Why isn't this pot changing the speed of my motors?

Discussion in 'The Projects Forum' started by jiniej, Apr 14, 2014.

  1. jiniej

    Thread Starter New Member

    Apr 14, 2014
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    Hi! I just made an account here because I've been tinkering around with Arduino and circuitry for a while and finally sat down to take it seriously. Yesterday I made this 'elevator' that raises and lowers a tupperware outside my dorms window... for fun.. I wanted to add a speed control and based on my limited knowledge I thought a linear taper potentiometer would do it, by slowly increasing resistance on the current running to the motors, the speed should decrease. My circuit looks like this:

    [​IMG]

    The behavior of my circuit that results from this is that the motors run on full power only when the pot is fully on, as in the wiper in the pot is touching the lead pin, fully connected(turned all the way left in the photo). Turning the pot even the slightest degree to the right, i.e adding any resistance, turns the motors off... why? I have no formal training in electrical engineering, but I am savvy and do know some... Help!1!1!
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    If you have a multi-meter , measure the resistance of the motor, then compare that resistance to the resistance of your 5Kpot.
     
  3. jiniej

    Thread Starter New Member

    Apr 14, 2014
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    hm... ok. don't have one but am gonna buy a used one today for like 5$.. will report back
     
  4. ericgibbs

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    Jan 29, 2010
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    OK,
    Its most likely that the motor measures a very low resistance, say 1 thru 100 Ohms.
    So as soon as you turn the 5K pot from zero ohms the motor will not receive enough current to run, its all been 'dropped' within the pot.

    There is also a high risk of damaging the pot if the current thru it exceeds the rated specification.
     
  5. Nykolas

    Member

    Aug 27, 2013
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    A 12V computer fan, used as an example, draws 100mA or more. Not accounting for dynamic load changes (speed variations, load changes, mech. friction, temperature), use Ohms law to get your max pot value to reduce the voltage to half: 4.5 V/0.2 Amp=22.5 Ohm. That battery will not last long! E
     
  6. strantor

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    Oct 3, 2010
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    Considering the max Ohms you can add in series provided by Nykolas, consider that your 5Komh pot likely is the standard 320 degree turn, and 5000/320 = 15.6 ohms. So a turn of only one degree puts you already near the max.
     
  7. jiniej

    Thread Starter New Member

    Apr 14, 2014
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    Yes! I have noticed the battery dies quick!!! Why??? What can I do to improve this circuit? Would a wall supply work best? (Well obviously yes it would, but is it my only option?)
     
  8. jiniej

    Thread Starter New Member

    Apr 14, 2014
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    So I need a smaller value pot?
     
  9. strantor

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    I'd say you need to rethink your approach. You could probably get it to work with a smaller value pot, but why? The pot you would need would be a high wattage pot, not cheap. You've already got an arduino capable of PWM output. Why not use that, with a $.50 transistor, to vary the speed of the motor?
     
  10. jiniej

    Thread Starter New Member

    Apr 14, 2014
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    Well mostly because i was hoping to use the arduino for something else... But yes, this does sound easier, I have a lot of different resistors and transistors, as well. What is the general setup you have in mind, strantor? Would I still need to use the 9V or could I power the motors as well as the arduino with the USB--> wall charger? If this is the case, I will switch to Arduino... Just for knowledge sake, which pot would I be looking for if I decided to go that route (As in changing my 5K pot in the diagram above)?
     
  11. jiniej

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    Apr 14, 2014
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    Something like a 100Ohm 1/2 watt pot?
     
  12. strantor

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    Yes you can power both from the same source. It would be a good idea to use decoupling/bypass capacitors especially if operating on a breadboard, as the switching on/off of the motors can cause sudden voltage spikes/dips in the whole circuit, which the arduino won't like. Also you should add flyback diodes across the motors to prevent blowing out your transistors.

    info

    that will depend entirely on how much current the motors pull. Nykolas quoted something like 20ohms IIRC; that sounds like a good first guess, but still it will depend on what you measure with your soon-to-have DMM. And my best guess is somewhere around 2W.
     
  13. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Modern automobiles are full of all sorts of handy electronic controls and sensors etc, for e.g. you could try a windshield wiper motor control from an one of these wrecker outlets.
    Some let you loose to strip what ever you need for a few $'s.
    An alternative is a 555 variable pulse width IC control output to a Mosfet to drive the motor.
    Max.
     
  14. crutschow

    Expert

    Mar 14, 2008
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    A simple (but power wasting) way to control the motor with a pot is to use the pot to control an NPN power transistor. Connect the wiper to the transistor base, the emitter to the motor, and the collector to the + supply (either a larger battery, such as 6 D alkalines in series or a wall-wort with sufficient current to power the motor). The two other pot leads go between the plus supply and common.

    You may need to go to a lower pot value such as a 1kΩ or 500Ω device depending upon the motor current required.

    Note that depending upon the voltage dropped and the motor current, the transistor may get hot so would need a heat sink. A common power transistor that should work for you is the 2N3055.

    A much more efficient way is to use a 555 configured as a variable duty-cycle astable oscillator, with duty-cycle controlled by the pot, driving an N-MOSFET, as suggested by Max.
     
  15. GopherT

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    Nov 23, 2012
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    No mater how you connect the motors or control the speed of the motors, a 9 volt battery is not going to last very long.
     
  16. atferrari

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    Jan 6, 2004
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    "B" means linear in all pots I have seen up to now.

    Maybe for yours could also mean "burnt". :p
     
  17. THE_RB

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    Feb 11, 2008
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    Or use the pot, an LM317 and another resistor. That gives a variable regulated voltage to drive the motor so you get a better linear speed control.

    The disadvantage is that the LM317 will drop about 2v so your max voltage to the motor is battery-2v, and the minimum voltage to the motor will be 1.25v (the minimum of the LM317).

    But it will give a good linear speed control from battery-2v down to 1.25v.
     
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