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    Measuring resistance value by using pic controller

    hai friends,I like to read the resistance value by using the pic controller.
    I used the voltage divider and I can get the resistance values also.But when I tired to find the low values like "1 OHM" like means it can't showing the values.
    I need to calculate "resistance in OHM value" also.
    How to measure the resistance values in OHM by using pic controller..?

    •   Alt13th January 2018, 08:47

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    Re: Measuring resistance value by using pic controller

    Measuring very low resistances in a voltage divider is very difficult unless you consume large currents. A better method is to feed current from a pin through the resistor and then a capacitor to ground. You can monitor the voltage across the capacitor to see how fast it charges and from that calculate the resistance.

    Alternatively, you can use a fixed current source and amplify the small voltage dropped across the resistor. If you do that, I suggest you use a four-wire measurement to get better accuracy. To do this, feed the current through two wires to the resistor and measure the voltage across the other two wires, it eliminates the voltage dropped in the wiring which might be quite high compared to the value you try to measure.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



    •   Alt13th January 2018, 10:12

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    Re: Measuring resistance value by using pic controller

    Hi,

    You can't measure Ohms directly.
    Either you measure V and I and calculate R according Ohm's law
    Or you use one known resistor and connect them in series with the DUT, and measure both voltages...then you can calculate the resistance value according voltage ratio.

    BUT the very first you have to do is to decide what
    * resistance range you want to measure
    * and what resolution you need.

    Assuming you have a 10 bit ADC...then this gives 1024 steps of resolution.
    As a raw rule you can calculate 1024 equal steps of resistance.
    Some examples:
    * 1 Ohm ... 1024 Ohm with a stepsize of 1 Ohm. --> you can't measure resistance between 1 and 2 Ohms, like 1.6 Ohms
    * 0.1 Ohm ... 102.4 Ohm with stepsize of 0.1 Ohms
    * 100 ohms ... 100.000 Ohns with stepsize of 100 Ohms
    And so on.

    If you need higherrange or better resolution..
    * you may use a higher resolution ADC
    * or switch ranges with an analog switch, maybe with the use of an Opamp

    For sure there are options to optimize on some parameters.

    So ... what's the range an resolution?

    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. Thank you.



    •   Alt13th January 2018, 10:31

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    Re: Measuring resistance value by using pic controller

    Given that you are using a microcontroller to do this job ( instead of a dedicated IC, eg. ICL7106 ), you could also consider implementing an auto-range feature, which consists of changing the full scale through analog keys or MOSFETs that would add parallel resistances to the low side part of the resistive divider as the measure lies above or below determined threshold.
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