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  • moto-chrome
    started a topic Measure 100 amps with 10 amp meter ?

    Measure 100 amps with 10 amp meter ?

    I have built a 12 volt lamp controller capable of 100 Amps. The problem is that the meters on the market can only measure 10 amps. I have heard it is possible to do using resistors. Does anyone know how to do this ?

  • BobbyIronsights
    replied
    Re: Measure 100 amps with 10 amp meter ?

    Originally posted by asdco View Post
    stewart warner makes a 100 amp analog (i have one in my shop). If you build a shunt that big you need a special metal, can't remember the name of it (i have a piece of that too lol)
    Are you talking about nichrome?

    Leave a comment:


  • asdco
    replied
    Re: Measure 100 amps with 10 amp meter ?

    stewart warner makes a 100 amp analog (i have one in my shop). If you build a shunt that big you need a special metal, can't remember the name of it (i have a piece of that too lol) its not expensive (i got 3 feet of it for 10 bucks) but you have to build up a bridge with it and it gets super hot when running. The way it works is; the bridge carries the entire current load and the amp gauge just takes a test reading off the bridge. the bridge metal has a known ohm drop per inch. you can adjust the meter by lengthining or shortening the distance between the amp wires. The amp wires hook parallel to the shunt. All in all, its easier to just find a stewart warner gauge, I think I paid around 50 bucks for mine.

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  • sdold
    replied
    Re: Measure 100 amps with 10 amp meter ?

    Right on, that's cool that it works. Dadkar2's shunt idea is a good one, I completely forgot about those.

    Steve
    Last edited by sdold; 11-01-2005, 01:07 AM.

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  • moto-chrome
    replied
    Re: Measure 100 amps with 10 amp meter ?

    Thanks for all the suggestions. As a cheap solution I followed SDOLD's suggestion. I used a cheap multimeter that can read 200 mV. One lead is connected to the start of my positive bar (1/2 in copper tubing) and the other is connected to the same bar about 5 ft away, at the same point where my anode jumper connects. The only disadvantage is that the jumper going to my anodes must always be connected (welded) at the same point, so I had to make it a little longer to be able to reach all plating tanks. Accuracy is only 95 % at the low and high end but pretty damn good from 5 to 95 A. A reading of 1 mV = 1 A.

    I will eventually buy a good shunt.

    Leave a comment:


  • dadkar2
    replied
    Re: Measure 100 amps with 10 amp meter ?

    If you want a really solid solution, It turns out you can buy a 100 or 300A shunt from Mouser. They are in the $45-55 range. Here is the link which will take you to their online store:

    http://www.mouser.com/index.cfm?hand...denumber=52917

    You can use these with any standard DMM or analog meter that can measure 50mV. The concern with the previously described solutions is handling the power in the shunt. These shunts are designed for the amperage you're asking to measure. The wire idea is really cool, but calibrating it will be more tricky than you might think at first. Also, it has a tempco of about .4% per degree C. As the wire self-heats, the value will change.

    If you prefer an analog meter, they're for sale too, on the same page!

    My search on Caswell's page turned up no products for sale or I would have recommended those first. It could be that Ebay might be a source for these...but you might not get what you think you're getting especially if it were abused. My batting average in buying electronic instrumentation on Ebay (and having it work to spec) has been below 50%.

    Regards,
    Ken
    Last edited by dadkar2; 10-26-2005, 09:25 PM.

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  • HobbyTalk
    replied
    Re: Measure 100 amps with 10 amp meter ?

    1 foot of 10 gage copper wire is .00118 ohms. From this you can easily figure amps by measuring the voltage drop across the wire.

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  • eprigge
    replied
    Re: Measure 100 amps with 10 amp meter ?

    In the end it doesn't really matter whether you have a DMM in current or voltage mode or a panel meter or what, you still need to buy/make a very low value high power shunt for the meter. You don't want anything much over 0.001 ohms just because of heat dissipation.

    Again I'd suggest using a length of 10-12 (or thicker) gauge wire as the shunt. If you can get a known value current source from a power supply, you should be able to make your own shunt to work very accurately with whatever meter you choose.

    There usually several power shunts made for this purpose on ebay also.

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  • seanc
    replied

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  • sdold
    replied
    Re: Measure 100 amps with 10 amp meter ?

    Yeah, no kidding, they are cheap

    I thought he was looking for a multimeter, since he said everything he was seeing was 10A, which sounds like the typical DMM. I might have been wrong.

    Maybe he could just get a panel-mounted meter, aren't they something like 0-50 microamps? And put some kind of shunt around that. For a hundred amps, maybe a 1-inch piece of copper bar would be about right

    Leave a comment:


  • seanc
    replied
    Re: Measure 100 amps with 10 amp meter ?

    Steve:

    Yep, a multimeter works great, but sometimes it's not convenient.

    In my case, I only have one DMM, and I use it for a lot of other things besides plating, so the dedicated amp meter relieved me of constantly switching the meter around.

    But the price of some of these offshore DMMs is so cheap anymore, it might be worth it to get a few of them for dedicated purposes.

    Sean

    Leave a comment:


  • sdold
    replied
    Re: Measure 100 amps with 10 amp meter ?

    Originally posted by seanc
    Steve:

    That's all a digital ammeter is: a voltmeter w/a built-in shunt.

    Sean
    Right, so if moto-chrome is going to use a multimeter, he can build his ammeter with the multimeter set as a voltmeter, across an external shunt, and not have to do any of the parallel resistance calculations.

    Steve
    Last edited by sdold; 10-21-2005, 01:33 AM.

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  • seanc
    replied
    Re: Measure 100 amps with 10 amp meter ?

    Steve:

    Originally posted by sdold
    There's an easier way to do it, just find a shunt resistor with a known (small) value of resistance, say 0.1 ohms.
    That's all a digital ammeter is: a voltmeter w/a built-in shunt.

    I got a surplus 200mA digital panel meter, which used a 1-ohm shunt (the actual meter was a 200mV full scale "movement"). Wired in additional 0.1 & 0.01 ohm resistors through a switch, and now have a switchable 20A, 2A, 200mA dedicated ammeter.

    Sean

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  • seanc
    replied
    Re: Measure 100 amps with 10 amp meter ?

    Originally posted by eprigge
    No, my numbers are right
    My apologies, you were correct. My dsylexic brain was mis-reading your value as 9/10. Too early in the morning and not enough coffee yet. Sorry.

    Sean

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  • eprigge
    replied
    Re: Measure 100 amps with 10 amp meter ?

    Originally posted by seanc
    eprigge:

    Your values are off.

    The formula for parallel resitance is: (R1 X R2)/(R1 + R2)

    In order to read 10x the original amperage, you need 1/10 the original shunt resistance. Your numbers produce just under 1/5 the resistance, and the meter would be way off.

    The correct value for the second resistor would be @ 0.11 x the first value
    No, my numbers are right, 0.11 is (close to) the 1/9th as I said... the formula for the second shunt resistor is Rs = Rm/(n-1) where n is the factor of division you want to get on your meter readings and Rm is the meter shunt resistance.

    I would hazard a guess that a DMMs 10A range shunt resistance is around 0.01 ohms. Any more than that and you'd have too much power dissipating inside the meter. You might be able to find out from the meter docs or the manufacturer instead of measuring it yourself. If it was 0.01 ohms then the second shunt would need to be 0.00111 ohms and would need to handle 9 watts to carry 90A.

    I would use a constant, known current source like a current controlled power supply to make the new shunt. It's such a low resistance you will probably just make it out of a length of copper wire. If you know you've got 10A flowing through the load, diddle with the shunt resistance until your meter reads 1.00. Voila. In the example above, about a foot of 10-12 gauge wire would be in the 0.001 ohm ballpark.
    Last edited by eprigge; 10-20-2005, 02:41 PM.

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