Back EMF and Diodes

People are constantly talking about protecting the Arduino from "Back EMF" from devices that use coils like motors and relays, but I've never heard anyone say the same for Servos. It seems to me that all a servo is is a motor with a feedback mechanism, so it seems that servos would be susceptible to this "Back EMF" as well. Maybe they're not, but I honestly don't know, so if anyone could shed some light on the subject, that'd be great :).

Also, I have an old Computer fan I intend to power using a transistor and a 12V supply. Does this fan require a diode for protection as well? I would think so.

As an afterthought, I think the "Back EMF" are produced when the power is suddenly cut from the object, such as when a motor is turned on or off. Does this mean Servos don't require a diode because they never lose power (until the entire board loses power)? And are not moving when the power is lost?

I hope that's not too confusing :stuck_out_tongue: ::)..

PS: If anyone could explain really what the diode does as well, that'd be great. People sometimes refer to it as "shorting out the reverse current", or something along those lines, which complicates it in my mind even more :P.

By continuing my Googling, I've so far found another post that explains it (to some extent):

Back EMF is a large voltage generated when a magnetic field collapses. That is when you turn the electromagnet off you will get a spike of several hundred volts generated by the coil in the reverse direction of the voltage you applied.
You use a diode to clamp or short out this voltage so it dosn not go on to damage anything else in your circuit.

Still doesn't answer all my questions though :P.

Model R/C servos do have motors, and also internal electronics to control and drive the motor. Any back emf protection diode would need to be installed inside the servo. Can't say I've seen a complete schematic of a hobby servo, but I would bet that they deal with EMF protection internally.

As far as your fan, if the DC power for it is being shared with any other electronics then yes it too should have a snubber diode installed.

Lefty

Model R/C servos do have motors, and also internal electronics to control and drive the motor. Any back emf protection diode would need to be installed inside the servo. Can't say I've seen a complete schematic of a hobby servo, but I would bet that they deal with EMF protection internally.

As far as your fan, if the DC power for it is being shared with any other electronics then yes it too should have a snubber diode installed.

Lefty

Aaahh, thanks. :slight_smile: I would have put a diode with it anyways (just in case), but it's nice to know that it's important. Can anyone explain really what the diode does? :stuck_out_tongue:

Can anyone explain really what the diode does?

The diode is wired across the motor coil is such a direction in that it is normally reversed biased (turned off) for normal current flow through the motor coil. On losing power the magnetic field collapses causing a voltage spike of reverse voltage polarity, which would then cause the protection diode to then be forward biased and start conducting. This effectively puts a short circuit across the motor winding causing the spike to be dissipated in the resistance of the motor winding.

Lefty

The diode is wired across the motor coil is such a direction in that it is normally reversed biased (turned off) for normal current flow through the motor coil. On losing power the magnetic field collapses causing a voltage spike of reverse voltage polarity, which would then cause the protection diode to then be forward biased and start conducting. This effectively puts a short circuit across the motor winding causing the spike to be dissipated in the resistance of the motor winding.

Lefty

OK, I think I get it now. Thanks for helping out ;D!