DC motor control

Discussion in 'General Electronics Chat' started by anne, Apr 24, 2008.

  1. anne

    Thread Starter Active Member

    Apr 20, 2008
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    This is related to another post I made but different enough to ask afresh.

    12vdc 2.0amp SMPS.
    1.0 amp 1900rpm@12vdc permanent magnet DC motor

    I'm looking at motor controllers in the catalog I'll order the motor from. They are kinda pricy and I don't need any other adjustments except speed. RPM/VDC will be set and never moved much outside of a narrow range.

    As an experiment, since I'm using a power supply dedicated to just this motor, I tried a common 110vac dimmer switch on the input of the SMPS.
    Running a .50amp muffin fan it worked exactly as I might have hoped.

    Is there anything wrong with this form of motor control for this low voltage low amperage purpose? It's so simple and seemingly elegant.
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    the SMPS is designed to maintain a fixed dc voltage on its output. If you vary its input voltage it adjust the pulse width so it outputs a constant dc voltage. But if you reduce its input too much it wont work at all. So you cant vary the dc output voltage of the SMPS by varying its input ac voltage. you can find a n SMPS with variable output dc voltage to control the speed of the motor.
     
  3. anne

    Thread Starter Active Member

    Apr 20, 2008
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    0
    I don't understand why it's working to control the muffin fan on my workbench then.

    Is there a simple method of locating an discrete tuneable device on the board of a typical smps for basic output control?

    I'm trying to use what I have at hand if at all possible, for cost savings, to keep it out of the landfill, and just for the challenge of it.
     
  4. mik3

    Senior Member

    Feb 4, 2008
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    it would be helpful if you have the circuit diagram of the SMPS to post it.
     
  5. anne

    Thread Starter Active Member

    Apr 20, 2008
    39
    0
    oops, very sorry. I went back to the bench to test the actual voltage and discovered that I'd used the wrong supply. I have a drawer full of 'wall wart' power supplies. Some are std issue and some are SMPS. They appear similar except for the weight.

    I was using a regular supply and you are quiet correct. I couldn't control the SMPS with the dimmer.

    I doubt if it's possible to get a circuit diagram for this 12vdc 2.0 amp SMPS. It's from consumer electronics. I get these from thrift shops for fifty cents apiece. They are generally pretty simple little boards. If I had just a bit more (or any really) electronics schooling, I image I might be able to find a spot on the board for trimming the voltage. Maybe just before the little board mounted wire-wound transformer?

    At least these aren't devices or currents that I can destroy my workshop with, and they're cheap enough. I'll just have to experiment.

    In any case. The moderators can delete this thread if they wish.

    and thank you mik3
     
  6. mik3

    Senior Member

    Feb 4, 2008
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    I dont think you can modify the circuit if it is using a microcontroller to control the circuit. But if you want take some photos of the board on the top and bottom and tell us the serial number of the product. Maybe we might find something.
     
  7. Caveman

    Active Member

    Apr 15, 2008
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    Why don't you just use the linear p.s.? And as far as the earlier question about that method of motor control. It is fine as long as you don't have significantly varying loads. If you do, the motor will change speed as the load changes.
     
  8. anne

    Thread Starter Active Member

    Apr 20, 2008
    39
    0
    Bulk and weight are very high premiums in the project I'm working on. In order to get a few amps reserve in a linear supply would be too great a trade off in this regard.

    I can pretty easily work around the motor control, it would just be a bit of a luxury on this project. I just thought it would be interesting to see if it was possible to add output control/trim to an SMPS, but not if I'm in too far over my head.
     
  9. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Use a MOSFET to clip the output of the SMPS into pulses and a control circuit to control its pulse width modulation and thus the average voltage across the motor terminals.
     
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