Shop OBEX P1 Docs P2 Docs Learn Events
Golf Challenge: Cheap Simple PWM Circuit — Parallax Forums

Golf Challenge: Cheap Simple PWM Circuit

ercoerco Posts: 20,256
in General Discussion
I need a bunch of small self-contained PWM speed controllers for cheap DC brushed motors. I've been able to reduce speed a bit using my favorite trick of series dropping diodes, but that loses torque.

Picture a small 3- or 4-pin board with DC + and - in, and motor + and - out. Onboard circuit has a pot to adjust speed 0-100%. Say 3-12V. Set speed & forget. No reverse. Seems like these should be a dollar from Ebay but I can't find anything close!

Alternatively it could be a 3-pin board with standard "servo" connections: +, - and motor out (common ground).

I'm about to cobble up half a dozen by hand using the serfeit (!) of 555 timer chips I've hoarded. With the diode trick I can get below 50% PWM, and it's got a decent current rating of 200 mA, so in some cases it wouldn't even need a transistor switch.

I bet the geniuses here can think up a few alternatives. Beau & PhiPi are pretty quick with offbeat solutions.

Whaddyagot?

Comments

  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2017-01-10 23:01
    Wouldn't one of these nice switch-mode regulator chips work as a motor PWM if you skip the choke etc and directly drive the motor and filter the feedback voltage?

    EDIT - actually, the speed control pot would be the "feedback" but otherwise it would just be an open-loop PWM driver.
  • ercoerco Posts: 20,256
    edited 2017-01-10 23:09
    Good call, first responder PeterJ! This video shows an LM2596, which may deliver 150khz pwm. I have some of those somewhere, might be worth trying. Thanks!

  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2017-01-10 23:15
    Yep, I'd skip all the traditional SMPS circuitry and use a SMPS chip as a bare open-loop PWM but you'd have to get that PWM frequency very low as the motors certainly won't like those high frequencies.

    The SMPS module is the easy route though if you are not worried about feeding a linear voltage to the motor. Direct motor PWM overcomes stiction and allows it to run smoother at lower speeds.
  • jmgjmg Posts: 15,173
    edited 2017-01-10 23:15
    erco wrote: »
    I need a bunch of small self-contained PWM speed controllers for cheap DC brushed motors. I've been able to reduce speed a bit using my favorite trick of series dropping diodes, but that loses torque.

    Picture a small 3- or 4-pin board with DC + and - in, and motor + and - out. Onboard circuit has a pot to adjust speed 0-100%. Say 3-12V. Set speed & forget. No reverse. Seems like these should be a dollar from Ebay but I can't find anything close!

    Alternatively it could be a 3-pin board with standard "servo" connections: +, - and motor out (common ground).

    I'm about to cobble up half a dozen by hand using the serfeit (!) of 555 timer chips I've hoarded. With the diode trick I can get below 50% PWM, and it's got a decent current rating of 200 mA, so in some cases it wouldn't even need a transistor switch.

    I bet the geniuses here can think up a few alternatives. Beau & PhiPi are pretty quick with offbeat solutions.

    Whaddyagot?

    If you need 12v and 200mA, the 555 is going to struggle a little, but there is little that comes close.

    Older Switchmode PWM drivers could do PWM, but you need one that has sawtooth access on the pins.

    If you want it to react like a servo, then you'd likely need a low cost MCU.
    Cheapest self-contained MCU that is not brain-dead is the EFM8BB1 from SiLabs.
    (or broadly similar but slower n79e715as16 )

    Both have on board CalOsc, PWM, UART, ADC so has no problems with PWM/pot/servo side, but it does needs a regulator and transistor/fet drive for 200mA/12v

Sign In or Register to comment.