Varying resistance on negative

Discussion in 'General Electronics Chat' started by Aaron_sfas, Jun 21, 2015.

  1. Aaron_sfas

    Thread Starter New Member

    Jun 21, 2015
    3
    0
    Hi guys. This is my first post, so I'll try to give as much info as possible, but apologies if I miss something.

    I have a question about why resistance may vary on battery negatives (the positive voltage is 120Vdc). All of these negatives are directly electrically connected (jumper wire between terminal terminal strips, as there are a LOT of components involved), but when I test their resistance with all circuits live, I get some rogue resistances.

    There are 4 terminal strips that all have negatives connected to them:

    I- 1
    I- 2
    I- 3
    I- 4


    If I test between 1 and 2, I get about 5 ohms. 1 and 3, about 10. Between 1 and 4, about 15... so you can see the pattern emerging of about 5 ohms per terminal (although I feel that figure in itself may be inconsequential).

    HOWEVER, if I electrically isolate all equipment, I get readings of about .2 ohms regardless of which terminals I test between (which is what you would expect).

    Could this be my meter (fluke 115) getting thrown by some transient signal when the equipment is live? I tried the meter on both OHMs and 'bell' function, with the same results either way.

    Any help would be greatly appreciated.

    Thanks
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    If the batteries are delivering current to a load, there will be some small IR drops which will confuse the Ohmmeter. Put your Multimeter in the mV mode, and read the voltage between the points you were formerly trying to measure resistance.
     
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  3. #12

    Expert

    Nov 30, 2010
    16,316
    6,818
    You can NOT measure the resistance of a live circuit.
    Well...Mike got here first, again. :(
    I will not bother to duplicate his advice.
     
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  4. Aaron_sfas

    Thread Starter New Member

    Jun 21, 2015
    3
    0
    Right. I figured seeing as the points I was testing between were (should be :S) at zero potential, it would be ok to test live. But duly noted.

    Thanks very much for that guys.

    Out of curiosity, will testing live in that manner damage my meter? Or just give me warped results?
     
  5. #12

    Expert

    Nov 30, 2010
    16,316
    6,818
    Warped? Every time.
    Blow it up? Sometimes.
    Either learn your meter or use the Harbor Freight Sunday Specials for $4 each. :D
     
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  6. Aaron_sfas

    Thread Starter New Member

    Jun 21, 2015
    3
    0
    Fair enough. I definitely know better than to test resistance on actives/positives. But this new information has been news to me. Thanks again for your help.
     
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