inductive spike simulation ltspice

Thread Starter

denison

Joined Oct 13, 2018
125
I have tried to simulate an inductive spike on ltspice. Had no luck. I have used an n type mosfet rated at over 200v. Very small current with this one. Easier than trying to use the voltage controlled switch.
The simulation shows no large spike which should be there on switching off or on. I have attached an asc file showing the connections.
Can anybody explain why the simulation does not show any large spikes?
I can see that the asc file does not show the result of the simulation. But from the schematic anybody with ltspice can run the simulation and will see that there is no large inductive spikes?
 

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Thread Starter

denison

Joined Oct 13, 2018
125
refer to the schematic in the attached file. the moc 3063 has a blocking voltage of 600v. the snubber recommended is 39 ohms and a .01uf capacitor.
I had an electric motor connected to the same mains as the controlled resistive load. Turning on the power switch blew the moc. Adding a mov across the mains terminals has fixed the problem.
I then decided to try to fix the problem with a snubber. I simulated a 1000v pulse for 5us and the recommended snubber passed the whole voltage. I have now tried a 1uf capacitor and 5 ohm resistor. The 1000 volt pulse is clamped at 147v. A 2000v spike is clamped at 296v. A 3000v spike is clamped at 444v. Try the simulations yourselves.
The 5us spike is only a guess. Can somebody suggest a more realistic length for the spike? Of course don't try simulating with the motor attachment as there is no figures for that. The spike is measured at the input of the moc 3063.
 

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crutschow

Joined Mar 14, 2008
25,395
The simulation shows no large spike which should be there on switching off or on.
There is no inductive spike when switching on.

My simulation shows a turn-off spike of 282V at the drain of a high voltage MOSFET with a fast fall-time 10V-0V gate pulse.
 

Alec_t

Joined Sep 17, 2013
11,639
Can anybody explain why the simulation does not show any large spikes?
You have used V2 pulse rise and fall times of 0. Spice doesn't like that and uses ~60mS instead. Try rise/fall times of 10nS, for example, and you'll see the inductive turn-off spikes.
 

Thread Starter

denison

Joined Oct 13, 2018
125
thanks Alec, that explains it. but there must be a turn on spike too because I blew my moc3063 on turning on the mains. can that be explained? Actually I have no more trouble now with a 275vac mov across the mains.
 
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