I'm interested in building a fan controller from a gm Coolant temperature sensor (thermosistor) and a npn transistor for my car, I know there are fan controllers out there, however I would like to make this a learning experience, and see if I can produce one cheaper than buying one.
My research so far has led me to this:Java Applet Circuit simulator

The potentiometer is to fine tune the fan on temp.
The dtsp switch at the bottom is to represent the the sensor. It should reach around 350ohm at 190 degrees, and it should be around 175 degrees at 500ohm.
The Lamp on the far right is to represent the fan motor it will be a fused 25 amp circuit.

Now that circuit has 2 major problems that I can see.

One being that the temp sensor loses impedance as it warms. That means with that circuit I have cobbled together I would have to run a spdt relay and wire the fan to the N.O. terminal, which I would like to avoid for the sake of keeping the coil on the relay from over heating.

The second problem is this circuit has no hysteresis, and I cannot make the fan cut run until it reaches a certain temperature. This is not as big a deal to me, I don't particularly need it to have a certain run time. Just so long as it does not run over the temp I have designated. I would guess a capacitor could maintain a minimum run time?

I have a very limited knowledge of circuits and components and any help would be appreciated. I plan to build it with breadboard and a plastic enclosure once I get it sorted out.

 

Can't see your circuit....

 

This thread may be closed soon, as they do not permit automotive projects to be discussed here.

 

Well that sucks about the automotive part.
Also it will not load the circuit when you click the link?
Here is a screencap:
Drop Box Screen Cap
Also I apologize for posting an automotive project. But in its essence it is no different from designing an in-expensive geo-thermal heating setup.

 

Use an op amp to set the desired temp..with hysteresis

 

The owners of All About Circuits has elected not to host discussions of automotive electrical system modifications/enhancements due to safety concerns, the potential of legal ramifications and the possible circumvention of vehicle regulations at the state and federal level.

This thread is against the AAC forum rules, Chapter 6, as seen here:

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Another good forum that shares many of the same membership is http://www.electro-tech-online.com/ . Good luck.

 

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