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Viewing an ohms change on an oscope???

Discussion in 'Beginner Electronics' started by scanner80, Aug 6, 2005.

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  1. scanner80

    scanner80 Guest

    I'm looking for a circuit I can build or equipment I can buy
    so I can time how long an ohms change is. I would like to build it if
    possible . I would like to view it on a scope.
    One example would be a starting ohms reading from a device of 1355 ohms. It
    will increase by approx. 20 ohms
    and then return to 1355 ohms. The time it will take is approx. 19 seconds. I
    will need to be able to see a change as small as 1 ohm if possible , but the
    most important thing is to see the reading change from 1355 and return to
    1355 ohms. I need to then measure the time with cursers on a scope.
    I know a respiration monitor can see and display an ohms change , but I need
    a way to exactly measure the change.
    I will be greatful for any help.
    Thank you,



    To measure resistance you need to apply a voltage and then
    measure the current. Another way is to use a "voltage divider", put your
    changing resistance in series with a known resistance, apply a voltage to
    the circuit and monitor the voltage dropped across the known resistor. As
    your changing resistance decreases current in the circuit will increase and
    the voltage drop across your known resistor will increase. This will all be
    linear so you will be able to make notations on your scope graticule so that
    you can directly readout the display in ohms.

    You will want to use a regulated voltage source. A look into Ohms law will
    provide the math for you to make the calculations..
  3. scanner80

    scanner80 Guest

    I just want to be able to measure the time it takes from when the resistance
    leaves 1355 ohms and returns to 1355. I can do it with a meter and a
    stopwatch, but I was looking for a more verifiable way to do it. I want to
    remove human error. This is for calibration purposes.
    Is there any type of simple timer or timer circuit that can will trigger as
    the ohms changes and stop when it returns?


    Ø The first is that the resistance change is too small to give a

    If a 30V source is used (My bench supply is 0-60Vdc regulated, BK 1623A) and
    1K for the known resistance a difference in 20 Ohms of the resistor under
    test will result in a rise of 100mV across the known resistance




    R1 (1355-1375 Ohms)



    R2 1K Ohm -à




    1355+1000=2355 Ohms


    12.7mA X 1000 Ohms = 12.7V

    And the high end

    1375+1000=2375 Ohms


    12.6mA X 1000 Ohms = 12.6V

    A difference of

    12.7-12.6 = 100mV


    Ø > will increase by approx. 20 ohmsseconds.

    If this whole process takes 19 seconds, triggering shouldn't be an issue.
    Remember, he was using a stopwatch to measure the time, so, just use a quick
    time base and watch the green line rise and fall.

    10mV/cm gives me full deflection and I have plenty of range on the Vert.
    position to bring the trace to the bottom of the screen. My scope is now
    measuring 2 ohms/cm. I don't know why this would not be linear. Scope is
    calibrated annually.

    As for not running the number last night, I was doing other things and
    fiddled with this while on break. And, I didn't want to steal your thunder.
    I knew you would come up with a more elegant way of doing this. I'm just a
    hack anyway..

    I do like your bridge idea...
  5. JeffM

    JeffM Guest

    ...reading from a device of 1355 ohms...
    Posting the same question individually to multiple groups
    is not only foolish (worse than linked cross-posting), it is selfish.

    Those people who do not visit each of the groups
    will not benefit by the intelligence of the responses at those other

    Despite those folks who warn about EXCESSIVE cross-posting,
    posting the same question individually to 4 groups
    is a really bad approach.

    This practice robs those people
    who do not habitually visit all the groups
    of the wisdom of others on the topic.
    It also doesn't let everyone know
    when the question has been answered sufficiently.

    Most importantly, it does not allow those in 1 group
    to gain from further details revealed in another group.

    If the query is relevant to multiple groups,
    put the names of ALL the groups in which you would like it to appear
    on the Groups line (the To line) THE FIRST TIME you post it.

    Proper use of the Followup-To line is also useful.


    Yes, just now, and your correct, (as usual), I had to add another divider to
    give me 12V to run into the B chnl. Then display (A-B).
    Wouldn't you consider an output change of 5mV for every 1 ohm change in the
    DUT to be linear? To me, a nonlinear response would be something like, 5mV
    for the first ohm of change, 7mV for the next ohm, 10 for the third, etc,
  7. scanner80

    scanner80 Guest

    First, I want to say I'm very sorry for the multiple postings.
    I did not realize I was causing a problem.
    I'm kinda new to newsgroups and did not know the correct way to do it.
    I also did not know there was so many helpful people out there and was just
    tring to reach as many people as I could.
    Also now I know to post at the end of the post , another mistake I made.
    Once again I'm very sorry.

    Second , I would like to thank you all very much for all your posts and
    research on this project.
    You have given me many great ideas.
    You guys are so smart, I feel like I'm not qualified to change a 9V battery.
    I work in a calibration lab calibrating Medical and test equipment. I also
    do a lot of the research. I get ideas
    of new ways to do things and try to work on them when time allows or at
    I'm not an engineer (as you could probably tell) I only have an Associates
    Degree. But I do the best I can and I do alot of research on my own.

    Once again THANK YOU ALL VERY MUCH!! . And thank for your time.
    I will use your info and work out the details.

  8. scanner80

    scanner80 Guest

    Wow, you are something of a butthead aren't you?
    I don't cal the equipment used on patients. I cal the test equipment . I
    have been to many oem training classes and (here is the key thing ) I follow
    a cal procedure.
    Duh! If you know anything about calibration ,you would know you need a
    calibration procedure. You seem like you did not know that. I forgive you.
    I do know about the equipment used in hospitals and the biomeds who use
    them. While the majority of them are good. I have seen things that would
    scare the h*ll out of you.
    My advise ? STAY HEALTHY!
    Once again, for all you good and nice people that offered advise in good
    faith , THANK YOU!
  9. Now I know why those instruments that we send out for cal, come back wacky.
    We make it a pracitce, for new equipment, to hang on as long as possible with out sending the thing in for cal.
  10. legg

    legg Guest

    Considering the costs involved in calibration servicing, it's always
    bothered me how many times simple metering is returned with a new
    calibration sticker, when the battery(s) were nearly dead and
    protective fuses were still open.

    Charging for replacements, where required to calibrate, would not have
    significantly inceased the bill. No great knowledge of electronics
    would have been required to read the equipment manual and perform
    these basic functions.

  11. Chris Head

    Chris Head Guest

    Hash: SHA1

    Sure thing. See:

    for details. Try not to blow anything up.

    Version: GnuPG v1.2.1 (MingW32)

    -----END PGP SIGNATURE-----
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