Specifically, I have two small, 5V fans salvaged from a very old laptop. One is model Nidec C34427-55 app and the other is AD0205LB-G50. Looking under the sticker on the hub, there seem to be some components but they are too small to see what they are, to my untrained eyes. I've looked around in the search engines but not found information on the fans' specifications. Is the flyback diode built-in or must I add one before connecting it to the board? If a flyback diode is needed will an old 5V LED be sufficient?
(50V/div
So, yes, these motors are highly inductive as you might imagine, despite the control electronics, and a free-wheel diode is needed... It does surprize me there isn't internal diode and that the device survived this.
This was a larger fan, I suspect the spike will be a lot less for a tiddly 25mm fan though.
I have a multimeter but no oscilloscope. I don't suppose that would be responsive enough to catch such a brief surge.
Instead, I have just now tried putting a 5V LED in position as the flyback with the Nidec C34427-55 controlled by a switch. When I turn the switch on or off it flashes dimly for brief instant. I suppose that is a bad sign and I should not connect that motor without a diode. I'll be able to test the other one in a similar way some time later this week.
Not on a timescale of fractions of a microsecond, no!
I'm sure fan motor circuits vary - for instance the amount of decoupling is likely to vary, some cheap ones without decoupling no doubt, which will be likely to produce higher voltage spikes.
Basically if in any doubt DC-switching a load, add a diode in case its inductance is significant, its cheap and without downside.
