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Blow oscilloscope measuring coil Back EMF

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ptluis:
Hi, I've been testing some coils, and i wonder if it's possible to damage the scope measuring Back EMF from coils like relays? I got results like spikes of 400v up to 600v, with short durations in ns range. I'm testing on an old scope that allows a maximum of 300v on the inputs. His not damaged, yet, but I wonder if there's a proper safe way of doing it? 10x probe was used. I don't have a 100x probe, maybe i could create a voltage divider between probe and scope?


thanks

RoGeorge:
Yes, back EMF can permanently damage electronics, and sometimes the coil itself can get damage from its own back EMF.

The voltage spike may vary wildly (for the same coil) depending on the initial conditions, I wouldn't risk measuring that.

MathWizard:
People do make higher voltage probes, with resistor dividers, and caps. Dave has a video on it too, not much too it. You could solder up something on a protoboard, and then uses your normal probes on that.

Fungus:

--- Quote from: ptluis on August 24, 2023, 08:51:37 am ---Hi, I've been testing some coils, and i wonder if it's possible to damage the scope measuring Back EMF from coils like relays?

--- End quote ---

Yes, of course it is!


--- Quote from: ptluis on August 24, 2023, 08:51:37 am ---I wonder if there's a proper safe way of doing it?

--- End quote ---

Any circuit with back EMF should have protection in it, it can also damage your relays.

A $0.02 diode will do the trick. Try putting in an LED so you can see it flashing.

joeqsmith:
If you divide the signal, you're loosing resolution.  Maybe a simple clamp after the probe to limit the voltage.   Recovery time may be a wash if you are over loading the scopes front end today and are happy with it.  :-//

If you are just playing with DC, there are some very inexpensive 100X probes out there.  Then again, I cut one open and wasn't impressed.   

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