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Pulling down 5V to ground

Paker

Apr 30, 2017
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My old car has a belt driven fan that spins whenever the engine runs. Not good on gas mileage. I would like to replace it with an electrical fan that runs only when needed.

This is how electric fans are controlled in many cars. A fan control module sits next to the fan. The module is a Normally On MOSFET module. It has an internal 5V source (Normally On), which turns on the MOSFET circuit, which drives the motor. Car computer controls the module with a single wire. This wire is connected to the internal 5V. When the computer grounds it, fan motor stops. The computer generates PWM ground signal based on engine temperature and car speed.

I need to fabricate a circuit that generates PWM ground signal. I found a computer fan control circuit built on TC648 PWM chip. The output pin has 0-5V PWM signal. But this pin can sink up to 45 mA. I need a circuit connected to this pin that can positively pull 5V down to ground. Or do I not? Can someone help me with this?

I have basic understanding of electricity, but not good enough to design an electronic circuit. Thank you. Pulse width is in the order of 10 ms.
 
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Bluejets

Oct 5, 2014
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Seems a long way around.
Most that I've seen just run a relay control from a temp sensing module.
If temp within a certain range the fan runs. Simple.

Cannot see why an ecu would have a mosfet continually powered as you say.
Car speed would have little input I would imagine as it is primarily a temp control.
Most auto circuits are earth return if that's what you are getting at.

I doubt anyone has the time or inclination to design circuits for you.
Help you out , yes.
 

Paker

Apr 30, 2017
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Thank you for the reply. As you noted, if this gets tricky, I will use relays.

ECU normally keeps the control wire at 0V. So mosfet is normally off.

Car speed has much to do with engine cooling. When a car moves at about 40 mph, air movement is adequate and electrical fan is turned off. It's how most cars control the fan module.

I need help only on the circuit that pulls 5V to ground. My plan is to use npn transistor, E=ground, C=control wire (5V), B=PWM chip output pin (5V). 1K resistor in the base. No resistor in the collector. Does this look okay to you? I think I need a fast switching transistor so minimize mosfet heat. Am I going in the right direction?
 
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Bluejets

Oct 5, 2014
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Car speed has much to do with engine cooling. When a car moves at about 40 mph, air movement is adequate and electrical fan is turned off. It's how most cars control the fan module.

Yes but most engineering concepts would not look at using 2 sensor input when one ( engine temperature) is sufficient. (and they usually do as far as I have ever seen)
 
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