I'm going to install a taurus fan on my fancy new aluminum radiator (copper one finally gave up the ghost). I'm using the attached schematic to wire it. However, I want to avoid the AC voltage spike when the fan spins down, so I need to wire a diode in somewhere. I'm not entirely sure where these bad boys need to be located. I'm going to use a 1N5404 diode as suggested in the diagram, but will placing it along ONLY the ignition circuit protect my electrical system or should I use two diodes, one on each switched feed to the fan to totally isolate it from backfeed? ...or should I use find some relays with built-in coil diodes... or diode each coil? So confusing..
You don't need the diode where they say. I think you need two where I've indicated. They keep the reverse dc voltage from back feeding into your system. I would also find relays with a built in resistor, that way it doesn't matter which way you hook up the coils and it will do the same thing as the diode. I think most automotive relays have them built in
Frank I due but not in the fan wiring .. Mine is in the wiring for the alternator it's on the trigger wire for the regulator from the ingnitions switch .. Jay I think it may not be a bad idea ..
mine is wired like the top of the pic. only diff. i have a temp. sensor in the intake, inline with the ground wire....185 on 170 off...
Well, one for each fan speed. One for switching between the two in an "automated" circuit. One for A/C switching, which I'm not using. And the 5th is for a tri-color LED on the dash that indicates the state of the fan (Green = low, Red = high, Blue = A/C)
I was under the impression it was an AC generation, which is the danger in a DC system. Been a while since I took electronics, but I should be able to just piggyback the diodes into each line and ground them out, which won't matter until the juice is turned off, which "energizes" the diode and grounds the circuit, correct?
If the motor is like a relay there is a voltage spike when it is switched off. The voltage spike is from the magnetic field collapsing across the coil and inducing a voltage. Your ignition coil fires the same way, producing a spark when the coil is switched off. In a relay the spike is always reverse in polarity, which would put a clamping diode in forward bias shorting the spike. I think the fan motors are permanent magnet motors which may act like a generator when spinning down. But then again they are controlled by a relay, so if the relay switches open there really is no place for the power to go, except maybe to prevent the contacts from burning as they open.
Looks to me like its just used to keep from energizing circuits that you may not want energized. IN other words, you dont back feed 12v to the accessory side from the ignition side.
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=1N5404-E3/51GI-ND Data sheet: http://www.vishay.com/docs/88516/1n5400.pdf
