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Pulse Width Modulated (PWM) Controller for 12 Volt Motors

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This electronic controller is designed to allow a user to vary the speed and power output of a typical 12 volt motor such as a fuel pump, water injection pump or cooling fan. It could also be used as a secondary injector controller. Other uses, robots and small electric scooters and carts. Anywhere a 12 volt DC motor needs to vary speed or power. This controller circuit allows setting a "Low speed" when full power is not needed and a "Hi speed" for use when full power is needed. An additional feature included is a "Progressive" feature that smoothly ramps speed up from Low speed to Hi speed based on an input signal of 0-5 volts. This circuit will be offered both in kit form and fully finished.

The inspiration was a request to control the speed of a large positive displacement fuel pump. The pump was sized to allow full power of a boosted engine in excess of 600 Hp. At idle or highway cruise, this same engine needs far less fuel yet the pump still normally supplies the same amount of fuel. As a result the fuel gets recycled back to the fuel tank, unnecessarily heating the fuel. This PWM controller circuit is intended to run the pump at a low speed setting during low power and allow full pump speeed when needed at high engine power levels.

Motor Speed Control (Power Control)
Typically when most of us think about controlling the speed of a DC motor we think of varying the voltage to the motor. This is normally done with a variable resistor and provides a limited useful range of operation. The operational range is limited for most applications primarily because torque drops off faster than the voltage drops. Most DC motors cannot effectively operate with a very low voltage. This method also causes overheating of the coils and eventual failure of the motor if operated too slowly.

Of course, DC motors have had speed controllers based on varying voltage for years, but the range of low speed operation had to stay above the failure zone described above. Additionally, the controlling resistors are large and dissipate a large percentage of energy in the form of heat.

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