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Can i adjust a 12V DC motor's RPM with a Dimmer?

  1. Jun 29, 2009 #1
    Dear electronic gurus,

    (2 part question) I want to check that:

    If i got a 12V motor (such as http://scientificsonline.com/product.asp_Q_pn_E_3041860" [Broken])

    would i be able to adjust its RPM if i got a:

    12V dimmer switch box (http://cgi.ebay.co.uk/LED-Lighting-...6:2|39:6|72:1683|240:1318|301:1|293:1|294:25")

    and a 12V battery pack
    and some circuit wires

    What am i missing from this list, in order to get a functioning motor where i can adjust
    its RPM?

    Secondly, what would i need if i wanted to make the motor spin the other way? Is there a special dimmer switch that exist that would let me do this?
    (I was told that its a matter of inverting the current?) I am aware that i could cross the wires on the motor, the other way -
    but i was hoping for an easy, less fiddly way of doing it)

    Thank you!
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Jun 29, 2009 #2


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    This circuit with a DPDT switch will let you reverse the direction without swapping wires. I used a buzzer (because that's all I had readily available in Eagle) to stand in for the motor, in case you wondered about the odd part numbers.

    Adjusting the speed is a little trickier and, no, you can't use a lamp dimmer. I don't have a time to go into the intricacies of motor speed control at the moment, but I'll post more later if no one else provides good info for you. In the meantime, look into PWM speed control, which is a simple and efficient method, though it requires a bit more circuit-building than your original suggestion.

    Attached Files:

    Last edited: Jun 29, 2009
  4. Jun 29, 2009 #3
    Thanks negitron. ill read up on that.

    Does that mean that my original circuit is ok
    (with just the motor, dimmer and battery pack) if i didnt want the motor to spin the other way
    but say still wanted to asjust the RPM?

    (or are you saying that adjusting the motor with the dimmer is not possible to begin with?)

    Sorry if this seems very inept of me.
    First time doing a project with electrical wiring.
  5. Jun 29, 2009 #4


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    The schematic in the link actually includes an SDPT reverse switch like the one I drew, so you can simply use that do do both. The switch only reverses the motor, the rest of the circuit controls the motor speed by adjusting the amount of average power that is delivered by using a technique called pulse-width modulation. You can also accomplish this with less efficiency using a rheostat (a lamp dimmer is NOT a rheostat, despite the superficial resemblance). Any old SPDT will do provided the contacts are rated to handle the motor current under full load. Ditto for the rheostat, if you choose to go that route, however, note that using a rheostat to control motor speed is often fiddly, particularly if you're careless in selecting an appropriate value.
  6. Jun 29, 2009 #5


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    That Ebay dimmer looks like a variable resistor type. They would probably say so if it was pulse width modulated.
    A variable resistor type would affect the motor torque badly at low speeds.

    Hobby shops that supply parts for model trains would carry proper pulse width controllers. These are much better.
  7. Jun 29, 2009 #6
    Hi Negitron,

    Ok. So i need a SPDT 'plus' a rheostat to accomplish what i want to do...

    You mention that the SPDT contacts need to be able to handle the motor's current under full load... (is this the voltage? as in the SPDT has to handle 12V)?

    would you be able to perhaps recommend an appropriate rheostat and SPDT switch that would work with either of the 2 kinds of 12V motors i am considering (links in first post)?

    Perhaps a bit of background to what i am trying to do. Im attemping to make a rotating plinth (like those in jewelry displays). Because i want it to turn very slowly, im trying to go for a motor with a low RPM - and then find a way to adjust it with, what i am now assuming, is a SPDT switch and rheostat combo. I also want the plinth to be able to turn the other way if need be. I dont really need to be fiddling with the RPM once ive got it right, but id want the option to control it.

    Thanks so much again.
  8. Jun 29, 2009 #7
    Hi vk6kro,

    So is a pulse-width controller the same thing as a rheostat?

    Would i use it along with an SPDT switch?

    Would a pulse-width modulator allow me to change the direction of the motor's spin, or just adjust its RPM?

    Thank you
  9. Jun 29, 2009 #8


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    No, a PWM modulates the average power delivered by turning the voltage on and off at regular, rapid intervals of varying width. The longer the voltage is on compared to the time it's off, the higher the average power delivered. A rheostat is simply a variable resistor. It controls the delivered power by turning some of the supplied power into heat. Heat is waste, which is why I said earlier that PWM is more efficient; it is likewise more complex to use because it required control electronics to generate and modulate the pulses.

    You need the switch to reverse the direction without swapping wires. The PWM or rheostat controls the speed and the switch controls the direction.

    You're probably better off with a gearhead motor, since they already turn slowly. They have a reduction gear train built in, which means they can deliver high torque (turning power). These are the sort of motors used in commercial rotating displays.
  10. Jun 29, 2009 #9


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    A rheostat is a resistor that you can vary. It works like a volume control in a radio, but only uses two of the three connections.

    The problem is that when you put resistance in series with a motor and the motor needs to draw more current with a heavier load, the voltage across the motor drops and you get a rapid decrease in power.

    Pulse width modulation is different.
    Imagine you could switch power on and off to a motor. If it was on half the time and off half the time, it would get an average of half the supply voltage and would run slower than if it got power all the time.
    You could vary this and get short "ON" pulses and thus get lower speeds.
    If you did it fast enough you wouldn't notice the motor stopping and starting.

    In its simple form, you would need a switch to reverse the direction of the motor.

    There is a more complex circuit (called a "H bridge") which lets you have a centre "stopped" position and increasing speed in opposite directions as you rotate the control away from the centre position.

    Pulse Width Modulation puts the full supply voltage on the motor when it is on so the torque of the motor is not affected by series resistance.
  11. Jun 30, 2009 #10
    Thanks again guys.

    Ok. So a rheostat seems out of the question, because i dont want to be losing any torque or voltage in the process.

    It seems like this h-bridge thing is exactly what im looking for:
    So it allows me to adjust the voltage (adjust the RPM) as well as let me reverse the current (for the motor to spin the other way) and it wont lose any voltage like a resistor/rheostat?

    I've found some custom built h-bridges on ebay - is it just a matter of plug-n-play with the motor?
  12. Jun 30, 2009 #11


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    With what you have said about your application, it seems that PWM and a H bridge might be a bit of an overkill.

    Probably you need to consider the mechanical aspects of what you want to build.
    It could even be as simple as an old battery powered record player mechanism.
    This would solve some of the mechanical details for you and there must be thousands of such players in basements and attics which will never be used again.
    These have DC motors which could be reduced in speed to about 30 % of their current speed without stalling

    A H-Bridge is really only 4 power transistors driven by slightly smaller transistors, so it is no big deal to put together. So don't pay serious money for one, especially if the PWM control is not included.
  13. Jun 30, 2009 #12
    Thanks again guys.

    And thanks for the link to the gearhead motor negitron - that really helps alot.

    I've come across this PWM (https://www.amazon.com/dp/B001T7H0V...LGULW&tag=shopzilla_rev_1044-20&linkCode=asn").

    Would this work with the motor you suggested? (Perhaps another link to one online?)

    If not, i was just thinking of ordering that motor from amazon, then bring it to a modelshop/electrical supply store, and ask them for an PWM that will make it work.

    vk6kro - Im not too keen on dismantling anything, and i dont have a an old record player (or anything that spins) other than my microwave - which im scared enough to touch as it is. :) Thanks though.
    Last edited by a moderator: May 4, 2017
  14. Jul 2, 2009 #13
    100 Hz is kinda low. Those do seem kind of expensive too.

    I wrote a small paper on making an H-bridge/CA module, if you want email me and I'll send it to ya.

    Now, the motors seems pretty small current. That means you have no need for hbridges that can handle 30 amps.

    Also, if you want it to rotate only one way that'd be trivial with a 555 timer and a mosfet/transistor.
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