Hi all, just a simple question.

I know that diodes are used to protect circuitry from back EMF for inductive devices (solenoid valves, DC motors).

I'm just curious if it's good practice to use diodes when there are no electronics. For example, if you are just controlling a simple DC motor with a push button or switch. I assume the switch contacts can wear down faster if back EMF is not dealt with.

Thanks,
Philip

 

the problem with motors, is that without EMF you have large currents. EMF is your friend with motors in regards to circuitry. power handling equipment for motors is upsized to handle the large inrush currents, prior to the development of EMF.

 

Don't confuse back EMF with inductance--these two effects actually oppose each other. Back EMF would be harmless to your switch, but the inductance might cause sparking. However, for a small motor in a circuit that won't be switched many times over its lifetime, it shouldn't be devastating.

 

Interesting fact: In some industrial applications contacts on switches and relays are made to arc on purpose. This cleans the contacts of corrosion that can build up over time. Don't however use them in an explosive environment...

Mike T.

 

Cool, just what I had thought. Thanks guys!

 

Depending on your age, or if you work on lawnmowers, you may know of "points".

Ignition in cars and small engines before electronic ignition is dependent on mechanical opening and closing of "points" to provide the pulse for the spark plug coil.

If the condenser (capacitor) failed, the points would fail, due to carbon buildup from arcing too often. The capacitor would bypass the high voltage reverse EMF voltage to ground rather than letting it build up to the level that it could jump the air gap between the contact points.

Sometimes arcs are desired, other times, the back EMF needs suppression, in the form of a filter capacitor or a diode. The diode has the side effect of clamping the voltage at a level, so sometimes Zener diodes are used. This also isn't a perfect solution for a bi-directional motor.

There are entire journals and books on this issue if you talk to Google about "snubber circuits".