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Electronics Slowing down a small DC motor.

Discussion in 'Modding' started by dullonien, 23 Apr 2012.

  1. dullonien

    dullonien Master of the unfinished.

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    As part of some models I'm making for a uni project (the same ones using the hard drive components for anyone who's seen my wanted thread in the sale forum), I'm using a small DC motor stripped from an old Nintendo Gamecube. I've incorporated a series of gears to step down the speed of the motor quite considerably, but I've been trying to create a simple circuit to fine-tune the final speed of the motor, and I'm a little ashamed to say, failing miserably.

    I'm powering the motor by a 9V battery (PP3), and so far I've tried incorporating a 10Kohm potentiometer into the circuit in an attempt to create a super simple solution, but it doesn't really work. The motor only switches on with the pot set to full, and as soon as I try to reduce the speed it just stops completely. Measuring with my voltmeter shows the voltage across the motor terminals at roughly 8.5V with the pot at full, then the voltage drops to 0.11V when I try to reduce the speed.

    Ideally I'd like as much control as possible, with the biggest range of speed control as possible. What's the solution? I know I'm probably missing something really obvious, but it's been a long, long time since studying anything to do with electronics.

    Thanks in advance for any help, much appreciated.
     
  2. Furball Zen

    Furball Zen Shut up and Mod

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    A fan controller should work.
     
  3. dullonien

    dullonien Master of the unfinished.

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    That could be an option, but I haven't got one to cannibalise.

    If anyone knows what I need to add to my existing circuit to make it work that would be great.
     
  4. SuicideNeil

    SuicideNeil New Member

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    A potentiometer is the best & simplest solution ( that's all a fan controller is basically anyway, with some extra gubbins thrown in for good measure )- sounds like you just need to try a few different ratings..
     
  5. dullonien

    dullonien Master of the unfinished.

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    Yeh maybe. I've tried a 1KΩ (which I fried when trying a 12v power pack, oops), I also tried a 7.5KΩ, which started to burn when I turned it too low (running from the 9v battery), and eventually burn't out completely.

    As stated I'm now trying a 10KΩ but isn't giving me any control at all, but maybe I need to go higher? There's the option on Maplins of 22K, 47K, 100K, 470K and 1M.

    However I don't understand why the current 10K pot stops the motor when turning it down. Turning the potentiometer down basically increases the resistance, but if that doesn't work then trying a larger potentiometer will surely just make things worse surely? But the lower values fried, so I don't quite understand where to go next.

    So thinking this through rationally, maybe I need to look into incorporating a fixed resistor into the circuit to reduce the voltage before it get's to the pot, then use a very low value pot (1KΩ) to fine tune the final voltage? Have I got this correct, and would the resistor need to be in series or parallel? Does anyone know what kind of voltage I should be aiming for before going into the pot? I'm not entirely sure what the voltage range of the motor it. Also if I do this, then it might not give me much control, only varying the final speed very marginally.
     
  6. Furball Zen

    Furball Zen Shut up and Mod

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    Pots on a Sunbeam Rheostat are 10K, but you cant afford one of these? Yes i know youre across the pond, but im sure you can find SOMETHING :)
     
  7. dullonien

    dullonien Master of the unfinished.

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    Completely forgot that I do have three of those stashed away somewhere. I might give it a try, but really the 10K pot I'm already using should be doing something similar. And Ideally I'd like my own circuit, because I've already designed the bracket (that's already been lasercut) to hold a certain potentiometer from maplins. I might give the zalman fanmate a quick try though. Thanks for reminding me.
     
  8. dullonien

    dullonien Master of the unfinished.

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    Right I'm getting completely confused now. How can I have forgotten so much from my time in school?

    I'm trying to work out roughly what value resistor to buy in order to reduce the voltage in the circuit from the current 9.08V down to about 5V. I think this would be a good place to start, and would get the motor to a more manageable speed. I can then look at adding a potentiometer to vary the speed.

    It looks like I may need to build myself a voltage divider (potential divider) using two resistors in order to reduce my voltage to the 5V I want. I think I can use a potentiometer as one of the resistors, thus varying the final output.

    Can anyone help determine the values of the resistor and potentiometer I need. From measuring the circuit of just 9v battery and motor I get the stated 9.08V across the motor terminals, and I get a resistance of 3.63KΩ. Using Ohm's law I believe this gives me a current of 0.00264A (264mA?).

    The formula for a voltage divider circuit is Vout = (R2 / (R1 + R2)) x Vin.

    If I use a 10KΩ pot as one of the resistors (not sure which one tbh), what value should I put in there. Ideally I'd like the pot to be able to vary the output voltage from say 3V to 5V. I have no idea if this is possible. Also the value of 10KΩ on the pot, is that the maximum resistance? How can I find out what resistance range it offers?
     
  9. Byron C

    Byron C And now a word from our sponsor

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    I'm pretty sure that to reduce the speed you need to reduce your voltage, rather than the current. The issue you may be encountering here is the stall current of the motor: there's usually a minimum amount of current they need in order to start spinning, and using a pot - or any resistor - will also reduce the current, therefore potentially dropping the current supplied by the battery below the stall point of the motor. 9v PP3 batteries aren't really able to deliver that much current to start with.

    Is the motor speed going to be controlled by something else, or do you just want to reduce it's speed? If you're going to want to control the speed of the motor in any way - say, from a PC or a microcontroller - then what you need is really a motor driver circuit. These circuits vary the speed of a motor based on a pulse-width modulated (PWM) input signal - effectively making the motor work in the same fashion as a servo.

    If however you just want to reduce the speed, the simplest way to do this is just to add more gearing - this is the way I'd prefer to do it, as you can get the desired effect of "slowing down the motor" and still have a good deal of torque.

    Alternatively, you could try using a voltage regulator on your existing supply or using a power supply with a lower voltage. Maplin has AA/AAA battery holders available with PP3 terminals on them, so you won't even have to re-wire anything. You could get a holder that takes 6 batteries and try reducing the number of batteries being used, therefore reducing the voltage in 1.5v steps (or 1.2v, if you're using rechargeable batteries). You can use less than the specified number of batteries in a battery holder quite easily: just bridge the empty sockets with a piece of stiff wire or, even better, a straight piece of metal like a discarded telescopic antenna. Just don't let anything touch that piece of metal/wire! ;)
     
  10. Byron C

    Byron C And now a word from our sponsor

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    Sorry, I just re-read your opening post - you want full control over the speed of the motor...

    I think the best way to achieve this is to use a motor driver that accepts a PWM input and then control the speed with something like a servo tester. Although depending on the overall project you're using it in, this arrangement may end up being more complex than the rest of the project!

    What you may want to do is first try it with a lower voltage supply - to make sure this gives the reduction in speed that you're looking for - and look into using an adjustable/variable voltage regulator. I've never used one myself but the first hit on Google for "variable voltage regulator" shows this product from TI, which has a sample application circuit in the data sheet. It can output between 3 to 40V and is rated for up to 1.5A - which should be more than enough to overcome the stall current of your motor.

    EDIT:

    0.00264A is 2.264mA - 264mA would be 0.264A ;)
     
    Last edited: 25 Apr 2012
  11. Boscoe

    Boscoe Electronics extraordinaire.

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    For the simplest solution just use a pot with a larger wattage rating to overcome the heating problem. Using this method or PWM or voltage regulator, whichever way you choose to lower the voltage to the motor, you will have to getting the motor spinning at a higher speed before you can lower the voltage to get the motor to spin slower. The better proper option would be to use gearing to lower the output speed of the motor way below what you want to maintain the motor speed at which you can control easily, you will also get more power that way.
     
  12. dullonien

    dullonien Master of the unfinished.

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    Thanks for the replies. I've already used gearing to reduce the speed of the motor, but not quite enough for my liking. I'll post a picture of what I'm doing a bit later, but basically the motor is powering a very delicate model and controlling the speed will hopefully result in less chance of breakage.

    I'll have a play around with a simple voltage divider circuit to see if that works, but BLC might be right that it's the current that's dropping too low and stalling the motor, which will mean that this circuit won't work. I don't want to spend much to get this working, because it's already cost a decent amount, so that may rule out PWM or voltage regulation. I also want to keep the circuit simple, because I've got limited time, and need this finished today if possible.

    I'll have a little play now.
     
  13. dullonien

    dullonien Master of the unfinished.

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    A voltage divider circuit doesn't work. If I use two 510Ω resistors, this drops the voltage down to ~3.5V, which is roughly what I'm looking for, but the resistors start to burn after a few seconds of use, so that's a no go really.

    I got a AA cradle from Maplins, so I think the easiest thing to do for now is to just use 2AA batteries and forget about being able to control the speed. If I get desperate, a PMW controller costs £15 from Maplins.

    If anyone knows of a way to do what I'm looking for with just a few resistors and a potentiometer then please let me know, but it looks like it's just not doable.
     
  14. dunx

    dunx ITX is where it's at !

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    You need to find the current drawn by the motor and multiply by the voltage across the resistor to get a value in Watts for the resistor...

    dunx

    P.S. GTX 460 land O.K. ?
     
  15. Kayvon

    Kayvon New Member

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    There have been a lot of posts here about the "easiest" and the "best" way to slow down a motor. Now, I can't tell you what the easiest way is, but I'll tell you the best way to slow down a motor without consuming as much energy as when it runs at full speed.

    You need a pulse width modulated (PWM) signal going into the motor. In other words, if you want the motor to go roughly half as fast, then you should still be giving it the full 9V (or whatever you were using), but only half the time. Turn it completely on, then turn it completely off. Want it to run even slower? Leave it off longer than you leave it on (that's the "pulse width" idea). The key is to do this switching very quickly (the "modulation" idea), meaning you'll need a PWM chip or a microcontroller to do it for you, and a fet or power transistor to support the output current.

    If you putting resistors in series instead, you'll eventually burn out your motor. And you can use gears to slow down the output of the motor, but you're still using the full amount of energy and noise from running at full speed. (The energy usage may be something you don't care about if you're not running from batteries.)
     
  16. Furball Zen

    Furball Zen Shut up and Mod

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    Hence the fan controller suggestion...
     
  17. Byron C

    Byron C And now a word from our sponsor

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    But don't most fan controllers just change the voltage? I haven't seen that many 4-pin case fans...

    What Kayvon & I described is different to the way that fan controllers operate.
     
  18. Furball Zen

    Furball Zen Shut up and Mod

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    They do it with transistors too. Sunbeam Rheobus is all you need, or the single version of it.
     
  19. dullonien

    dullonien Master of the unfinished.

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    As I promised, here's a few pics of the finished model. The speed is ok using two AA batteries, even if it's not controllable.

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  20. Margo Baggins

    Margo Baggins I'm good at Soldering Super Moderator

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    make a video please :D
     

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