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    Measure potentiometer value using a constante current source

    Hi,
    I need to measure a potentiometer value from a joystick. The usual way to do this is to use it as a voltage divider.

    But I remember from university that I use a constant current source to measure the potentiometer value but I couldn't remember the advantages of that application beyond this way if the power source varies I still get the same value.

    Could you explain me if that application will give me best results? per example about noise?

    PS: I'm thinking to use a lm334

    Thanks :)

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    Re: Measure potentiometer value using a constante current source

    The output wire taken from the potentiometer is lengthier & the output is too low (maybe in lower potential level) then the voltage output from the potentiometer will absorb noise. So you will not get the accuracy. So this problem is avoid using constant current source...


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    Re: Measure potentiometer value using a constante current source

    Quote Originally Posted by udhay_cit View Post
    The output wire taken from the potentiometer is lengthier & the output is too low (maybe in lower potential level) then the voltage output from the potentiometer will absorb noise. So you will not get the accuracy. So this problem is avoid using constant current source...
    thanks for the explanation :)

    To use only one current source for the 3 joystick, i need to use a micro with differential ADC, right?
    Last edited by metRo_; 23rd April 2013 at 13:37.



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    Re: Measure potentiometer value using a constante current source

    I disagree. take the basic set up, constant VOLTAGE source, three pots wired across it. Whatever the slider is doing OR the input impedance of the stage its following, the controls are fully isolated.
    Now replace the constant voltage with a constant current. The voltage drop across the pots is the resistance of the three in parallel times the (single) source current . Now if a pot is adjusted, the impedance of the following stage, takes more or less current. This change in current causes the voltage drop across the pots to change, which causes the voltages picked of by the other pots to change resulting in interaction between the pots. So you will need three current sources easier to use a single voltage source.
    Frank



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    Re: Measure potentiometer value using a constante current source

    Some original joysticks used the Pot to control a VCO and thus frequency was the output, however DMM use constant current ( 0.0001mA to 1mA) to drive the probes and thus Resistance is the voltage on the probes.
    A best design is easily achieved with good test specs™
    A better question deserves a better answer. ™
    ... so include all your acceptance criteria ( values, % tolerance) and assumptions in your question or any design.

    ... Tony Stewart EE since 1975
    - slightly north of Toronto



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