I don't know a whole lot about electronics, but after over-thinking this project, I'm getting more and more curious. What I have is a 12v 30a (havn't measured but that's what I've read) cooling fan in my truck. The fan has 2 power leads but is a single speed fan. I would like to run full power to one lead and partial to the other, depending on temp. At first all I wanted to do was find a resistor to drop the speed of the fan in half. After reading on resistors, I can across NTC thermistors. However, I can't seem to find a thermistor that would be rated for that power. My other thought was a rheostat style relay, but I can't find anything like that. Something where the power output of the high power is determined by the power input of the low power, which would be controlled by the thermistor. In other words, instead of a simple off and on relay, a variable style controlled by the input. I guess what I'm asking is...

1. for forgiveness for probably not knowing what I'm talking about and confusing everone!
2. Is there such a relay, or am I looking for something different?
3. Am I over-complicating this (I think so but it's fun) and should I just put some sort of resistor in and call it good?

 

We are a bit iffy about projects like this. The fan has an unknown function - the two power leads seem to indicate something. Maybe you can get to a source for the wiring diagram to see what that second poser lead might be.

One problem is the result of a failure in the circuit - will it let the fan run at full speed, or leave it (and shortly, your engine) dead? Underhood electronics and wiring have to be very rugged to survive the temperature ranges and so on.

And perhaps you could tell us the purpose of the lower fan speed?

 

I originally thought the fan was a 2 speed, which is what I wanted. I found out that the fan was out of a v6 mustang which has it's speed controlled by the computer. The wiring diagram specifically says "not used" on one of the + terminals. I had everything set up for a 2 speed fan before I realised this.




Those are the diagrams I followed for the wiring. It works perfect except for high speed on both leads, because of the fan. If what I'm asking for fails, there is still the high speed circuit for a fall back. As far as why... why not? The lower speed would mean less power used, as well as longer fan life. It also makes more sense to me that the fan only use the power it needs to do it's job, which is why the thermistor is so appealing. I also just want to do it to do it. I like to tinker and learn and find it easier by doing if I can get pointed in the right direction.
I understand if you don't want to help for liability issiues or what-not, I know people try some crazy things out there. I would at least like to know if the variable relay that I'm describing is out there.

 

Unfortunately the resistor method would not save as much power as may be expected. At the moment if the fan uses 360W and you want to reduce that to 180W with a resistor, the resistor would waste 75W which would make the total 255W.
Also a resistor capable of 75W is very big and requires heatsinking.

 

In other words, no good. PWM would be the way to go.

That way you just turn the signal on and off very fast to change the motor speed.

PWM DC MOTOR CONTROLLER google search:

heres a result:
http://www.aaroncake.net/CIRCUITS/motorcon.asp