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How to measure mA draw?

Discussion in 'Electronic Basics' started by AB, Feb 22, 2007.

  1. AB

    AB Guest

    Hi all,
    Have a newbie question for you. I have this burglar alarm that has
    some very specific current draw requirements. It works on 12VDC. One
    set of warning horns cannot exceed 120mA current draw from a
    particular set of output terminals. I have a collection of small
    piezo type horns, but no specs for them, so I'm not sure how much
    current they draw at 12VDC. I have a small multimeter. Can anyone give
    me a primer on how to go about measuring current draw in mA for a
    particular device? Thanks.
    AB
     
  2. Connect to a 12 V source and measure the current. BTW, piezo devices don't
    draw much.


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  3. The usual connection for current measurement (and 120mA should be fine
    with your multimeter, as most of them seem to support that much) is to
    connect your meter in series with the device.

    Without the meter, you have two connections like this:

    (+) ------ (+)piezo horn(-) ------ (-)

    I've labeled the polarity of the horn above and the remaining (+) is
    the plus side of your 12V battery and the (-) is the other side.

    To test the current draw, insert your meter this way:

    (+) ------ (+)meter(-) ------ (+)piezo horn(-) ------ (-)

    or else this way:

    (+) ------ (+)piezo horn(-) ------ (+)meter(-) ------ (-)

    And make sure you connect up the leads to the A or ampere connections
    and set the meter to measure current with the range appropriate for
    120mA, at least. (There may be smaller ranges and you don't want to
    injure the unit, so make sure the range you use supports at least
    120mA or more.) Also, verify the meter's polarity as well as the
    horn's.

    Jon
     
  4. chuck

    chuck Guest

    The multimeter needs to be in series with the siren.

    One lead of the siren to the positive 12 volt source. The other lead of
    the siren to one lead of the multimeter. The other lead of the
    multimeter to the negative 12 volt source.

    Make sure the multimeter is set up for measurement of current. If it is
    digital, it will not matter which multimeter lead (black or red) goes to
    the negative 12 volt source. With an analog meter, it does matter and it
    should be the black lead.

    When measuring unkown values, it is good practice to begin your
    measurement at the highest range on the meter, which in this case may be
    20 amperes. A piezoelectric siren will probably not show any indication
    when the meter is on the 20 ampere scale. You can move to more sensitive
    current ranges until you get a decent reading. The idea is to avoid
    damaging the meter by passing too much current through it on any
    particular range.

    Chuck
     
  5. And never ever ever connect it to a voltage source when in current measuring
    mode, the meter is basically a short circuit. Just replaced a fuse in my
    meter yesterday. :-(
     
  6. Well.... that's right! Though the OP may not know what you mean.

    To the OP, Anthony means don't connect your meter, when switched into
    current measuring mode, directly across the 12V terminals of the
    battery. Make sure you have your horn or other current limiting load
    inserted in series with it, as well. That's because without the load
    added in series, there is very little to otherwise limit the current
    flowing and the result is that your meter looks about like a wire to
    the battery and a LOT of current starts to flow, with the result that
    your meter will either be damaged or else will pop a protective fuse.

    If you've ever made the mistake of touching a jumper cable across the
    two terminals of your car battery, you will be able to understand the
    result we're talking about here.

    Jon
     
  7. Or ohms modes.


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  8. For some reason that doesn't seem to bother my old Micronta. I'm not sure
    how they do that though. My first meters sure didn't like you doing that.
    :-O Usually the application of 120VAC in Ohms mode is what did most of them
    in.
     
  9. Chris

    Chris Guest

    Hi, AB. If you're concerned about the alarm electrical load, have
    your burglar alarm drive a small relay. If you put a back-biased
    diode across the relay coil, you should be able to turn on a relay
    that can switch several amps with no problem.

    Look for a 5V relay with a coil whose resistance is greater than 42
    ohms. Here's one from Radio Shack that will do the job:
    5VDC/1A SPDT Micro Relay $4.49 USD Model/Catalog #: : 275-240

    This relay can switch up to 1 amp.

    Cheers
    Chris
     
  10. AB

    AB Guest

    Thanks to all for the great info. I feel confident going forward now.
     
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