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Old 11-07-2011, 08:41 AM   #5175 (permalink)
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Originally Posted by JoeG View Post
Hello all,
I'm just putting the finishing touches on my 500A Revolt controller that I got from Paul & Sabrina. It's seems to be working fine and has turned the wheels on my still unfinished 91 Geo metro conversion using a 12V battery. I haven't got the traction pack yet for final testing.
For cooling I've mounted 3 CPU coolers with heatsinks and fans.
My electrical knowledge is limited and I've been unable to come up with a simple way to have the fans automatically come on at 100deg F(40C) or so.
It seems that all the radiator fan controllers start at 160F, which is higher than I would like.
Does anyone have any suggestions?
Are these two, three or four pin fans?

Two pin fans take just +12V. A few have external thermal sensors on a short wire that influences the speed, but most spin at full speed all of the time.

Three pin fans add a tachometer output so that a fan monitoring circuit can sense how fast they are spinning, and detect when they have failed. The output is open collector, and needs a pull-up to the high logic level.

Four pin fans add a PWM input for speed control. The input expects a 25KHz PWM signal. It has a has a weak internal pull-up to 5V, so if nothing is connected to the pin the input looks like a 100% PWM and the fan runs at full speed.

The input requires little current, and can easily be driven by controller output if you have a spare output pin that is 5V tolerant. Many modern chips run at 3.3V or less, and use a simple open-drain MOSFET as a buffer. But the Revolt uses 5V and could directly drive the fan speed control.

But in the end you'll probably find it simpler to replace the fans with ones that have thermostatic control sensors than add smart fan control to a controller that doesn't have it built in.
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