Is there such thing as a resistance follower?

Discussion in 'The Projects Forum' started by gte, Apr 17, 2014.

  1. gte

    Thread Starter Active Member

    Sep 18, 2009
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    I need something like a voltage follower circuit, only instead of a comparator duplicating voltage, it duplicates resistance.

    I have 2 computer modules that measure resistance by outputting 5v and then reading the value that does not drain through a sensor which varies resistance based on conditions that it senses. I only have room for 1 sensor and want to feed the value to both modules (common ground).

    Is there something that can monitor resistance or voltage to the sensor and simulate the resistance to the other sensor, in essence a resistance follower? This way both modules could read from 1 sensor, while having their 5v inputs isolated from each other.
     
  2. inwo

    Well-Known Member

    Nov 7, 2013
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    I don't know!

    It shouldn't be necessary though.

    Computer is reading voltage. IMO
     
  3. gte

    Thread Starter Active Member

    Sep 18, 2009
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    It reads from the voltage it supplies, so when I infuse the second supply voltage to the circuit, it automatically skews the voltage value higher. That's why I have to isolate the 2 circuits.

    I thought about trying to put a pull down resistor into the the circuit if both supplies were tied together, but since the sensor is constantly varying resistance (aka voltage) depending on what it senses I don't believe this would work.
     
  4. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Not exactly sure what you are trying to do, but it sounds like you are trying to save a sensor. You could switch the voltage from one to the other, taking a measurement of each?
     
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  5. inwo

    Well-Known Member

    Nov 7, 2013
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    Op should be able to disconnect the pull-up resistors in one computer.

    Or post internal schematic. If the input impedance was known, an external circuit could be designed to drive both.
     
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  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Why can't you just apply one fixed voltage (5V) to the circuit, then read the sensor value using both computers?
     
  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    If you can't modify either of your two inputs, maybe you can overpower one. Hook up the sensor to one exciter/input, pick off the signal through an opamp voltage follower, and drive the 2nd input with this very low output impedance. As long as everyone is within voltage and current limits, you might be able to drive the 2nd input in spite of its 5V excitation potential. After all, it can't be a low impedance source or no signal would develop across it.

    ak
     
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  8. gte

    Thread Starter Active Member

    Sep 18, 2009
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    I can't mod the modules unfortunately.


     
  9. gte

    Thread Starter Active Member

    Sep 18, 2009
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    I thought about trying to use a voltage follower, but I couldn't come up with a way in my head as to how it would work.

    How could the voltage follower sink voltage from the second input, so that it would match the voltage value of the first input?

    Do you have a circuit diagram that would help me picture this in my head?




     
  10. zhomeslice

    New Member

    Apr 19, 2014
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    I think the pull up resistor in each receiving circuit is different resulting in different temperatures for the same voltage. Take a fixed resistor similar to the resistance of the sensor now and replace the sensor with it for each receiving device the displayed temperature, what ever it is, should match. Measure the voltage on each and they will probably differ. If they don't you could use an op-amp voltage follower to fix a voltage irrelevant of the pull up resister in the second receiver. The op-amp input should not interfere with receiver 1's readings (attach thermister here) and the op-amp's output will force the requested voltage for reciever 2 irrelevant of the pull-up resistor within the reciever.

    simple op amp voltage follower: Lint to page

    [​IMG]
     
    Last edited: Apr 19, 2014
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  11. zhomeslice

    New Member

    Apr 19, 2014
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    I took a minute to draw this up:
    [​IMG]
    This scmatick should clairify what I was saying with my other post I hope this is much simpler than you had imagined Parts should be readily available at radio shack.
     
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  12. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Yup, that looks like it. Thanks for the effort.

    What is the value of the thermistor (usually given as the resistance at +25C).

    And the biggie, do you have any idea what the output impedance of the Sense terminal is? Usually it is equal to the thermistor resistance at the temperature of interest (or the middle of the temperature range of interest) as this minimizes linearity errors in some circuits.

    Also, is the Sense output pulsed or steady state? Sometimes thermistor circuits are pulsed to minimize self-heating errors in the sensor.

    ak
     
  13. zhomeslice

    New Member

    Apr 19, 2014
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    AnalogKid, it looks like we are on the same page. the signal I assume is constant but i have seen battery powered temperature sensors that pulse the thermistor to save power as well as prevent errors from self-heating. if this is the case the pulse would follow through the op-amp and ruin any chance for a good reading there. additional components would most certainly be needed.
     
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  14. gte

    Thread Starter Active Member

    Sep 18, 2009
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    Thank you for this. The signal is constant and it does not pulse.

    So the op amp comparator has negative feedback and pin 6 is tied directly to In-? And the negative feedback allows pin 2 to sink current/voltage to V- keeping it at the same voltage value as In+?

    This looks pretty simple to build, is there any particular op amp at Radio Shack that I should use? Maybe an LM741?



     
  15. t06afre

    AAC Fanatic!

    May 11, 2009
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    Can you post any link for more information on those devices. It could be that the sensor is digital like the DS18B20 sensor
     
  16. gte

    Thread Starter Active Member

    Sep 18, 2009
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    I don't have a link or spec sheet for the sensor, but the sensor is from the late 80's or early 90's in origin I believe, if that helps?
     
  17. gte

    Thread Starter Active Member

    Sep 18, 2009
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    zhomeslice, analogkid, is my understanding of how the comparator op amp works correct?
     
  18. AnalogKid

    Distinguished Member

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    Yes. It's called a voltage follower, similar to an emitter follower but without the base-emitter voltage drop. In round numbers, the amount of current the output can sink or source is independent of the circuit but directly related to the part. That is, a newer opamp such as a 5534 can source and sink more current than a 741 with less distortion and error, but the circuit is essentially identical. And it is important that your device be an opamp, not a comparator. You need a linear output stage, and while many comparators are repurposed opamp designs, some have true digital outputs with no linear active region.

    ak
     
    Last edited: Apr 28, 2014
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  19. gte

    Thread Starter Active Member

    Sep 18, 2009
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    I built this tonight on a breadboard and tried it and I must have done something wrong as it did not work. Strangely enough I would not see the voltage drop at all? I could put a 1k resistor inline with sense2 and watch the voltage drop, but I did not see a drop when I fed voltage into pin 3 of the voltage follower.

    I did however notice that when I connected sense2 to pin 6 (before I had connected sense1 to pin 3, but everything else connected in the circuit), I would watch the voltage drop from 5.04v to 2.00v? Is that to be expected? I would think not, but I triple checked everything and it was connected as per the schematic. After that, I would make the final connection of sense1 to pin 3, which was 1.65v and sense2 would still stay at 2.00vDc. I supplied the voltage follower with 10vDc of power. I used an lm741 ... any guesses as to what I'm doing wrong here?
     
    Last edited: Apr 25, 2014
  20. gte

    Thread Starter Active Member

    Sep 18, 2009
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    I was able to find this out about the sensor and module if this will help? Both sense1 and sense2 are identical.


    [​IMG]
     
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