Diodes

[PAFred]

In an effort to resolve a difference of opinnion could someone difinitively validate clarify the correct purpose for a diode being placed across a relay or soleniod coil.

Is it there to protect the coil from damage of the collapsing field;

Or is it there to protect circuit boards from the backward EMF of the collapsing field

Just a friendly disputed point of view here.

 

[MacGyverS2000]

The coil could care less about the collapsing field... it handled the current going forward, it will handle it trying to continue going forward.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[zappedagain]

I agree. In other words, it is there to protect the drive circuit as the back EMF can be quite high.

 

[VEBill]

1) ...protect the coil...
2) ...protect circuit boards...

2

 

[waross]

Both. A very high back EMF may damage the coil as well as other equipment.
In some applications one aspect may be more important than the other. Many coils will withstand the back EMF, some won't.
In some applications the diode may introduce a time delay on drop out.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[marks1080]

This is very common for the purpose of introducing a time delay on drop out, as waross mentioned.

 

[itsmoked]

It should be noted that you didn't say what the driving force of the relay was. A diode would only be used on a DC driven coil.

Keith Cress
kcress -

http://www.flaminsystems.com