Weird Problem


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Thread: Weird Problem

  1. #1
    Join Date
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    Exclamation Weird Problem

    Hi All
    i have being experiencing this weird problem with my circuit and for the life of me i just cant get my head round it.
    i have a simple potential divider circuit like this

    5V|---------/\/\/\/\------/\/\/\/\------GND
    10K X 20K pot

    i am trying to get 2.53V so when i power up the circuit and vary the pot i get the
    2.51v when i check it with a voltmeter
    Now here comes the problem , if i take the probe of the volmeter off from point X and put it back to the same point x the voltage drops to 2.34v
    and i have to adjust it up again with the pot .
    but again if i disconnect and reconnect the voltage drops again.
    i havedone this about 20 times is there something weird going on here ?

    Isaac

  2. #2
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    Angry

    5V|---------/\/\/\/\------/\/\/\/\------GND
    ..................10K.....X..... 20K pot

  3. #3
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    Default

    What are you using for a power supply?
    Dave
    Always wear safety glasses while programming.

  4. #4
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    Thanks for your response its a Variable bench power supply 0-30V @ 1A

    Isaac

  5. #5
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    If the power supply is stable, thought maybe you were using batteries, I will have to ask if there is a load on the divider.

    Other than that, I will have to guess that the pot becomes unstable with a little heat.

    Why not use fixed resistors?
    Dave
    Always wear safety glasses while programming.

  6. #6
    skimask's Avatar
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    Quote Originally Posted by mackrackit View Post
    Other than that, I will have to guess that the pot becomes unstable with a little heat.
    And it's a 20K pot! I'd bet a bump from the other end of the table would knock it out of whack.
    Try a smaller value pot.

  7. #7
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    isaac, The maximum recomended input impedance for all Microchip Processors is 10K....

    Dave Purola,
    N8NTA

  8. #8
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    Quote Originally Posted by Dave View Post
    isaac, The maximum recomended input impedance for all Microchip Processors is 10K....

    Dave Purola,
    N8NTA
    its not for the input the pic
    but i suspect its got to be the pot would try a lower vaue like 12k

  9. #9
    skimask's Avatar
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    Quote Originally Posted by isaac View Post
    its not for the input the pic
    but i suspect its got to be the pot would try a lower vaue like 12k
    Same thing most likely applies. Lower the values of the resistors.

  10. #10
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    Daves' suggestion of using a fixed resistor is good too. I would replace at least one pot
    with a fixed 1%, and if using an A/D input to read this I would drop the pot to around 5K
    with the fixed value set to limit total current.
    Regards,

    -Bruce
    tech at rentron.com
    http://www.rentron.com

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