Advanced Engineering Tech - Adjustable fan switch?

I have some electric fans I bought to put on an old ford truck I have. I want to put in a knob (like a potentiometer) to control the fans so I can turn them up and down when I need to.

What would be the best route I can take to achieve this?

Could I use a Rheostat for this job?

???
http://www.ls1tech.com/forums/advanced-engineering-tech/396223-what-forum-please-read-before-posting.html

Nobody asked for a fucking smart ass. This being the engineering section I would think there would be an electrical engineer that could help me with my question.

I need a variable resistor that starts at high resistance and goes to zero, and can handle 250+ watts.

I haven't been able to find one that is decently priced. If I have to, I would take some suggestions and be open to making one.

God damn buick dousche bags.... All I asked for was help.

FUK YOU, you are the douche bag. If you can't wire up something that simple.. you need more than an EE to help you.

Buick douchebags?? You must have had your dumass handed to you by a Buick!

I asked a question, and its not even a question really. I have an idea of what I'm going to do. I left what I asked pretty open ended so I would hopefully receive induvidual answers that were not based off of my own ideas. That way I could see someone thing from someone elses perspective and maybe see a different solution.


Oh yeah, your on the money dumb ass. I've never raced a buick. Just seen how all those hens start acting when they get together.

hook the 24 vac common up to ground.

F.U.... no F.U.... no you are :lol:

anyway, it's actually a technical question when you get into it for what's being specifically asked.

where is 24 vac coming from ?

the problem with a rheostat is it'll be huge and cost you a few hundred bucks to handle over a 100 watts, plus you would have to run 10-gauge wire from the relay under the hood into the cabin where your rheostat is then all the way back to the fan motor.
DC motors can easily be varied by adjusting the voltage to them, but the problem most of the time is you only need a slight change in voltage. Most rheostat's are like 1/2 ohm to 5000 ohm or more, so you're going to turn the rheostat knob a couple degrees and your electric motor is going to shut off because off a lack of voltage. For instance, if you're fan motor draws 10 amps, then 1 ohm of resistance will have a voltage drop of 10 volts with only 4 volts getting to the motor assuming input voltage of 14 volts, and chances are your fan draws up around 15+ amps making the voltage drop worse; you would need more like 0.1 - 0.3 ohms resistance to get a drop of 1 to 3 volts to lower motor speed.

what you want is PWM - pulse width modulation.
search for PWM speed control DC motor.
that is how it is done on electric fans, such as on my 2003 jeep GC which I had to replace the PWM relay last year which cost $70.

http://www.flex-a-lite.com/auto/html/vsc.html
http://www.frsport.com/DIF-10005-Thermostatic-PWM-True-Variable-Speed-Electric-Fan-Controller_p_10337.html

i do like your solutions and ideas but they are very pricey.

you could go with having stages like fan one on fan two off. both on for high. that is cheap and simple. i guess that is how most cars are already set up. but is true variable control worth 200 dollars? i would think most of the time you are too hot because you are sitting still or you are ok because you are rolling. so there might not be the needs for fine granularity. also with pwm i am sure your electrical system will love the big spikes unless the box takes care of it with some ee magic. ;)

possibly look into putting a switched load in line with the 12 v source. mebbe you could look into taking out the switch from a box fan (yes i know its ac) or some other consumer appliance that just uses a switch for speed control. it wouldn't be extremely adjustable but would have a couple of steps and not screw you out of two Benjamins. it would need manual intervention though.

for just a friendly convo there is nothing wrong with this topic though.

what you can also do is what is done on the 4th gens....
for full speed the two fan relays open in parallel so each fan gets full 12v power.
but for slow speed the relays open in series so power goes through fan1 then through fan2 then to ground, increasing the total resistance of the circuit and making the fans run at a slower speed. Why not set up two relays like that to give 2 fan speed settings, low and high?

The truck I am putting it on is a trail rig. Thats why I want full control of the fans. Sometimes I may need more, less, or none. And if I want to leave them on to cool the truck off while its not running this wouldnt drain my batteries so much.


The PWM controller is exactly what I'm looking for. Another member recommended me the same thing and pointed me to some on ebay. Most of them are made in China but they are fairly cheap 40-80$. Even if I did blow out I could probaly make more and beef it up from the one I buy if it doesnt last.

This whole time I've been thinking of how to use a potentiometer's reading to make something that will control the voltage. Maybe simular to an amplifier??

Anyways, I guess I can shop around for a PWM and find out. Thanks for the help.


Edit: I've found some good sites on PWM controllers and how to make them. I will continue reading thanks for the heads up.

so why would you even need to control the fan speed anyway? Your truck is going to create a surplus of power as it is. Why not just save yourself the $$ and drop in a $1-2 switch and call it a day?

I already said that I want that control on the trail. Surplus of what power? Amperage? Not when it has a winch and KC's going it won't.