Using shunt resistors to measure large currents

CALLD
CALLD Solar Expert Posts: 230 ✭✭
in Off Grid Solar & Battery Systems #1
Does anyone know how to calibrate a shunt resistor to use with a pocket sized digital voltmeter?

I want to measure DC current up to about 125 amps sustained and brief surges/spikes up to 300 amps. Would be nice if I could use my pocket sized digital multi-meter on the millivolt setting and calibrate it to show amps at a rate of 1amp = 1 millivolt across the shunt. If I could somehow ensure that the resistance across the shunt was exactly 0.001 ohm would this work?

First problem would be how on earth does one get a shunt of such a low resistance to be that accurate within the required tolerances, and still maintain the value with 100+ amps hell-bent on heating it up?

Second problem is obviously increasing the resistance to give the volt meter more to work with will just exacerbate the heating effect and result in more power losses.

Is this something that is possible to DIY reliably or should I just cough-up and go to the professionals for a high-end ammeter?

D
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #2
    Re: Using shunt resistors to measure large currents
    CALLD wrote: »
    Does anyone know how to calibrate a shunt resistor to use with a pocket sized digital voltmeter?
    <snip>
    Is this something that is possible to DIY reliably or should I just cough-up and go to the professionals for a high-end ammeter?
    Calibrate a shunt resistor? A precision calibrated shunt resistor is cheap.
    Why buy a high-end ammeter, when you can buy an inexpensive shunt to use with your voltmeter?
    http://www.solar-electric.com/batteries-meters-accessories/metersmonitors/mka-100-100.html
    http://www.solar-electric.com/batteries-meters-accessories/metersmonitors/mkb-500-50.html
    CALLD wrote: »
    First problem would be how on earth does one get a shunt of such a low resistance to be that accurate within the required tolerances, and still maintain the value with 100+ amps hell-bent on heating it up?

    Second problem is obviously increasing the resistance to give the volt meter more to work with will just exacerbate the heating effect and result in more power losses.

    I think your first and second problem are the same problem. I don't know the technology behind modern shunt design that allows them to hold their resistance constant while heating up.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • CALLD
    CALLD Solar Expert Posts: 230 ✭✭
    #3
    Re: Using shunt resistors to measure large currents
    vtmaps wrote: »
    Calibrate a shunt resistor? A precision calibrated shunt resistor is cheap.--vtMaps

    Thanks yes after googling a bit I see that. Think I just need to get one rated at the highest possible wattage. Will need to stack a few in series to get to one milliohm as the ones on offer are rated at 0.15 - 0.25 milliohms, with the potential problems being losses in accuracy at the series connections! The shunts will be permanently connected between battery and inverter so allowances will need to be made for a possible elevated working temperature. Perhaps I can do some calculations to ensure the elevation in temperature remains at insignificant levels under average loads...
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #4
    Re: Using shunt resistors to measure large currents
    CALLD wrote: »
    D

    I want to measure DC current up to about 125 amps sustained and brief surges/spikes up to 300 amps.
    Depending on how brief these "surges/spikes" are, you may have to use an oscilliscope, as regular volt meters take time to process changes, causing inaccurate readings of brief spikes.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #5
    Re: Using shunt resistors to measure large currents
    CALLD wrote: »
    Will need to stack a few in series to get to one milliohm as the ones on offer are rated at 0.15 - 0.25 milliohms, with the potential problems being losses in accuracy at the series connections!

    more selection:
    http://www.murata-ps.com/data/meters/dpm_shunts.pdf

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #6
    Re: Using shunt resistors to measure large currents

    Why not use a DC current Clamp DMM? Much easier, quicker, and safer (just clip the jaws around the wire in question).

    And you can find meters with "peak hold" function.

    Not cheap for a Fluke or similar quality meter.

    If you want something cheap, this one is "Good Enough" for our needs (except missing the peak hold function). Not quite pocket size-But there are other brands/models available that are a bit smaller.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Mountain Don
    Mountain Don Solar Expert Posts: 494 ✭✭✭
    #7
    Re: Using shunt resistors to measure large currents

    That sears meter is a great deal. For us DIY RE users I believe it is ideal.
    Northern NM, 624 watts PV, The Kid CC, GC-2 batteries @ 24 VDC, Outback VFX3524M
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  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    #8
    Re: Using shunt resistors to measure large currents

    I think your reinventing the wheel there , mate. The industry standard is the 50mV 500A deltec shunt. They are rated at 0.25% accuracy, and have a near zero thermal characteristic.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


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  • jcheil
    jcheil Solar Expert Posts: 722 ✭✭✭
    #9
    Re: Using shunt resistors to measure large currents
    zoneblue wrote: »
    I think your reinventing the wheel there , mate. The industry standard is the 50mV 500A deltec shunt. They are rated at 0.25% accuracy, and have a near zero thermal characteristic.

    I use several of the Deltec shunts. They are pretty much dead-on accurate and only $27 each and have a very nice mounting block.
    http://www.solar-electric.com/mkb-500-50.html

    Although I am still in search of an external digital "panel" meter that is calibrated for 50mv at 500a. Right now I use a digital panel meter that just shows MV. Which is good enough for me because I simply use that reading to make sure that the my two parallel batter banks are charging equally.
    Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html
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