Multimeter mA and A fuses display continuity?

Discussion in 'Test & Measurement Forum' started by Mr. Multimeter, Apr 12, 2019.

  1. Mr. Multimeter

    Thread Starter Member

    Sep 17, 2018
    61
    4
    While pobing a multimeters internals, I discovered when I touch the 400 mA fuse and the separate High Current (20A) fuse I get continuity .!! .This seems like a big faux pas, any thoughts as to why this is happening? These should be two completely separate circuits..seems like a dangerous screw up.
     
  2. MrChips

    Moderator

    Oct 2, 2009
    18,895
    6,052
    Why would that be a screw up?
    Current measurement is made by measuring the voltage across a shunt resistor.
    Both low current and high current probes share the same COM terminal.
     
  3. Mr. Multimeter

    Thread Starter Member

    Sep 17, 2018
    61
    4
    Roger that, but the Test Leads were not connected to COM, one lead was probing voltage input the other lead on the 20A fuse, these 2 circuits should not meet..?
     
  4. Mr. Multimeter

    Thread Starter Member

    Sep 17, 2018
    61
    4
    Sorry, I was not clear enough.
    One probe was connected to the Volts Input Jack, from there If I probed either the high current or mA jack, I had continuity.
    Can a via be shorted to cause such an occurrence?
     
  5. sagor

    Member

    Mar 10, 2019
    91
    22
    Most meters that read current do so via a shunt (via a fuse) to create a voltage. Thus, all current ports on most meters would connect to the "volt" port, with some low resistance (current shunt).
    Your meter reads the volts across the current shunt, so it makes sense the current ports are connected to the voltage input as well.
    A lot of this depends on the meter make and model, and what design they use. Digital meters tend to share a sinngle voltage point for current and voltage. Analog meters may actually use a current shunt across the actual meter, and not measure voltage.

    On a digital meter, the resistance between current ports and the voltage port may be a small fraction of one ohm, very small...
     
  6. BR-549

    AAC Fanatic!

    Sep 22, 2013
    4,936
    1,346
    You are still not clear enough. What function is meter selected to? On most meters.....this is used to check fuses without taking meter apart.

    Read data sheet on meter.
     
  7. Mr. Multimeter

    Thread Starter Member

    Sep 17, 2018
    61
    4
    I found the problem, there was a bad ptc and 2 vias were charred from a coworker who obviously never used a meter before , I replaced the PTC and was able to fix the traces , its now working 100%, no continuity from the voltage input to the uA/A inputs as per norm. Thanks for your input.
     
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