First let me say I know I've asked a lot of electrical questions lately and your responses have helped tremendously, THANK YOU. I REALLY apprecaite all your help and patience with this "challenged" individual.

I've read most if not all the posts about the electric fan conversion installation. Only saw one reference to the diode install. I plan on installing the diode about 6" from each fan motor. It's an radio shack 276-1144 diode that is black and has a silver band on one end and red tape on one end(banded end) and white tape on the other end. If I undersatnd it correctly the silver banded end goes to the negative side, right? What gauge wire should I use off the 10 gauge for the diode and led wires? Next what is the best way to achieve a good connection. I'm thinking if using a 3 way butt splice as I'll be running additional wires to the dash for the LED's. My concern with that setup is the 3 way butt splice "hole" is quite a bit larger than the diode wire unless 18 gauge wire is sufficent.

As I plan on wiring this on a bench, another option possibly would be to "blend" all the wires together and solder them.

Any and all help and suggestions would be appreciated. TIA Pete

Pete,

Not sure what the diode is doing myself as I don't have one nor have I seen one in the factory wiring diagrams I have looked at. But if I hazard a guess the diode may be used to provide fan braking when the fans are turned off. And if this is the case, you can also achieve this simply by how you wire up the control relay. Standard relays have what is referred to as DPST switches (Double Pole, Single Throw). If you connect the power supply to one of 87's and the fan to 30... then you can connect the other side of the fan to the other 87. When the relay turns off it will connect both fan motor wires together completing a braking circuit.

This works for permament magnet electric motors because they are also generators, and they generate current in the reverse direction to that which makes them go.

Played around with this affect many years ago as a kid with slot cars. Darn cars slowed/stopped in about 1/3 the distance with brakes wired this way.

Steve

Steve the expaination is from a post on the fan install. I tried to find the schematic that is somewhere on the site but couldn't. It had wiring using the relays and also wiring diagram for having the fan actuate when the A/C kicks on. In the diagram it shows a triangle pointing to the left into a vertical line. This all points to the + side of the fan motor. I'll try and include a "copied" picture but don't know if it will work(didn't work). It's not the exact diagram but similar and shows the diode except my diode munber is different.

The diagram shows how to use the fan controller to fire the Bosch relay. A reverse current blocking diode (or freewheeling diode) is a good idea on any motor control relay. Once power is removed from a DC motor it becomes a DC generator and the reverse voltage spike can shorten the life of the relay. The diode will bleed this spike to ground.


Here is a link here on TC that shows the wiring schematic I'm using.

http://www.chevelles.com/forums/showthread.php?t=89141&highlight=windstar+fans

"It's to eliminate the voltage surge from the motor when the relay opens. A motor is basically a coil of wire so when you break the current to it, it will produce a large voltage of the opposite polarity than what was applied before. A little like how the ignition coil produces such a high voltage." From Peter F.

For some reason I also remember, if memory srves me correct, that it also prevents back flow(whatever that is) to the relay as the fan spins down.

Hope this makes sense. Pete

The diode does indeed protect against counter-emf. Counter-emf is generated whenever power is removed suddenly form an inductor. Inductors resist instaneous changes in current, just as capacitors resist instantaneous changes in voltage (this is why a capacitor can act like a small battery).

A motor is an inductor, just like a coil (including an ignition coil), and so resists instaneous changes in current, also a turning dc motor is also a dc generator, so there will be voltage across the motor until it stops turning (the "backflow" mentioned, not a big deal, though). The main concern is removing power from the motor results in the motor trying to hold the same current flow through it, except that the relay contacts are open and there is no path for current to go. The current has to go SOMEWHERE, and the easiest place is often to arc across the contacts. It is the same reason a set of ignition points will burn out, unless itself protected by a condensor (capacitor). Fun, huh?

It will also provide some braking. It will not prevent the motor from turning as you are driving but will cause it to spin to a stop much faster than simply coasting to a stop. Simplest way is to throw a short on the windings, which the diode essentially does (but only in one direction). Sometimes a resistor network is also used to dissipate the current as heat. The technique is sometimes called "Dynamic braking". Faster braking can be accomplished by "Plugging", which instead of throwing a short on the windings puts a voltage in the opposite direction.

Braking is actually quite hard on the motor, especially plugging. It beats up the motor quite a bit, more on high-torque motors than high-speed (the high-torquers will keep trying to spin longer, for one thing), for example, braking is really hard on drill motors.

Anyways, the main reason we are interested in the diode is to help out with counter emf, not braking. For the same reason good practice puts a diode across the coils of the relays, although I suspect the GM relays have built in diodes across the coil (many relays do).

The diode does indeed protect against counter-emf.

Anyways, the main reason we are interested in the diode is to help out with counter emf, not braking. For the same reason good practice puts a diode across the coils of the relays, although I suspect the GM relays have built in diodes across the coil (many relays do).

Dan are you saying instead of putting the diode across the + and - wires at the fan motor they'd be best at the relay(bosch 70 amp)? If so between what terminals.


Further research yielded this. The cathode end(banded end) goes towards the + side and the triangle end, anode(like I can tell, yeah right) goes toward the -. I can definately tell the banded end so therefore that end goes towards the + terminal, right?

Man this stuff is confusing to me. :clonk:

TIA Pete

Dan are you saying instead of putting the diode across the + and - wires at the fan motor they'd be best at the relay(bosch 70 amp)? If so between what terminals.

Not necessarily better, more like in addition to, although again, the relay may already have it built in, since it's standard practice to do so. If you did do it, it would be striped end at 86 (+), other end at 85 (-).


Further research yielded this. The cathode end(banded end) goes towards the + side and the triangle end, anode(like I can tell, yeah right) goes toward the -. I can definately tell the banded end so therefore that end goes towards the + terminal, right?

You are correct, to forward-bias a diode (allow it to conduct) the non-banded goes to the + terminal of the source. Conversly to reverse-bias (or in this case to block conduction of the applied voltage from the relay, otherwise the diode would short out the voltage) the band goes to the positive terminal. 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.

Another way to remember your diode ends and symbols, etc, is to remember that the band matches the line in the symbol, the one at the tip of the triangle. And if you think of the triangle as an arrow, that shows you the way to hook it up in order to forward bias it (if you are like 99% of the population who mistakenly thinks of current flowing from positive to negative, but that's another whole can of worms). Since we tend to think of positive flowing to negative if you want a diode to conduct you place the diode so that it "points" from positive towards the negative, and orient the band to match.

Now I'm Relly confused. :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

And this is exactly why I suggested the DPST relay configuration Pete. With that solution there is no forwards/backwards involved. It actually sounds like it does pretty close to the same thing by allowing current to flow when the motor is turned off.

I figure if a modern OEM design uses the NC portion of a relay to connect both sides of the fan together, then that is probably a very workable solution. That is how Ford does it.

Of probably a bigger concern than the diode is how you handle startup. I have Taurus fans and they can sink 70A at startup. Ford used two relays both controlled by the ECU. I always interpreted this as high and low speed and in fact I think the wiring diagram may have even labeled it that way. But I now highly suspect that there is no low speed, instead the low speed is really startup speed because of the inline resistor that reduces startup current to 35A. Note that running the fans pull 15A no matter which speed, high or low, is turned on. So bottom line is that resistor is minimal in it's affect on a running fan circuit while it cuts startup current in half.

Steve

Now I'm Relly confused. :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.

Hank,

Well put!

Pete, yes, to confirm, banded end towards positive terminal of the relay. You don't really have to sweat out the theory behind it, unless you are interested. But putting the diode in that way will prevent arcing across the contacts when the motor shuts off.

Hank says the GM relays have a similiar relay inside the case, so that is why the circuit diagram we are discussing doesn't show them, although they are in there.

Hope I didn't do more damage than good!

Banded towards positive terminal!

THANKS!!!! :thumbsup: I'm really sorry for all the questions but NOW I think I understand. You have been great and I appreciate your patience with me and my dumb/stupid questions. I only wish I could offer help half as much as I've received from this wonderful site. Pete