Hi everyone,
I need to control the flow of a curcit on a car i am working on (not my T).
I need a Diode that will restrict the 12 volt dc current to one direction and that will not reduce the 12 volt dc power I need to run a Fan.
what is happening is i have hooked up a fan to cool a radiator. The ground is hooked to the frame and the + is hooked to a fuse that is activated when the switch is turned on. once i start the car all is fine and works great fan works engine runs, but when I turn the key off the fan slows down and the engine continues to run. i have checked the fan after the switch is cut off and it is acting like a alternator it is make dc volts while it is slowing to a stop and some how feeding the distributor and keeping the engine running untill the fan slows enough to stop make dc voltage.
Did any of that make since?
HELP
Interesting that the power coming out of a slowing down permanent magnet motor can keep the ignition going. But anyway, yes a diode would solve the problem. However, there will be a drop of about 1v across the diode unless you use a high power schottky diode (eg. 30A rectifier diode from a computer power supply). The voltage drop will be about half then. Any diode so used will require heatsinking. In reality the voltage drop should not matter; given the battery will be up around 14V with the engine running.
The alternative is to use a relay in series with the fan, but the relay coil would have to be fed from some source independently switched from the igntion.
Chuck,
In my experience with power diodes, the forward voltage (the drop through the diode) will be less with a oversized diode. I assume that your fan draws around 10 amps so I'd choose digikey.com part number 497-4412-5-ND ($1.59) which is a 30 amp silicon rectifier in a TO-220 package. The tab is isolated electrically from the diode inside, meaning that it can be mounted on the fan shroud (use white thermal grease) so that the fan can blow on it.
My guess is that the voltage drop will be some 0.75 volts and at 10 amps that means that 7.5 watts of heat must be removed - John is right - cool it.
Seth
I forgot to mention, if your car is negative ground and you choose to place the diode into the hot (positive) side of the fan motor, then the motor will connect to the left lead (the cathode) and power will connect to the right lead (the anode).
A data sheet for the part can be obtained through DigiKey - just type in the part number at the top of their homepage.
Seth
The only way I can see that the fan would cause the engine to continue running is if it was wired to the ign terminal of the switch, or to the coil. Why not just wire it to the ACC side of the switch?
I have done some more wire probing and it is wired on the fuse panel that is only active when the switch is in the start and run position. I have checked all the grounds and added 2 more. All the curcits are wired correct. each on its own fuse. I thought it was feeding back thur the Alternator, but I unhooked it and got the same results. I will either go with the diode or a lighted 30 amp toggle siwtch since this will be a continuous running fan.
Chuck, Yamaha had the exact same problem with their Rhino 660 ATV. If the fan was on and you turn it off it would continue to run from the generating power of the fan, just like yours. The solution is a diode in the circuit, but I can't seem to find a part number or other information. Try your local Yamaha ATV dealer.
I had the same problem on a T speedster. Moved the wire to the fan from "ign" to "acc" on the back of the ignition switch and it worked perfectly.
I like John H's alternative idea the best. Use a relay to open the circuit when the ignition is off. The voltage drop and diode heating problems are eliminated that way.