PWM for motor speed control

parts-man73

A friend of mine is asking for help with a project. He's got a windshield wiper motor from a car, and has it mounted to a bracket and somehow it moves a fishing pole. The purpose is to move the lure in such a way that's more inviting to the fish. He's taking care of the mechanical aspects, but asked for help with the electronics.

He wants to have an adjustable speed for the motor. I had first thought to pulse the motor, going 1 revolution, stop, pause for some time, repeat. There is a switch on the motor that pulses once per revolution to facilitate this. But the motor coasts for about 1/2 a revolution when the power is removed. I don't think this is the answer. And my friend requested that the motor rotates at a constant rate, but that the rate be adjustable.

My second thought, use PWM to vary the speed of the motor. I want to use parts that I already have on hand. I have a IRL2910 MOSFET that I think might be up to the challenge. I also have a few TIP120G Transistors on hand. I want to provide the Pulses with a Propeller as well as providing input via a switch to change speeds, and output via a 7 segment LED to indicate current speed. A propeller is overkill, as the simplest microcontroller could handle this task, but well....

Some specifics for the motor. I checked current drawn by the motor...

• with no load - about 1.2 amp
• start up current is approx 3 amp
• current when stalling the motor jumps to around 5 amps
• using a 12 volt deep cycle battery to power this project.

I would like some suggestions on what to use, the MOSFET or the Transistor? or other?

Thanks - Brian

Phil Pilgrim (PhiPi)

To stop the motor from coasting, you can simply short its leads together once power is removed. This provides a braking action and can be accomplished with either a relay or a solid-state device like a MOSFET.

-Phil

parts-man73

Thank you, That might be a good answer too. I just shorted the leads of the motor together (with power removed) and the motor stopped dead.

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Brian

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