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[SOLVED] Back EMF Diode Configuration

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HardwareChap

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I've seen two latching relay circuits with different back EMF diode protection configurations. I can understand how one circuit works, but I'm not sure how the other does. In the circuits below, I understand how "Circuit B" works, when the FET turns off the voltage on the connection to the drain (and anode of D3/D4) rises, this forward biases the diode and the current then circulates though the coil and diode. In "Circuit A", I can't see how the back EMF is dealt with

Capture.JPG
 

Circuit A will only work when the diodes are Zener types. Remember a Zener works like a normal diode in forward direction so the clamp voltage is approx Vz +0.6V in both directions, assuming the same diode types are used for D1 and D2.

Remember also that if MOSFETs are used, and that would be the most common type for relay driving, there is an internal PN junction across drain and source.

Brian.
 
Hi,

I ssume the Ls are coupled.

If so, then neither A nor B will work.

There are discussions about it in the forum.

Klaus
 

HardwareChap - can you give us more information please.
Are the two coils part of a single inductor (magnetically coupled) or are they independent coils. It makes a difference to the way the current flows in the de-energized coil and hence how it can be clamped.

Brian.
 

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