Making digital potentiometer

Discussion in 'General Electronics Chat' started by jakovn, Jul 12, 2009.

  1. jakovn

    Thread Starter New Member

    Jul 12, 2009
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    I m trying to control 3V, 40mA laser brightess digitaly but not with PWM
    When I use analog potentiometer I get the whole rangle with 0-40 ohm resistance
    Is there a digital pot that fits requirements or something simple I could make. 16 steps is enough
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    You could use some fets that switch some resistors.
    The resistor values could be 40, 20, 10 and 5 Ohms.
    When all resistors are parallel the value will be 2.66 Ohms.
    When only the fet of the 40 Ohms is switched on the resistance will be about 40 Ohms.

    Greetings,
    Bertus
     
  3. millwood

    Guest

    I doubt that you will be able to find a digital pot capable of that kind of current. and low resistance digital pots are quite rare too.
     
  4. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Here is an example of a high-current "digital" pot:

    [​IMG]

    The original is from the National datasheet for the LM317. I am suggesting this as a concrete example of the approach suggested above by bertus, which could be adapted to mosfets.

    John
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    How about a programmable current source?
     
  6. ELECTRONERD

    Senior Member

    May 26, 2009
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    In your schematic example, do you simply connect the LED's to the different transistor outputs? The resistor values connected to each collector will vary the current to each output, correct? So I could have a rotary switch connected to an LED to vary the current. This doesn't seem digital at all...
     
  7. Wendy

    Moderator

    Mar 24, 2008
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  8. gotumal

    Active Member

    Mar 24, 2008
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    You can make use of PWM to vary the intensity continuously. Apply PWM through a suitable RC to gate of a FET. That way you can control the ID and hence the intensity of LED.
     
  9. millwood

    Guest

    no reason to convert the pwm back to an analog voltage through a rc network. just apply it direct to the gate so the fet stays cool.
     
  10. gotumal

    Active Member

    Mar 24, 2008
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    Well, it depends on application. In photometric application it would not matter since persistance of visions doesn't perceive the ON-OFF operation of LED but the average current through it.

    While in radiometric application where the constant intensity matters for few tens of microseconds, you need a very high frequency PWM. Instead, RC does this job of averaging.
     
  11. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    While trying to refine the term, "digital," I stumbled on this ap. note from Analog Devices:

    [​IMG]

    It seems to be right on point for the subject of this thread.

    John
     
  12. ELECTRONERD

    Senior Member

    May 26, 2009
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    There we go, that looks digital to me!
     
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