DC Ammeter Shunt?

Discussion in 'General Electronics Chat' started by pntrbl, Apr 21, 2008.

  1. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
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    1st post. I've been nosing around the internet looking for an Electronics related BBS and this site appears to be where the smart guys hang out. Which of course begs the question, "What am I doing here?" :D But I'll do my best.

    I had actually hoped to be a little more prepared before posting, I'm just about thru reading Chapter 1 at All About Circuits, but as usual something has come up.

    What's the best way to connect a shunt to a DC Ammeter? It's a 100 Amp meter with a 50mv shunt that I figger must be .0005 ohms? A half a milli-ohm? Is that right? If the resistance is that small I'm thinking a couple of long 22 ga wires would hardly be appropriate.

    So ....... I've got some 8ga wire, but I'm wondering if some brass strips would be even better? Hobby shops stock .005 brass shimstock I could cut up and sandwich. Some copper bar stock seems like it'd be good idea but I'm not knowing where to source any.

    Thanx for any responses. I'll do my best to fool you guys into thinking I'm at least half way intelligent ....

    SP
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    Why you want to place a shunt to a 100amps meter?
     
  3. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
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    I thought I was supposed to? LOL! The shunt goes in parallel with the meter movement to drop just the tiny amount of voltage necessary to generate enough current to peg the meter at 100 amps? I thought I learned that but I'm open for correction.

    What I'm actually trying to build is a gas powered battery charger that runs on demand. Measuring largish DC currents is gonna be a requirement.

    SP
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    Ok but you dont need to put a shunt resistor to measure currents. The design of the meter itself is all right. You use a shunt resistor if you have an ammeter which can measure a max current of say 10 amps and you want to measure currents up to say 30 amps. The ammeter it self can not pass through it 30 amps so you use a shunt resistor to shunt (divert) the excess 20 amps through it and leave the other ten through the ammeter. Of course you have to use the correct value of shunt resistor as to take the right measurments.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    1,728
    If he has a 100A ammeter and wants to measure current in a range higher than that, he will need to add shunt bars.

    Brass is a pretty lousy conductor, actually - about 5x the resistance of copper, which is about 1.5x the resistance of silver. Tin/lead solder is actually pretty bad compared to copper, too - about on par with brass.

    But I digress. Getting back the original question:
    How much current do you think you're going to need to measure?
     
  6. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
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    The gas powered part of this deal is an electric start Briggs 5.5 so first I'll need to measure how much current it draws to size a relay. Just guessing but I'm expecting maybe 10 amps.

    Eventually I'll have that belted to a GM automotive alternator that should put out somewhere between 50 and 100 amps. As the battery charges that current should decrease and at some point I'll have to figure out how to use that decreasing current to kill the ignition and shut the Briggs off.

    So knowing the flowing is gonna be important.

    There's a very good chance I'm not explaining myself well enough on this. Happens all the time. What I've got is a Chinese panel meter that came with the 50mv shunt in the box. Even got a wiring diagram. If I was to put a 100 amps straight thru the meter I gotta feeling those little wires in there are gonna go poof!

    Realizing how small an amount of resistance the shunt is at .5 milliohms my thought was the connections to the meter are gonna be real critical for any kind of accuracy. But now that I've thought about it some more .......

    It's in Chapter 1. A parallel circuit has differing currents in each branch and a constant voltage across the branches. The constant voltage will be the wee bit the shunt develops and who cares what the resistance is in the other branch as long as it moves the needle? Turns out the meter movement is 25 ohms so if I add a couple tenths in connection losses what difference is that gonna make? Not much. I'm getting all fussed up about nuthin' ....

    Well, that was my 1st dumb question! :) Thanx for letting me think it thru.

    SP
     
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