Potentiometer output votlage

Discussion in 'General Electronics Chat' started by Dritech, May 4, 2014.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Hi all,

    I will be using a potentiometer to output voltage as close to 13mV steps. I want the potentiometer to output the same voltage even when the system is reset or when other loads are connected to the same power line.
    To help achieving this I was planning to connect one terminal of the pot to a 4.7V zener diode instead of connecting it directly to the 5V supply. This way if a voltage drop occurs (when connecting other loads or for any other reason), the voltage to the pot will remain stable.

    Will this work? is it possible to have a repetitive output voltage as close as 13mV?
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Can you post a schematic of your idea?
    That way we can have a look if it needs improvement.

    Bertus
     
  3. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Attached the the schematic. D will be 4.7V and R is for now unknown (Still have to calculate it). The pot will be used for monitoring the motor angle of rotation. I am not going to use an encoder because I am limited with space and budget.

    Note: Dividing 4700mV with 360degrees equal 13mV/degree.

    Now I have the following questions:

    1) Is it possible to get repetitive voltage values for the same angle when using the attached schematic? Are there additional measures that I can take to increase accuracy?

    2) The cable length from the wing (signal terminal) to the MCU will be approx. 60cm. Will that have any effect of the signal?

    3) Does the pot value make any difference when used for position monitoring?

    Thanks in advance.
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,649
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    Hello,

    A standard potmeter will not reach the full 360 degrees.
    It will be more like 270 degrees.
    Also how often does the angle change?
    Wear can be a problem.

    Bertus
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You can get a 360° continuous turn Pot, but you would need to initially 'zero' it.
    Pot value should not be that critical, especially if the input impedance is fairly high in comparison to the pot.
    Max.
     
  6. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    That was the type I had in mind, at Digikey they are around $16.00.
    Max.
     
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The chosen potmeter will have an effective range of 340 degrees.
    I hope this is enough for you/
    Have a look at the datasheet for more info:
    www.bourns.com/pdfs/6539.pdf

    Bertus
     
  9. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Yes. I was considering using a gear to get the full range.
     
  10. MrChips

    Moderator

    Oct 2, 2009
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    and you can eliminate the resistor R and zener diode D.
    If you use a 5V regulated source that should be stable enough for your requirements.
     
  11. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Thanks. Will the cable length effect the signal? and does the pot value effect the performance?
     
  12. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Personally I would have used a 5k minimum, your resolution will also be a bit finer, but if the input is a micro then the cable and input impedance should not have any effect as far as I see it.
    Max.
     
  13. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Thanks for the reply. I will be using another potentiometer for a 180degree rotation. Which potentiometer from the two below would you suggest please?

    http://uk.rs-online.com/web/p/potentiometers/7435686/

    http://uk.rs-online.com/web/p/potentiometers/7899456/
     
  14. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The first, but at 50k you may be tempting a noise issue, the lower impedance would be preferred, I would go 10k max.
    I have a new surplus quantity of the second in 91 series 2.5k, you are welcome to one for postage.
    Max.
     
    Dritech likes this.
  15. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
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    Are you doing a ratiometric measurement? That is, are you reading the voltage on the wiper and converting it directly into degrees, or are you looking at the ratio of the wiper voltage and the +5V, and converting that fraction to degrees? If you are going straight from the wiper value to degrees, the absolute value of the +5V is important. As it is, you need it to be regulated to within 0.26% to hold your sampling error to +/- 1 degree, and that is not counting in any errors in the A/D.

    ak
     
  16. ramancini8

    Member

    Jul 18, 2012
    442
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    If I understand the problem an LSB is 13 mV. A 1% change on a 5V supply is 50mV. I don't think a 5V regulated supply or zener diode regulated supply will be accurate enough. This requires a precision reference, and that in turn usually dictates high load impedance, i.e. a high resistance pot.
     
  17. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Also be aware that the linearity is +/-2%...if that's important to you.

    Ken
     
  18. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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  19. alfacliff

    Well-Known Member

    Dec 13, 2013
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    for a 360 degree potentometer, why not use a ten turn pot? adjust the body for the desired range of resistance.
     
  20. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    The OP never stated the real application for this. If it's 0 to 360 degrees or <0 to >360 degrees, it makes a difference.
    Dritech,

    It's often easier the help solve your problem than help solve your solution. ;)

    Ken
     
    Last edited: May 6, 2014
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