Would you use a diode in this situation?

The moderator of this board has given me the following diagram which I will modify to interrupt my stock fuel pump relay wire (instead of the starter).  I have repeatedly read the "Basics" section on diodes, but I just can't grasp what they accomplish when used with relays. 

Would you use diodes with this setup?  What, in the most basic terms, do I risk if I don't use them?

Personally I don't like doing ignition kill ( or fuel pump kill ).  Contrary to belief relays do go bad.  What would you do if the relay went out when you are driving?  The engine would stall and you would be stuck ( especially if the relay is not accesible ).

So, I would risk shortening the life of each relay if I don't use diodes.  I know you referred me to that information before, but I just have to say that I still don't understand it--why the spike occurs and what it can do.  If it is a low draw wire I'm interrupting (a signal wire), what is the big risk?  Is the voltage spike more of a concern when the relay in question is being used to switch on and off high-amp device, like headlights or stereo amps?

And I can't quite picture the method of installing the diode you suggested.  It sounds like complicated, precise work.

Because I am using relays actually made by Bosch, I presume they are of better quality and less likely to fail than some other brands.

As far as the issue of potential failure, I'll carry a couple of extra relays in my car, and rig everything so I can easily bypass the relays if I need to.

 I appreciate all the advice.  This is all new to me, and I have difficulty grasping all of it.

Lets see if I can explain this and not confuse you or me any more.

The coil of the relay is an inductor, just like one half of a transformer. when you put voltage across the coil, it creates a magnetic field and energizes the relay. Now comes the bad part, when you remove the voltage the magnetic field collapses. This collapse creates a voltage spike, which has to go somewhere. This spike could go to the other relay in the circuit above, either one, and burn out the other coil or it could burn out it's own coil.

This is the reason that you put a diode across the coil of the diode, especially in the circuit that you have shown above. The diode will take the voltage spike through itself, this gives the spike a path other than the coil.

The amount of current through the contacts of the relay do not have any affect as to the amount of the voltage spike, it is determined by the inductance of the coil in the relay. It also does not matter how good quality the manufactor is unless they put diodes inside the relay across the coil.

I hope this explanation has not confused you more, please let me know if you need more information.

Tom

Thanks, Tom, that explains it better. 

From what you say, the above setup is more likely to suffer such a problem than a setup involving a single relay.  You recommend that I use diodes in this particular setup?

(Some people have told me this spike is a very rare problem (in general--not as to this specific setup), and it seems like installing a diode will be a hassle, so I had thought I wouldn't use one.  I couldn't find relays with internal or built-in diodes, and it sounded to me like this wasn't too much of a risk.)

Thanks to everyone who has responded.  My cognitive abilities are a bit impaired recently, so I've just had a hard time figuring this out to my satisfaction.

Lee280zx,

9 times out of 10 you will not have a problem. I personally like to put the diodes in for security.

However, what people are telling you is correct, the spike is a rare problem and not putting a diode on will not put your system at a risk.

In the dual relay setup that you showed above, if one relay should create a spike, that spike would go into the coil of the other relay and the + wiring. What it would do to the other relay is a question that I could not answer. There are other varibles that need to be answered to figure out what it would do the the wiring and relays. You are not wiring this setup to any electronics that are sensitive to voltage spikes, so not putting in the diode on the relay would not be a problem.

Good luck with your project.

Tom

Putting a diode or resistor across the relay coil is called suppression.  Many (but not all) Bosch relays already have built-in resistor or diode suppression so you won't need to add one externally.  It will be drawn on the schematic engraved on the side of the relay casing.  Hope that helps.

Thanks Tom and Jason!

I'll look at the side of my relays when I get them.

Lee