rheostat or PWM?

Discussion in 'General Electronics Chat' started by enginerd15, Apr 14, 2008.

  1. enginerd15

    Thread Starter New Member

    Apr 8, 2008
    I want to simply control the speed of a small (< 1/100 hp) DC 24V gear motor. A rheostat seems like it should work and so does a PWM (pulse width modulator). I understand the basics of each, but is there more to consider when picking between the two? What are the pros and cons of each?

    Any advice? Thanks,
  2. DC_Kid

    Distinguished Member

    Feb 25, 2008
    sure. there is obviously more to it than just "pwm". there will be needed circuitry to be able to change the pw. the frequency of pwm would need to be chosen with care (30Hz, 100Hz, ??, etc), and the frequency might need to change when pw changes, all kinda depends on the motor being driven, etc. a rheostat sounds simpler, but it too has some things to worry about. will the motor be ok with lower voltage drive?
  3. SgtWookie


    Jul 17, 2007
    Rheostat = extremely simple + extremely inefficient.
    PWM = moderately simple + extremely efficient.

    Rheostats tend to get expensive in the higher wattage versions. Even with a 1/100 HP motor, that's still about 7.5 Watts. Trying to "go cheap" will result in a burned-up rheostat, particularly if you run the motor at low speeds for all but brief periods - since that's when the rheostat will have to dissipate the most power.

    Over in the Projects Collection forum, I posted a "simple PWM" circuit months ago. You could use that, but you would need to add a voltage regulator of between 10-16V to power the 4093 CMOS IC. A 7810, 7812, or 7815 regulator would be simple to add.