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Is my DMM broken (cannot read current)?

Discussion in 'Electronic Basics' started by Martin Cote, Jun 25, 2007.

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  1. Martin Cote

    Martin Cote Guest

    Hi, I just bought a cheap DMM (Elenco M-1000C), and I believe that the
    low-range current reader is broken.

    The DMM has a 10A plug, which seems to work properly (it shows
    reasonable results in high-current circuits). However, switching to the
    mA plug doesn't seem to work at all. It always shows me 0mA when I'm
    expecting 9mA (the range is set to 20mA).

    Also, when I use the 10A plug, a LED of my circuit lights up when I make
    the contact. It doesn't with the 20mA range.

    Am I doing something wrong with the low-range current reader? I really
    think the DMM is not functionning properly, but I would like to know
    what you guys think.

    Martin
     
  2. If you are lucky, there is an internal fuse in series with
    the low current ranges, to protect the current shunt
    resistor from being blown up if you connect the current
    measuring leads across a voltage source or anything that can
    drive much more than full scale current through the
    resistor. If you are less lucky, the resistor itself, acts
    as the fuse, and that range is destroyed.
     
  3. Phil Allison

    Phil Allison Guest

    "Martin Cote"

    ** Even the cheapest DMMs have a fuse for the 200mA range.

    Open it up and have a look - expect to see a 20 x 5mm, 0.5 A fuse.



    ......... Phil
     
  4. You are probably right. I have a $2.99 meter (Are there any
    cheaper than that?) and it has that fuse replacement
    information on the back.
     
  5. Martin Cote

    Martin Cote Guest

    That's right. I'll replace it an see how it goes.

    Thanks,
    Martin
     
  6. Martin

    Martin Guest

    Also, all the current measuring ranges (and resistance, and diode-
    check, etc) are basically a short circuit. If you touch the meter
    leads to the wrong place for even a fraction of a second with the
    selector set to that, you will be replacing that fuse again.

    And be ESPECIALLY careful with the leads plugged into the 10 amp
    socket, thats an unfused piece of wire connected internally to the
    sockets ... hook that up to the wrong thing, and you will be making
    big sparks, blowing breakers, or setting wiring on fire.
    For the 10 amp socket, it doesn't matter what the selector switch is
    set to, with the leads plugged into that socket, the leads form a
    direct short.

    So make SURE the setting is correct BEFORE you hook up the meter.
    (and buy the fuses in a bulk pack ... cause sometimes one forgets ...)

    John alluded to this, but I just thought I'd make it blindingly
    obvious.

    Martin Bakalorz
     
  7. Guest


    How much trouble could it have been for the manufacturers to add a 10A
    fuse to the 10A input...?

    I guess that's what the home circuit breakers are for...

    Thanks for the info

    Michael
     
  8. Phil Allison

    Phil Allison Guest


    ** A fair bit, actually.


    ** No way on earth.

    Simple, cheap DMMs ( ie intended for hobbyists ) are not safe to use on AC
    mains wiring.

    They should *never* be used by an electrician - for example.

    Only DMMs which carry a safety category rating are safe to use around high
    voltage and high energy circuits like the AC supply.

    They all have things like shrouded plugs on the leads, fused high current
    ranges ( using special fuses that cost more than a cheap DMM) and no access
    to the battery compartment without a tool.




    ........ Phil
     
  9. Phil Allison

    Phil Allison Guest

    "Martin"

    ** Totally false.

    Those ranges are usually high impedance and protected against overvoltage
    accidents.


    ** Not true with the vast majority of DMMs.





    ......... Phil
     
  10. Jasen

    Jasen Guest

    protected against fires maybe,

    I gave my meter 600V of B+ while on the 10VAC range while reaching
    towards the filament terminals, this and bent the needle against the
    stop and took out the diodes and a few resistors, the fuse survived
    intact.

    I eventually got the needle straightened out and balanced again.

    Bye.
    Jasen
     
  11. Phil Allison

    Phil Allison Guest

    "Jasen" <>


    ** Follow the fucking topic !!!!!!!

    You donkey cock sucking, 2 yo kiddie fucking,

    pile of autistic KIWI EXCRETA
     
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