Why did the multimeter leads arc?

Discussion in 'General Electronics Chat' started by foolios, Nov 21, 2009.

  1. foolios

    Thread Starter Active Member

    Feb 4, 2009
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    I was about to go out to the car to check how many amps were being drawn from a parasitic component in the circuit. Battery has been dieing if I leave it connected for a few days. The 12VDC test light connected from the positive battery post to the positive cable(after pulling cable off) shows that there is a draw which causes the test light to come on. Pulled all the fuses one by one to see which one would cause the light to shut off. But none of the fuses being out stopped the draw.

    Anyways, what I did was I set the multimeter to 10 Amps. I know I've been able to get around 14 amps from the car battery when when the car is running which shows that the alternator is doing its job well. So, like a goofball I decided to see the reading on the a/c wall outlet thinking o ya, this circuit is 15 amps.
    Well, you know what happened, I had the multimeter set to 10ADC. A nice arc shot from one lead to the other as soon as I when from the hot to a grounded screw. I was surprised that the power didn't shoot through the meter and blow it up.
    Turns out there isn't even a 10A a/c setting for this multimeter.

    So, I have two questions:

    1) Why did the arc jump to the other lead instead of completing the path through the multimeter?

    2) Why is 10 amps of AC different from 10 amps of DC when it comes to taking a reading with a multimeter.

    I know I shouldnt have done that, but I don't understand why. Since I'm around to learn from my mistake I would really appreciate an explanation.

    Thanks in advance

    EDIT: Come to think of it. A car battery is advertised as having 300 cold cranking amps. Now I'm really confused as to why the ac amps were too much for the dc amps reading...
     
    Last edited: Nov 21, 2009
  2. ifixit

    Distinguished Member

    Nov 20, 2008
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    The ammeter is only to be used to measure current flow in series with a circuit that is carrying current that is =< the range setting of the meter... in your case, 0.010 Amps (10mA). Since there was more than 15 Amps available at the wall socket... you blew your meter up. :eek:

    The same would have happened on the battery with more than 300 Amps available.

    If you want to measure that phantom current draw then put the ammeter between the battery terminal and the cable which will put it in series with the load. WARNING! you can blow up your new meter if that current is much more than the meter can handle.

    Note: You may have only blown a fuse inside the meter.

    Good Luck
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    Rule of thumb: keep your meter set to 20VDC around an automobile.

    Otherwise, you will either blow fuses or "smoke" your meter.

    I'm not kidding.
     
  4. foolios

    Thread Starter Active Member

    Feb 4, 2009
    160
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    Thanks for the advice and info
    ::"image removed due to hosts image bandwidth constraints"
    I believe this one has 10A, not 10mA or am I mistaken?
     
    Last edited: Jan 23, 2010
  5. foolios

    Thread Starter Active Member

    Feb 4, 2009
    160
    1
    doh, now I know what the problem is. I was writing 12 amps from the car battery, but it's not amperage, it's voltage.
    I had that all messed up. Ok, now I see where the problem is. The car battery prolly puts out 300 amps at 12 volts. I can't measure that amperage with my multimeter only the voltage.

    But I would still like to understand the difference between the ac amps vs the dc amps. What is it about the design of the multimeter that does not allow it to measure a/c amps the same way it measures d/c amps? Is it because of the voltage? The a/c line has 120V whereas most dc circuits prolly use a range of 3v - 12v.
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    First, if you accidently short-circuit a good, fully-charged car battery, it is capable of supplying more than 2000A!!! It will usually vaporize whatever causes the short...

    Second, on your parasitic drain problem. Here is how I do it. Loosen the battery cable on the positive pole of the battery, but leave it connected. Set your multimeter to DC AMPS, 10A scale. Hold the red meter probe on the positive battery pole. Hold the black meter probe on the outer part of the positive cable connected to the battery. While holding both probes in place, break the connection from the battery cable to the battery pole.

    The parasitic leakage current that was formerly flowing through the battery connection is now flowing through your meter, without breaking the circuit as the cable was disconnected from the battery.

    If the current is over 10mA (0.01A), you have a problem you need to track down. You might have to switch the meter to a lower DC full-scale current setting to accurately measure the leakage. In all cases, start at the 10A setting and work down...

    I have had a parasitic leakage problem in several vehicles and airplanes over the years. One turned out to be leaky diodes in the alternator (there is no fuse between the big stud on the alternator and the battery positive pole (only a "fusible link").
     
  7. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Your meter in the photo does not measure AC current.
    This is common for elementary meters.
     
  8. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
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    You might want to read through chapter 8 in the DC ebook and specifically section 8.5, as it will explain the correct use of an ammeter.

    Another measurement option to be aware of is the use of clamp-on ammeters. There are models that can be used to measure DC current (they use Hall effect sensors) and they can be mighty handy around a car when you need them. Here's an example: the B&K 316, which is stated to be able to read 1 mA on the 10 A circuit. I've found they're a bit noisy to read down to 1 mA, but I'll trust one to around 10 mA. I used one to troubleshoot a parasitic load problem in a 2003 Dodge truck that belonged to a friend. The nice thing about clamp-on ammeters is that you (typically) can't hurt them if the unknown current is much larger than the selected range. The typical DMM will blow a fuse inside and you may be suprised how much replacement fuses cost (especially if your DMM is made by Fluke). :p

    I've also used my beloved HP 428B to measure currents around the car. It's a clip on ammeter that can read down to 1 mA full scale. I was very lucky to bag one in perfect condition a number of years ago on ebay for $50 delivered -- it looked brand new when I got it. Of course, I had been watching for one for 6 or 7 years. For those that may be unaware of this tidbit, HP continued to manufacture this instrument up until the mid 1980's or so -- a vacuum tube design still in production long after vacuum tubes were fossils in most electrical test instrumentation.
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    You were trying to measure the max current available from the utility power transformer that powers your neighbourhood. It might have a short circuit current of 2000A or more.

    An electrical outlet is fused at 15A so that it can supply no more than 15A [/b]continuously[/b]. Its instantaneous current is probably 2000A or more.

    Didn't your leads burn out (not arc)? Maybe they arc'd when they burned out.
     
  10. foolios

    Thread Starter Active Member

    Feb 4, 2009
    160
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    Thanks for the info all.

    I may not try the battery trick. Have had enough of a scare already.
    But I'm definitely going to read that section.
     
  11. foolios

    Thread Starter Active Member

    Feb 4, 2009
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    Wow, didn't realize that.

    Yep, they arc'd and one of the probes burned up a little on the end. I guess that amperage explains why the arc. There was enough there to just jump to the other lead, I guess there was enough voltage to make it jump through air instead of following the path through the meter.

    But why can I measure the batteries DC amperage and not have the full current force that the battery can provide force it's way through like the ac wall outlet did? I see that with the wall, and the way that the multimeter works, it's letting the current pass through it as if it was the connection without a resistance. I'm confused as to why there are differences between these two circumstances.
     
    Last edited: Nov 22, 2009
  12. Audioguru

    New Member

    Dec 20, 2007
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    You used your current meter to short circuit the electrical power of your entire neighbourhood.
    If you short circuit a charged car battery with your current meter then it would blow its fuse if it has a fuse, or the leads would immediately burn.

    But you didn't short circuit your car battery with your meter, you connected the meter in series with the load and measured the current produced by the load.
     
  13. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Every electrical experimenter is entitled to one suprise.

    But Please do some more reading before any more practical work.
     
  14. foolios

    Thread Starter Active Member

    Feb 4, 2009
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    Totally understandable now. Thanks for making that sensible to me.
     
  15. foolios

    Thread Starter Active Member

    Feb 4, 2009
    160
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    I've been looking at multiple sources for how to use the multimeter.
    The following quote has irritated me so much. Most everything I read describes current moving from negative to positive. But not this article:
    http://www.wikihow.com/Use-a-Multimeter

    at the line #6:
    Open the portion of the circuit that is to be tested (one lead or the other of the resistor). Insert the meter in series with the circuit such that it completes the circuit. An ammeter is placed IN SERIES with the circuit to measure current. It cannot be placed "across" the circuit the way a voltmeter is used (otherwise the meter will probably be damaged). Polarity must be observed. Current flows from the positive side to the negative side. Set the range of current to the highest value.

    Why is it being described as such? Shouldn't we be following electron flow theory and not conventional when we are applying this to the physical?
    Just seems so dangerous to be mixing this...
     
  16. SgtWookie

    Expert

    Jul 17, 2007
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    Yes, it can be confusing.

    I tend to do everything in electron flow rather than conventional flow.

    Many people use conventional flow most of the time.
     
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