Rookie multimeter question - measuring DC current

Discussion in 'General Electronics Chat' started by majhi, Jun 19, 2015.

  1. majhi

    Thread Starter Member

    Jul 2, 2014
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    I'm trying to test the DC current of a couple devices and I'm getting bizarrely high readings. I've never used a multimeter to measure current before so I'm probably doing something wrong. My multi is currently set to the 10A setting. I plug in a 12v 500mA wall wart and measure it and it reads 2.9. Then I try a simple rechargeable AA battery rated at 1900mAh and it reads 4.3! Lastly, I try a 12v 1A wall wart (unregulated, though, so it puts out around 16v) and it reads 5.8. I thought there was a pattern at first because for the first two power sources, the readings were 2.4 higher than rated for both, but the last one threw me off. What am I doing wrong?
     
  2. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    Current must be measured in series with a load not in parallel with the PSU!

    The connection described in the OP accomplishes nothing more than determination of the maximum current available from the power source (regardless of loaded EMF, PSU strain, etc...) not to be confused with maximum (safe) current drain! --- At best the reading is meaningless, at worst, injury, death, fire or damage to the PSU and/or instrument may occur...

    Most sincerely
    HP
     
    Last edited: Jun 19, 2015
  3. majhi

    Thread Starter Member

    Jul 2, 2014
    49
    1
    Ooohhh... whoops. At least I didn't fry anything.
     
  4. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    Most meters are not fused on their 10 Ampere range --- So... if it didn't melt, it's probably ok :cool: --- It may be advisable, however, to verify the PSUs output EMF prior to placing them back in service...

    Best regards
    HP:)
     
  5. dl324

    Distinguished Member

    Mar 30, 2015
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    Also keep in mind that when measuring current your multimeter has resistance that can affect readings.
     
  6. WBahn

    Moderator

    Mar 31, 2012
    17,715
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    Common misconception and really big no-no.

    We learn best from our mistakes. Be glad that you got to learn the lesson without the cost that many people have endured.
     
  7. majhi

    Thread Starter Member

    Jul 2, 2014
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    Wow, I had no idea that it could be that bad... I'm glad you guys cleared that up for me, thank you. I will avoid doing this in the future.
     
  8. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Why is that more and more discussions are held where the OP TS shows no schematic?

    Are we sure that he understands "across" "series" "parallel"?

    Sorry but it seems the trend...
     
  9. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    Point taken!:D

    @majhi

    To clarify the nature of the hazard; From a safety standpoint ammeters are best regarded as 'dead shorts' (note, for instance, that the shunt arrangement of a typical VOM set to its 10 Ampere range is accomplished via an unfused length of 14 AWG solid Cu conductor). --- Although lower current ranges generally exhibit greater resistance and fast acting fuses, misapplication may, nonetheless, present an arc-flash hazard in certain situations...

    Please consider use of 'Clamp Around Probe' style ammeters where high power/energy is available...

    Best regards
    HP:)
     
    Last edited: Jun 20, 2015
  10. majhi

    Thread Starter Member

    Jul 2, 2014
    49
    1
    So essentially what I did was the equivalent of hooking both alligator clips of my wall wart together? Yikes.

    As for the schematic, I do understand the importance of schematics when troubleshooting a circuit and I put something together in ExpressPCB to illustrate when I can, however all I did was hook a multimeter to a power source. So what exactly am I supposed to make a schematic of in this case? That comes across as sarcastic, but that's not my intent - it's a real question.
     
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  11. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    Correct...

    I think @atferrari 's concern is that, owing to the wide range of skill level on these fora, there might be 'communication issues' (CIP [dubious] cognizance of the terms 'parallel', 'series', etc...) Whereas images are, in essence, a 'universal' language:)

    Not at all!:):):)

    Best regards
    HP
     
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  12. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
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    There's a neat trick for cylindrical cell battery compartments - solder terminal posts to both sides of a narrow strip of double sided PCB material, that can then be slipped between one of the cells and the terminal.

    When I tried it on a cheapo Ni-Cd charger I got way low readings - the meter resistance (and or the leads resistance) was too high.
     
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