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# Relay, back emf, and diodes Watch

1. Hi,

Here is a picture from my text book and they say diode D2 is there to protect the op amp from the back emf of the relay when the relay stops working.

What I don't understand is how this D2 works. I am confused. When in normal operation, current flows in the direction of D1. When the relay shuts off, current flows in the direction opposite (I guess).

So does D1 do the protecting?

If D2 were not present, then the current will bounce back from D1 and go around the circuit to the input terminals of the op amp?

What are the directions of the back emf current?

As always, all feed back is welcome. Thank you for giving me the opportunity to post this question and your efforts in reading this. I appreciate it. A lot.

Thanks again.
2. (Original post by DerHund)
Hi,

Here is a picture from my text book and they say diode D2 is there to protect the op amp from the back emf of the relay when the relay stops working.

What I don't understand is how this D2 works. I am confused. When in normal operation, current flows in the direction of D1. When the relay shuts off, current flows in the direction opposite (I guess).

So does D1 do the protecting?

If D2 were not present, then the current will bounce back from D1 and go around the circuit to the input terminals of the op amp?

What are the directions of the back emf current?

As always, all feed back is welcome. Thank you for giving me the opportunity to post this question and your efforts in reading this. I appreciate it. A lot.

Thanks again.
The "back" emf in the relay coil attempts to keep current moving after it has been switched off. This emf can be very large and will produce a "spike" that can damage the other components.
The normal "on" current would be down through D1 and and the relay coil. No current will go through D2 in its reverse direction.
When you switch the relay coil off, the back emf tries to push a large current in the downwards direction. (The bottom of the relay goes positive, it acts as a source of emf) This current is diverted through the diode D2 which has a low resistance for current in the clockwise direction in that lower circuit.
3. Hi Sir,

I am still a bit puzzled though.

When you switch the relay coil off, the back emf pushes the current in the opposite direction?
4. (Original post by DerHund)
Hi Sir,

I am still a bit puzzled though.

When you switch the relay coil off, the back emf pushes the current in the opposite direction?
The back emf in a coil is described by Lenz's Law. It states that when there is a change in the magnetic field, an emf is produced that opposes that change.
When you switch off the current in the relay coil, its magnetic field disappears.
The back emf tries to stop this happening by producing (re-creating) a magnetic field in the direction of the original field. This means the "back" emf and current generated are in the downwards direction (same as original current) and will attempt to push a large spike of current through the main circuit.
The diode D2 has a very low resistance to current in its forward direction, and diverts this spike of current through itself and creates a clockwise current in the lower circuit, thus protecting the main circuit.
5. Cool!
One thing though: why not just use a normal copper wire instead of the diode. Would some current bounce back?
Thanks so much for your input!
6. (Original post by DerHund)
Cool!
One thing though: why not just use a normal copper wire instead of the diode. Would some current bounce back?
Thanks so much for your input!
If you used normal copper wire it would "short out" the relay in both directions and the relay wouldn't work. The diode only does something similar to this in the one direction that matters.
7. Thank you. Good luck in your exams.

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