Originally Posted by no1dc
Now I'm Relly confused.
"When counter-emf is generated it is in the opposite direction (+ and - is switched) so the diode is actually forward biased and conducts, shorting the windings. Make any sense? What you are doing is placing the diode reverse biased to normal applied voltage which is also forward biased with respect to any counter-emf."
The reason for the diode in the windstar fan install is due to the "EMF" stuff, right? So what I need to do in this particular installation is install it with the banded side towards the negative terminal so as to block the emf stuff, right? I'm really sorry if this all sounds ignorant but I'm trying to learn and in the process do this right the first time. Again thanks for your patience. Pete
Well Dan72 did a pretty good job of explaining it but it can be confusing.
When the banded end, cathode, - end, is more negative than the non-banded, anode, + end current will flow through the diode. You do not want this to happen when the relay is closed and you're trying to turn on the fan. If it is hooked that way the diode will short out the relay and blow the fuse instead of running the fan. If you turn the relay around so the banded, -, end is connected to the + side of the relay no current will flow through the diode, the current will flow through the fan. The diode does nothing, as if it was not even in the circuit.
Now when the relay opens the fan generates the counter EMF. The force is in the opposite direction, the negative wire now has a more positive potential then the positive wire. Remember the fan is now disconnected from the battery. So the banded, -, negative end of the diode is now connected to a source, the fan, that is more negative than the other end of the diode. The banded end is hooked to the positive wire of the fan but the fan is creating voltage in the opposite direction so the positive wire is more negative than the negative wire. When this happens current will flow through the diode. Basically shorting out the voltage created by the counter-EMF.
Braking, etc. is all secondary. The real purpose, in this circuit, is to dissipate the counter-emf so that it does not arc across the relay contacts.
I tried, once you understand it it's easy. Explaining it is not quite so easy.
GM does use relays with the diode mounted inside the case. I have not seen or worked with the Ford system so I can't say anything about the dual relays and there purpose or in the Ford design. Startup voltage surges are usually of minimal duration. You need to consider them but they do not generally last long enough to blow fuses, burn relays or fry wires. Not always but usually they are only present for a fraction of a second.