Relays with a diode

Hey I am a little confused about something so I appreciate help.

If you have a relay  you would connect a diode in parallel with the coil between teminal 85 and 86, say 86 is the positive side so the cathode is on that side. RIght??    Now  you flip your switch off and you have a reverse voltage spike from the relay coil.  Since its a reverse voltage I assume it come back from the relay coil through terminal 86 (positive) back down your wire.  I'm confused about this, how does connecting it in parallel help?  It would seem wiring the diode inline with the positive terminal with the cathode facing the relay would work.   That way the cathode prevents voltage from passing unless of course it is more than the diode can handle.

Also what would be the best way to connect wires to a relay?  Would you use female disconnects? If you soldered how would you insulate the bare wire so it didn't touch the other contacts?

Help

Ok ill try to clear it up if possible.

If we apply a voltage to a relay we build a magnetic field in the coil of the relay. This produces the magnetic field to operate the relay. (im sure you understand this part). Since the relay coil has an impedence it will have a voltage drop on it (the supply voltage) and a current through it.

The current through an inductor (like the coil of a relay) has a current flowing through it that can not change instantaneously. So even if voltage is removed there will still be a current through the inductor.

The current through and inductor is equal to L (di / dt), the time derivative of the current times the inductance.

Since the inductance is constant it really doesnt matter here. The derivative however shows that the voltage will be proportonal to the change in current. When we remove the voltage from the coil the current decreases drastically. Meaning the voltage across the coil will become a very large negative voltage.

So now we know that when power is removed from the relay we will cause a very large negative voltage. Now why is a diode used??

Capacitors used to be used to lower the voltage a bit since the voltage on a cap can not change instantaneously, now most people would rather use diodes. Why? It takes a little understanding of the diode.

Silicon diodes mostly used today have an electric field potential inside of around 0.7 V, this voltage is not a constant however, it will change depending on the current through the diode. The thing is it changes very little, to rise the voltage just a few millivolts may require amps of current. So the voltage across a forward biased diode will never change much from 0.7V.

On a reverse biased diode the diode appears to be an open so the voltage drop is zero and the current is zero.

Tie it all together:

We now see that if we place a diode across the coil opposite of the supplied turn on voltage the diode will be reversed biased and not alter the circuit.

But we see that the reverse voltage caused by the decrease in current is going to forward bias the diode. Since the diode becomes forward biased the voltage across it will not exceed around 0.7 V, and the diode will pass the current from the magnetic field. So the reverse voltage stays below 1V and doesnt harm electronics.

Hope that helps, sorry it was so long...

markcars, I use diodes on all relays because as you said they are very cheap, dont see why you wouldnt.

the easiest way is usually just to get a socket for the relay that already includes the diode, watch your polarity though. Partsexpess has them: http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=330-076

Here is my question regarding diodes and relays

All the diodes that I have seen and used have an amperage rating. The biggest amperage rating I have been able to find is a 3.0 AMP diode. Will a diode rated at 3.0 amps only block 3 amps before it does nothing anymore?

I soldered a 3 amp Diode across the 85 and 86 pins of a relay that was attached to a circuit with a 25 amp circuit breaker. When I applied power to either side of the relay coil, the diode caused power to show up on the other side of the relay, therefore causing a short when ground is applied.  Why would this happen, does it have to do with the rating of the relay?

thanks