Yep was going to say exactly this. Anything involving collapsing magnetic fields, eg solenoids, relays, motors, absolutely simply has to have a flyback diode.
Incidentally you can demonstrate this by wiring up the relay so it oscillates i.e. use the NC contacts in series with the coil. Then stick your fingers across the coil. It'll give you a nice shock. Not enough to hurt you but enough to go "hmm, I'm not going to do that again" like licking a 9v battery.
Attempts to measure these spikes on a cheap scope years ago ended up blowing the scope input FET up. Whoops. Good job it was a university owned scope :D
> Attempts to measure these spikes on a cheap scope years ago ended up blowing the scope input FET up.
Yeah - they make special high-voltage probes for this kind of thing (some can go up to 10 Kv and beyond - just depends on how much money you want to spend).
I believe that the main difference between a standard probe and an HV probe is one of resistance; I think the HV probe puts a large value (mega-ohm) resistor into the mix (not sure if it's in series, or between the probe input and ground - see my further note below).
If you have a scope, it's handy to have one around "just in case" if you can afford it. I got lucky myself; I found one for a few dollars at a local Goodwill thrift store (the strange stuff you can find there...)
Note:
Hmm - I decided to look a bit more into this - I guess things on HV probes can get complicated quickly!
So - a basic probe is just a voltage divider with large value resistors (like I alluded to earlier); but as the frequency increases, lots of other weird and fun stuff come into play (and in the comments section of that article, someone mentions special chemicals that had to be added to certain special probes he used in the past!).
Yeah the Tektronix HV probes used to come with an aerosol can and you had to fill them up.
Technically frequency compensation is required across all voltage dividers for scopes so not to accidentally create a low pass filter with the parasitic capacitance in the cables and input circuits. It's all quite fun.
Disclaimer: was an obsessive compulsive scope collector for a while.
Something that also exists (but are mainly used on pure or nearly pure electrical systems - think old-school, pre-microcontroller/cpu ladder logic relays and controllers) are "quencher resistors"; these were resistors placed across the coil to absorb the voltage spike. You sometimes see them on automobiles, mainly older vehicles.
