Voltage controlled PWM, is there a way ??

Discussion in 'The Projects Forum' started by acelee27, Mar 26, 2007.

  1. acelee27

    Thread Starter New Member

    Jan 11, 2007
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    Hi, thanks for any help in advance, much appreciated :)

    As part of my project i am trying to control the speed of a 12 volt DC motor (4 amps max) using pulse width modulation. However the PWM device needs to be controlled via a control voltage ( 0-12 DC) that is outputted from a frequency to voltage converter.

    I have tried hard to find a ready built PWM device that would fit the bill, however they all seem to be controlled via a potentiometer. Does such a device exist or is there another simple way round it.

    Thanks again, for taking the time to read.
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    What is the part number of the PWM device you are refering to?

    hgmjr
     
  3. acelee27

    Thread Starter New Member

    Jan 11, 2007
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    The PWM device i have been looking at is from maplin (UK site), the part number is RN41U. Its description is 'Panel Mounted Voltage Regulator Module'

    Thanks
     
  4. hgmjr

    Moderator

    Jan 28, 2005
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    I have viewed the picture at Malpin website.

    It is PC board with with a fairly large potentiometer mounted in the middle of it.

    Have you considered desoldering the potentiometer leads and injecting your own control signal?

    hgmjr
     
  5. acelee27

    Thread Starter New Member

    Jan 11, 2007
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    I have not considered that, i assumed that the signal from the F/V converter wouldn't be strong enougth. I could try amplifiing it through a transistor ??. I will try it to see what the outcome is, its all trial and error for me at this point.

    Thanks for the help :)
     
  6. hgmjr

    Moderator

    Jan 28, 2005
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    Yes. You will require a buffer between your control voltage and the PWM module. I was thinking more along the lines of an opamp. Is that something you would entertain?

    hgmjr
     
  7. acelee27

    Thread Starter New Member

    Jan 11, 2007
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    I have to confess i don't fully understand what an opamp is. I will do some background reading on them to get the basic knowledge. Is there an obvious one you could suggest that i could look into ?

    thanks
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
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    Look into "voltage controlled oscillator" while you're at it.
     
  9. hgmjr

    Moderator

    Jan 28, 2005
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    In addition to beenthere's recommendation you can review the AllAboutCircuits' Opamp Tutorial for basic opamp information. The style of presentation in the AAC tutorials is very good as a source of good solid information unencumbered by any complicated math.

    You may want to explore some of the other subjects covered.

    Remember that the forum members are here to answer any questions that might arise as you read the material.

    hgmjr
     
  10. kender

    Senior Member

    Jan 17, 2007
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    Absolutely. Analog PWM predates digital PWM. Analog PWM is a backbone of switching power supplies. There are lots of chips (pin-to-pin compatible even) that internally do the following:

    1. Generate saw tooth wave or triangular wave, or something similar.
    2. Take an analog voltage and compare it to saw tooth.
    3. Output the result of the comparison, which is PWM proportional to the analog voltage from the previous step.

    For an example see Texas Instruments TI5001. It's intended to work in a closed loop as a part of a power supply, that's why the analog input pin is called FB (feedback). However, it should work open loop too.

    Also, search for "PWM" on DigiKey.
     
  11. CaliusOptimus

    Well-Known Member

    Aug 14, 2005
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    0
    velleman makes a pwm kit that would likely fit the bill. it's rated at 6.5 amps and has a high impediance control voltage terminal that ranges from 0v - 5.1v

    just add a voltage divider to your 0-12v output, and it should work perfectly.
     
  12. kender

    Senior Member

    Jan 17, 2007
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    Here's a schematic for an analog POT-PWM-motor.
     
  13. Dave

    Retired Moderator

    Nov 17, 2003
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    Hi kender,

    The picture is too small to read, can you make it bigger?

    Dave
     
  14. kender

    Senior Member

    Jan 17, 2007
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    0
    I've replaced the JPG with PDF. The schematic appears in normal size on my computer.
     
  15. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
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    We've seen this before, I think it's a browser specific issue with some images. PDF works fine.

    Dave
     
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