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Random q #437
I want to measure the output on a motor on a oscilloscope. I've had it a while but not used it at anything more than 12v. I am worried about using it on this in ductive lost because of the high voltages that are generated when the motor stops and the magnetic field collapses. What precorsions should I take to protect the scope or am I being paranoid.
It should have some kind of protection but it's not guaranteed. I made up a simple probe with a 20v zener for automotive work for mine. I put a pretty big one on Just in case. Can't say if it helped or not but the oscilloscope lived.
Good idea! It actually to test the alternator but editing my post on this tab is a pain and I was thinking motor as I typed!
The alternator should have a reverse diode built in to stop high voltages as these can easily damage whatever it is powering e.g. car electrics.
I know I'm using the scope to get an idea of ripple!
Actually my previous post was rubbish, as the alternator slows the field shrinks and there is no massive back EMF as the load still goes into the rectifier bridge. It's only coils like relays where the load gets suddenly disconnected that have back-EMF.
As for measuring ripple, where?
Pre-rectifier it will be a sine wave (ish).
Post rectifier (no load) with capacitors disconnected, half sine wave.
With capacitors connected but no load, it will be DC.
With varying load, you will see varying ripple.
Post rectifer. Diode test[b] seems [/b] ok but I'm getting odd effects on crank sensor which run by alternator cable. Something is up, I want to probe about but don't want to kill my[s] toy[/s] tool. It will not be purely a half sine as there are three coils in most alternators.