Help a civil with an electrical control problem 1

[JBR87]

Hi guys and gals,

I'm a Civil so bear with me on this one. I've recently installed an electric fan in my car to replace the mechanical unit it once had and am running into a problem. When installing the fan I could not find a wire thick enough to connect to the fan so I just hard wired it to the battery (fan is on a thermostat controller so it's not on all the time). But the issue i'm having is that when I turn the car off and the fan is on it keeps running I want the fan unit to only have power when the vehicle is running.

I was thinking of using some sort of switch that trips when it gets a 12V current from the ignition (I had this with an amp I used to have). A buddy of mine suggested a IGBT or a MOSFET. Would this work? If so say I want it turned on by a 12V signal, when the fan turns on there is a very high amperage peak of 60-70 amps but the constant draw is about 30-35 Amps with a 15 volt max. What should I use?

 

[X49]

Is there any particular reason you don't want the fan to stay on after the car is shut off? How long does it stay on? I have seen stock vehicles that keep the radiator fan for a short period after the car is shut off. At 30 amps, it would take a couple of hours to completely drain your battery, so if the fan stays on for 20 seconds it won't hurt anything.

 

[JBR87]

Well the thing is the fan will turn on and off like 5-6 times staying on anywhere from 2 minutes to 30 seconds when the car is off and more depending on how hot it is outside. I figure it's the heat rising in the rad to where the temp sensor is. I just don't like it turning on and off 15-20 minutes after the car has been shut down.

 

[Skogsgurra]

Getting used to it is one solution. Using a relay that is activated from the key (or some other potential that goes on when engine runs).

I would not use a semiconductor device for this. No need, less reliable and more costly than a relay. There are many made for this and similar purposes.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[cranky108]

Maybe a little difficult, but you can run it directly off the alternator.
However on many older cars that wasen't possible. (I only have older cars, because I can't understand all that crap under the hood).

Another option is to use a vacuum relay, so it only runs when the engine is pulling a vacuum.

 

[ScottyUK]

Gunnar got it: just use a relay.

There is probably a spare position in the fusebox for a relay which you could wire out, or just install it in a convenient and *dry* location. Virtually every radio/CD player has a switched live signal which is only live when the engine is running.


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If we learn from our mistakes I'm getting a great education!

 

[waross]

Many modern alternators only have one wire and it is connected directly to the battery and hot all the time. Others are controlled by the onboard computer and tapping into that circuit is not recommended. Oil pressure switches are used a lot on diesels to supply a signal when the engine is running. Yes there were a lot of alternators that had a control line that was switched with the ignition. On diesels they were often controlled by an oil pressure switch.
You may be able to find a suitable feed on your alternator, but this type of alternator is by no means universal. Use caution.

Bill
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"Why not the best?"
Jimmy Carter

 

[LionelHutz]

Geeze,

Just use a relay such as this one;

http://parts.digikey.com/1/parts/748175-relay-auto-spst-12vdc-70a-qc-cb1ah-12v.html

or

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=255-2079-ND

Find a 12V accessory power wire. If it's an older car there is probably an "Accessory" spade terminal right in the fuse block. Otherwise, use a fuse tap or something else to get the power from the accessory fuse. This way, the fan will also turn off while you try to crank the car.

Oddly enough, this doesn't sound anything like a problem a civil engineer would have while working....

 

[JBR87]

Thank you Lionel,

That's just what I needed.