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Basic Electronics » Spark when using a flyback diode

December 03, 2010
by Wheresmind
Wheresmind's Avatar


I've been wanting to control a DC motor with my nerdkit. After watching 'Motors and Microcontrollers 101' I learnt about voltage peak (and the spark) that happens when you open the circuit and the need for a flyback diode. So I bought a few diodes and tried one of them (1N4004) with a little motor that I found on a portable fan.

I thought the flyback diode was there to prevent the voltage peak and thus the spark, but when I tried it the spark was still there.

Diode connected to motor

Have I misunderstood how the flyback diode works ? Am I using the wrong diode ? (I tried with a 1N4001 and the spark also happens)

I have done these tests with 2 D Batteries with which the motor runs fine.


December 03, 2010
by mongo
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the 1n4001 diode has a breakdown voltage that is pretty low. I use 1n4007's because they have a 1000V breakdown voltage.

The flyback diode generally goes across an inductive load, anode connected to the negative side, (reversed biased). It is to quench an inductive spike from backing up into the circuit and causing damage. You will still get a flash across a contact when it breaks. That is different that the inductive spike the diode is intended to kill. If you want to quench that flash across a contact, an R/C snubber is the best bet. I don't have the values on hand but they are usually around 100K across a .47uF capacitor and connected across the contacts.

December 04, 2010
by Wheresmind
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I thought the flash was due to a spike in the voltage due to the inductive load of the motor.

If I get this correctly, you are telling me that there is another reason for the spark beeing there. So the next question would be : how do I know if the diode is doing its job correctly and I won't damage the other components when I connect my motor + flyback diode on my board ?

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Did you know that a flyback diode is important when driving a motor or any inductive load? Learn more...