Having trouble interpreting current from my devices

Discussion in 'General Electronics Chat' started by electabuzz, Dec 5, 2014.

  1. electabuzz

    Thread Starter New Member

    Dec 5, 2014
    2
    0
    I'm currently building a circuit that switches on every few hours to power a little DC motor. It works great with a 9V battery but I would prefer it to work from a wall outlet.

    I pulled out an old but unused 12V, 3A adapter. I can't figure out if it is damaged or not (it's LED on light works) but if I throw it in series, solely with the DC motor, it doesn't seem to power the motor.

    Another thing that has me confused is that the 10A setting on my multimeter will give my 9V battery a reading of "~2.5" though I get a reading of "~0.3" from my adapter which should work up to 3A. (the voltage reading from both is as expected, 9v and 12v respectively)

    To be concise, my questions are:
    1. Can anyone explain the readings from my multimeter? Am I just reading the meter incorrectly? 2.5A from a 9V battery seems absurd and so does 0.3A from a 3A rated adapter when compared to 2.5A from the battery. (Measurements were taken consecutively, using the same settings)
    2. Given that the amperage from the adapter is so low, is this why it is not powering the DC motor? I was thinking maybe it is because the current and voltage are too great. However the multimeter says I'm drawing significantly greater current from the battery and I even put two 9v's in series and the motor worked fine. It still works...
    3. What would an appropriate volt/amp adapter be to power a circuit that runs perfectly on my 9V battery? (Hopefully I have one laying around here, if not I might just order one off eBay)


    Any help with this would be great, guys and girls.
    Apologies if my question seems elementary or if I'm missing something blatantly obvious.
    I'm really excited to get my little circuit going and I'm stumped on this power problem.

    Thank you!
     
    Last edited: Dec 5, 2014
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    do NOT put a current meter across a battery or power supply, it will give you a meaningless reading. current meters go in series with a load, to measure what the load is using. measure the voltage across the battery or supply with the load connected to see if the load is too great. the wall wart probably has a fault, or might have a current limiter inside.
     
  3. electabuzz

    Thread Starter New Member

    Dec 5, 2014
    2
    0
    Thanks for the reply!
    It seems that i'm getting meaningless readings even with 9V battery in series with the DC motor... very strange. It hops around radically.

    I find it very odd that it can give readings of 2.5A from a battery. Chalk up my surprise to my inexperience with a multimeter.. very interesting.
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    That should be in parallel with the motor.

    Adapters come in various flavors.

    You might have one that puts out 12VAC, in which case the motor wont like it, nor will a Voltmeter in DC volts mode. Try measuring the no-load voltage out of the adapter with the meter in AC Volts mode.

    You might have one that has a full-wave bridge rectifier built-in, but no filter capacitor. That should run a DC motor, but maybe slower because of the ripple. Try measuring the voltage out of the adapter with the meter in DC Volts mode.

    You might have one that has a full-wave bridge rectifier built-in with a filter capacitor. That should run a DC motor, but maybe faster because of the filtering . Try measuring the voltage out of the adapter with the meter in DC Volts mode.
     
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