What is the right size pot for my little hobby motor?

Discussion in 'The Projects Forum' started by Timmy Jones, Jan 20, 2008.

  1. Timmy Jones

    Thread Starter New Member

    Jan 20, 2008
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    I have a little hobby motor, a small one from radio shack. I want to make a variable speed control on it from very very slow to as fast as it will go.

    I used a 10v/700ma output walwart for a device I have here at a home and a connector from radio shack so I could connect it to the motor.

    In between one of the terminals I soldered a mini volume control pot #271-215 from radio shack to the #1 terminal and the middle terminal. The other terminal (3) I just left unused.

    I plugged it in and it adjusted the speed as planned but within a few seconds the pot started smoking an emitted a bright light so i quickly unplugged the walwart.

    The pot says Rated Power .05 watts. Switch rating 3amps at 30vdc. It is a 10k ohms with audio taper.


    Question:

    What am I doing wrong? I am trying to make a variable speed control on the motor from 0v to 2v. The motor is from an device that used two AA batteries for power.

    Do I need a different power supply or a different pot?

    Thanks for any help you can give me.

    Timmy
     
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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  3. hgmjr

    Moderator

    Jan 28, 2005
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    You have been officially initiated into the electronic world. Everyone has to release some smoke from a component at some time. Get use to the smell. You will need that smell memory in the future when you encounter another pending smoke release.

    The current drawn by the motor was too great for the poor little potentiometer to endure.

    It is generally not a good thing to use a resistor in series with a motor to control the speed. The series resistor has the effect of robbing current from the motor when the motor is loaded. The result is a loss of torque.

    hgmjr
     
  4. Timmy Jones

    Thread Starter New Member

    Jan 20, 2008
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  5. Timmy Jones

    Thread Starter New Member

    Jan 20, 2008
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    Thanks for the initiation. :) If not a pot then what would be best for variable speed from 0 to maximum?
     
  6. hgmjr

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    Jan 28, 2005
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    To handle the motor speed control properly requires the use of a technique referred to as PWM (Pulse Width Modulation).

    If you are interested in this approach, I would suggest you google search for the phrase pulse-width-modulation with keywords motor and speed-control.

    hgmjr
     
  7. hgmjr

    Moderator

    Jan 28, 2005
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    Here for example is just one of the many documents I located when I googled on the subject of pulse width modulation.

    hgmjr
     
  8. Timmy Jones

    Thread Starter New Member

    Jan 20, 2008
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    I saw your article and I appreciate your showing it to me and respect your extreme intellect but i do not have even the aptitude to grasp that stuff without making it my full time occupation and even then.... anyway.. is there an easy way for me to simply put some kind of speed control on this little motor? from 0 speed to maximum?

    Thanks again much appreciated

    Timmy
     
  9. hgmjr

    Moderator

    Jan 28, 2005
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    Another approach that would work would involve the use of a 3-terminal regulator such as an LM317 or equivalent. These devices can be obtained from a Radio-Shack or on-line from Digikey, Mouser, or Jameco. The price is a couple of dollars.

    In effect what you would be doing is building a variable voltage source. This would provide the low-impedance drive to the motor and permit you to use a standard potentiometer to adjust the output voltage. The input voltage would come from your wall-wart.

    Is that something you would be interested in?

    hgmjr
     
  10. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    hgmjr: LM317s don´t regulate from zero in usual connection, it would need something better than regulated voltage source.
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Do you know how to solder and read a schematic? If not, it will be difficult for you to make your own speed control. If you do, we can give you a circuit for a simple PWM.
     
  12. techroomt

    Senior Member

    May 19, 2004
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    i am not positive if it will work, but i would try a rheostat rather then a pot. a standard wall control light dimmer.
     
  13. Ron H

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    Apr 14, 2005
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    I think most wall control dimmers will only work with AC, and only on voltages much higher than 3V.
     
  14. hgmjr

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    Jan 28, 2005
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    Based on the availability of the 10V DC power supply, I thought that Timmy could use the LM317 as an adjustable power source for powering his motor. Granted an LM317 would provide an adjustable voltage that would range between a low of 2 volts and a high of 8 volts but I thought that would be a simpler approach than to tangle him up in the design of a PWM circuit. Specially since he had expressed a reluctance to tackle the PWM approach which is clearly the superior of these two methods.

    hgmjr
     
  15. Audioguru

    New Member

    Dec 20, 2007
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    RadioShack doesn't have a clue about electronics.
    Their spec's are useless.
    Max current?
    Average current?
    RadioShack doesn't know anything about current.

    Even for an LED they don't say its brightness. Duh!

    RadioShack has kids stealing discarded junk from the floor of factories then sells it with a huge profit.
    No wonder they are gone from Canada. Their replacements are called. "The Source by Circuit City" (good American ripper-offer store)
    I think they are called Maplin in the UK.
     
  16. SgtWookie

    Expert

    Jul 17, 2007
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    Radio Shack DOES sell a small rheostat; I don't recall it's rating offhand, but it was several Watts. Wouldn't be very efficient, but it would work to control the motor's speed - right up until the batteries got tired of fighting that rheostat ;)
     
  17. scubasteve_911

    Senior Member

    Dec 27, 2007
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    How about a simple power opamp? It isn't as efficient as PWM, but you can rig one up very easily and it is more intuitive for the beginner.

    You can get a sample of one from Fairchild semi.
    http://www.fairchildsemi.com/pf/KA/KA334.html

    or the L272 from ST, you can get through digikey.. Save your time and energy and stay away from radioshack, they're awful...

    Steve
     
  18. Ron H

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    Apr 14, 2005
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    A rheostat won't increase the load on the battery. A pot would.
     
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