Typical ON/OFF Transient Voltages from switching a DC Motor?

[LabSpokane]

I'm switching a small, 12V DC Motor (pulls approx 6W max) with a MOSFET that's rated to 60V IIRC.  I realize that the MOSFET is tolerant of a certain amount of transients and I have overspecified it, but I'd like to toss in a snubber just for belt and suspenders sake. 

Before I risk blowing an input to my scope in ignorance, what is the typical range of transients that I might see - just as an order of magnitude? 

10's of volts? 
100's of volts? 

Should I fuss with a 700V differential probe on the front as an "insurance policy" against blowing an input? My scope is rated to 300V max on the inputs I believe.

Thanks in advance.

[tom66]

It depends entirely on the motor. The transient is caused by two things: back EMF and by the inductance of the motor.

You would be able to easily eliminate this concern by placing a diode in reverse bias across the motor, which would absorb the meat of the pulse. Anything left would be determined by how fast the diode can begin conducting, and the inductance of the motor and diode leads, but is likely to be difficult to measure on a typical scope.

I would honestly be more concerned about the MOSFET as most oscilloscopes are specified to withstand brief transients whereas a large transient can easily break the MOSFET. Usually drain source short but in some cases if you get a large enough pulse on the gate due to the drain gate capacitance you'll blow the gate source insulator and short across G-S.

A quick look at my DS1074Z spec sheet says 1kV transients are acceptable although it does not specify duration or pulse rate.

[LabSpokane]

Thanks.  That's what I thought, and have always done with relay coils, but the motor switching is new ground for me.