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How do I figure out what size resistor I need to slow down a recycled printer motor down to it's absolute slowest speed?

The motor will be driving a small belt and a paper towel tube. This project is for Halloween. I'm trying to create a mechanical fade circuit for a fluorescent tube. The motor came from an old printer and runs at full speed when 12v is applied. I want to get the motor down to a speed where the tube rotation is as slow as possible.

verence3 years ago
A simple resistor will most probably not work. The motor is designed to run with 12V.

With a lower voltage it will run slower, but not linearly - i.e. it will run not run at 50% speed with 6V and 25% speed at 3V or 10% at 1.2V. At some lower voltage (but far over 0V) it will stop completely. And more severe: It will not start to run if you supply a voltage only slightly higher than the stopping voltage.

What you could try (without a guarantee for success) is a kind of pulsed supply - supply the motor with a switched 12V voltage - either PWM (pulse with modulation) or PPM (pulse pause modulation). Your motor will run much smoother (and with a higher torque) at lower speeds.