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Measuring Current

Discussion in 'General Electronics Discussion' started by hooperdoski, May 29, 2012.

  1. hooperdoski

    hooperdoski

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    May 29, 2012
    Hi! I'm new to the forum and hope someone can help me with something I have always wanted to understand.

    I am not into electronics, but am a hobbyist gearhead that has never understood how to read milliamps with a multimeter so I am hoping someone here can tell me how to read milliamps.

    This morning, because of a fairly new 12 volt battery having low voltage every morning, I decided to check the current draw (battery drawdown). I used the multimeter, exactly the way it’s shown setup, which I’ve attached a picture of. I connected the red lead to the disconnected negative battery cable and the black lead to the negative battery post. The first readings I got, with nothing on that I could see, kept bouncing between 07.60 (+ or -) and 08.30 (+ or -). I then kept the multimeter connected for about eight minutes and when I went back to check the multimeter again the reading was a steady 0.137.

    I’m interrupting this to mean that something was causing a voltage draw of 7.6 to 8.3 milliamps and that when whatever was causing the bouncing reads shut off the milliamp draw dropped to 0.137 milliamps. Am I right?

    FYI: The Function Selector was set to AA for 0.4 /4/mA and 4/10mA. The Black Cable was plugged into the Common Socket and the Red Cable was plugged into the 10A Socket.

    The little box before the AA in the previous paragraph should have been a symbol the looked like a backwards "U".

    Thanks in advance.
     

    Attached Files:

    Last edited: May 29, 2012
  2. BobK

    BobK

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    Jan 5, 2010
    If you have the meter on the mA range, the red lead is in the wrong place, it should be in the other red jack, which is marked with mA (as well as V and Ohm), however that is likely to blow a fuse, so put the meter in the A range instead.

    Bob
     
  3. hooperdoski

    hooperdoski

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    May 29, 2012
    Bob, here is exactly what the multimeter manual says in describing how you set the multimeter up to measure DC/AC current:

    "If you do not know the amount of current you are measuring, always connect the red test lead to +10A MAX."

    That seems to me to be telling me I I did it correctly. Do you still disagree?
     
  4. CocaCola

    CocaCola

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    Apr 7, 2012
     
    Last edited: May 29, 2012
  5. hooperdoski

    hooperdoski

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    May 29, 2012
    Cola, it didn't work. I'm running Windows 7 Pro. Maybe that's why. However, if the "U" works that's fine.

    Thanks for the info.
     
  6. gorgon

    gorgon

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    Jun 6, 2011
    From the labeling of the meter, both the uA/A and the mA/A position will measure 10A max if the red lead is plugged into the 10A max hole. If you measured a steady 0.137 in this position I would think we are talking about a 0.137A draw from your battery, or 137mA.

    This is 3.3 Ah over 24 hours, and would empty your battery in 2weeks+, if fully charged.

    TOK ;)
     
  7. BobK

    BobK

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    Jan 5, 2010
    Okay, I am wrong, I didn't notice that that range was marked ma/A.

    Bob
     
  8. hooperdoski

    hooperdoski

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    May 29, 2012
    Gordon, I'm not 100% sure I understand your answer. If the 0.137 equals 137 milliamps then what would the 08.60 reading equal. In other words I had thought the 08 part of the reading would mean eight milliamps and the .137 would equal 137/1000 of a milliamp.

    I ask this because a normal car 12 volt system drawdown is usually five milliamps or lower, which is like the car's clock running, but 137 milliamps seems like it would drop the 12 volt battery voltage down below 10 volts in two or three days.

    In the car I checked the accessory battery (yes, it has two batteries) and it is charging up to 13.75 volts when the engine is running, but once the engine is turned off the battery drops down to 12.20 volts and the next morning it reads 11.20 volts and I get a message stating "Conveniences not Available at this Time" so it's losing one volt overnight.

    Does that sound about right with regard to what you're saying and why is there such a minute spark when I disconnect the red test lead from the negative cable after I'm done?

    Thanks.
     
    Last edited: May 29, 2012
  9. hooperdoski

    hooperdoski

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    May 29, 2012
    Gordon, a better way to put it is, I though the 08 in the 08.30 example above meant the 08 was eight amps which would cause a spark that would make a popping noise, even though that didn't happen when connecting the circuit in series.

    Hope that makes sense.
     
    Last edited: May 29, 2012
  10. gorgon

    gorgon

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    Jun 6, 2011
    As I can read from the picture, the meter say 0.000. This should correspond with the 10A scale (9.999). So your measures, if they are 0830, could be 0.83 A. You said it become steady at 0.137, and this is 137mA. As long as you use the 10Amax lead input on the meter, you are measuring with a 10A scale, no mA or uA here.

    Normally 13.75V is a low charge voltage, 14.4V or higher is more normal as far as I can remember from my 'car days'.
    If the battery falls to 12.2V it indicates a low charge and / or too high load.

    TOK ;)
     
  11. hooperdoski

    hooperdoski

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    May 29, 2012
    Gordon, I'm sorry to be such a pest.

    The measurement wasn't 0830. It was 08.30, but I just took a close look at the screen and although it can't be seen in the picture there is a small "A" following the 0.000 so I'm thinking the the eight is eight amps. But connecting the common to the battery made a very small spark. However, that may be because whatever is coming on on the connection of the common wire may start up at a lower amperage and switch to the 8.3 draw a 1/4 of a second after the current flow starts.

    With the 8.3 reading I'm afraid to set the function switch to mA/A with the red lead going to the V.U(upside down "U").mA socket. However that socket says it will handle 400mA. Do you think I would be safe trying that socket with the function selector being set on the mA/A function?

    Also, I know you are right about the charging by the alternator should be throwing out 14.4 or more voltage, but I kinda flicked that off because the battery being checked is for the convenience fuctions. But even that low charging voltage does seem weird at that battery. I will check the alternator charging voltage tomorrow.

    Please bare with me a little more because like I said in the beginning, "a hobbyist gearhead that has never understood how to read milliamps with a multimeter" and I feel I'm real close to getting it with your help.

    Thanks.
     
  12. gorgon

    gorgon

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    Jun 6, 2011
    If you suspect the measurement to be more than 400mA even momentarily, do NOT use the mA input on your meter(see below). The Ampere inputs are normally protected with a fuse, and that will break if you load it with much more than the rated current. The reason for this is that your meter has a calibrated shunt inside it, that will heat and be destroyed if the fuse is not protecting it from overcurrent.

    Since you have 4 digits on your meter, and if you have 3 decimals, you don't really need to measure in the 400mA range. As long as you want mA numbers you'll get it in the 10A range.
    You said that you got a stable 0.137A measurement after a while, and that is more than enough information for you. It equals to 137mA and you'll be no wiser if you find that in the 400mA range the value is 136.6mA?
    So what's the point in risk blowing the fuse for that?

    If you on the other hand insist to use this 400mA range, you can use a trick like below to get away from the inrush current it looks you have.

    1) Put the meter in 400mA range and use the V/mA/uA/Ohm input hole.

    2) Short the measuring leads before applying power, with a piece of cable in parallel.

    3) Connect the meter, with the short, into the power loop.

    4) Wait for the current to settle.

    5) Remove the short from the meter, without disconnecting/reconnecting the meter leads.

    6) The meter should, if you've done it right, show the correct mA in the 400mA range, as long as the current is below 400mA!

    TOK ;)
     
    Last edited: May 30, 2012
  13. hooperdoski

    hooperdoski

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    May 29, 2012
    Gordon, I'm one of these guys that has to see it before I believe it 100%. This morning I saw it by using the same setup as I used yesterday on the car with the problem. I tested another vehicle of mine that I knew had an electrical system with no known problems. It read 0.002 (two milliamps).

    Thanks so much for staying with me on this. You not only got me on the right track, but you also taught me how to read milliamps, which has been a frustrating problem for years. I wish I could repay you in someway.

    Thanks so much again ..... Bud
     
  14. gorgon

    gorgon

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    Jun 6, 2011
    No problem, you're welcome ;)

    Good luck hunting the current leak.

    TOK ;)

    BTW You could try to disconnect the dynamo/generator (I don't know what you call it) from the battery and measure again. There could be a leak in one of the diodes in the rectifier bridge. A common fault, and maybe also the reason for the low charge voltage.
     
    Last edited: May 30, 2012
  15. gcb

    gcb

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    Mar 26, 2012

    That's interesting... Does the bridge resides in the alternator itself? Assuming you meant the alternator (not native speaker of English here)

    pet peeve on the other subject: do not write uA(even less UA), microA is better if you can't type Greek letters.
    Also it never hurt to understand it http://en.m.wikipedia.org/wiki/Metric_prefix
     
  16. gorgon

    gorgon

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    Jun 6, 2011
    The bridge recitifer is normally mounted on the rear of the alternator.

    Regarding the 'peeve' around prefix 'µ'. It's normal to use the letter 'u' to replace the 'µ' character. To my knowledge 'u' has no other use in this field, and historically the letter 'µ' has not been available on most printers.
    I agree on the comment if you extend it to the general use of upper lower case for prefixes and units, 'm' and 'M' being the most frequent. Most of these can be decoded from context, but signal either laziness or low level of knowledge.

    TOK ;)
     
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