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Conneting multiple ohmmeters

  1. Jul 8, 2016 #1
    This might be a forehead-slap moment but I'll ask anyway...

    I'm working on a project building a panel of ohmmeters that will simultaneously measure the line-to-line resistance values a 3-phase generator and was running into trouble connecting all three ohmmeters to the three-phase measurement point, where two would read accurately and the third always reads a combination of the other two. So I ran a quick experiment, taking two ohmmeters with separate 9V power sources. When each is connected individually to a 4.7 ohm resistor, they read 4.7 ohms. When you connect them both to the same resistor simultaneously they both read 9.4 ohms. I'm hoping someone more familiar than me with test equipment can tell me why they display cumulative resistance when both connected to the single resistor (I was under the impression an ohmmeter would always display the lowest possible resistance between the test leads).
     
  2. jcsd
  3. Jul 8, 2016 #2

    berkeman

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    Staff: Mentor

    To measure a resistance, some meters inject a current and read the resulting voltage...
     
  4. Jul 8, 2016 #3
    An ohmmeter means one (1) ohmmeter.

    think about it, crudely as,
    A nine volt nominal source across a 4.7 R -> current through the resistor is 2 A.
    Now connect a second nine volt nominal supply -> each source sends 1 A through the resistor, for the resistor to have still the nominal 9 v across it.

    The second time, the guts of this "ohmmeter" will interpret the lessor current as a higher resistance than before.
     
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