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Speed variator for car A/C fan motor

A small circuit meant to replace the current limiting resistors actualy used to control the motor speed.

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In old cars (also in rencent ones but less often) fan motor speed is controled using current limiting resistors. A rotating switch selects a resistor whose value will affect the current arriving to the motor and thus limit its speed. These resistors are arranged in network and placed near the fan so they can be cooled and not burn.
In my girlfriend's car, the fan motor is getting old and consumes more current than it was used to. This causes the resistors to heat more and the temperature protecting fuse to blow.
This leaves us with two solutions. Either changin the motor to a new one, or making a new driver that will produce less heat (I hope).
Of course we went with the second solution, otherwise this project would not be.

I will use a PWM controller (MC33060A) that will drive a MOSFET, then the motor.

3 potentiometers will allow to fine pitch the 3 speeds coming out of the selector.

Two major issues are :

  • There are only 4 wires coming out of the selector : 3 carrying the battery voltage (one at 12V depending on the speed selected, and the 2 others floating) and the 4th goes directly to the motor's (+) pole. No ground, no other way to get the main Vbat.
  • And the PWM controller input has its own 5V reference, so I can't put one potentiometer per selector output to set each PWM ratio. I could do that, but the battery voltage is never exactly 12V, it can vary from 11v to 14v resulting in a changing of the fan speed.

I have several ideas in mind to get rid of these. They wil be detailed and tested in the logs.

  • First draft

    Muphins11/16/2014 at 19:20 0 comments

    Here is the circuit diagram of the actual breadboard prototype:

    It work pretty well for a first draft but the are bad timings and slopes at the mosfet output and it reduces the precision in speed selection. I can put a bypassing capacitor before the BC547 in parallel with the 22k res, but it doesn't really solve the problem. I will upload oscilloscope photos later.

    Regarding the PWM controler diagram, we see the output is already controled by a transistor, so I will also try to drive the MOSFET directly.

    The results in the next log.

View project log

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esot.eric wrote 01/29/2015 at 01:43 point

Necessity is the mother of invention :)

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