coil will essentially transfer all of it's energy to the capacitor, and E = 0.5*L*I^2 = 0.5*C*V^2 which means capacitors voltage would depend on coils inductance, and a relatively large coil inductance and current and a small capacitor will result in relatively high voltage on the capacitor and across the MOSFET ... \$\endgroup\$ – Microsoft Linux TM Jan 11 '19 at 20:53

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