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This question already has an answer here:

While tyring to figure out how to best control a 12V device from a raspberry pi or arduino I ran into the following image: enter image description here

Now I'm wondering, what is D1 doing there? What's the use of that one?

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marked as duplicate by Brian Drummond, Community Oct 29 '16 at 15:05

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When the transistor switches open, energy stored in the motor can cause a back emf and destroy the transistor so the diode provides a current path for that energy to prevent this happening.

The same applies to switching any magnetic component such as a relay, a solenoid or a transformer.

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