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Current Shunt

Discussion in 'Electronic Basics' started by Chris W, Feb 26, 2008.

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  1. Chris W

    Chris W Guest

    I was considering putting a current shunt in my car to monitor current
    draw from the battery. I found a 200 A current shunt and was wondering
    if that would be enough. Obviously during normal operation I'm not
    going to be pulling near that. But during start I could draw
    significantly more than 200 A. The shunt I am looking at is ....

    http://www.allelectronics.com/cgi-bin/item/SNT-200/search/200_AMP_SHUNT,_50MV=200_AMPS_.html

    I'm guessing since it is rated at 200A, it can handle 200A continuously.
    If it can, it doesn't seem like short bursts of high current during
    start would cause it to heat up too much to cause any problems. If the
    200A shunt isn't going to handle the start current, I guess I could find
    a way to have it measure current for everything but the starter motor.
    Then I could get away with a 100 A shunt.


    --
    Chris W
    KE5GIX

    "Protect your digital freedom and privacy, eliminate DRM,
    learn more at http://www.defectivebydesign.org/what_is_drm"

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  2. Sjouke Burry

    Sjouke Burry Guest

    I have done this without a shunt, just tap to points 10-20 cm apart
    on the main wire, that gives you enough millivolts drop to
    (inaccurately) measure the current.
     
  3. John Fields

    John Fields Guest

    ---
    Since your starter's only going to be running for a few seconds you
    may very well be able to replace the 200A shunt with a 100A shunt
    and let the starter current run through it.

    Check this out:

    http://www.emproshunts.com/eng.aspx
     
  4. Tim Wescott

    Tim Wescott Guest

    Normal automotive practice is to run a big heavy cable straight to the
    starter (or to a relay on an inner fender), with a smaller wire going
    off of that to all of the rest of the electronics. If an ammeter is
    installed at all the shunt is in that 'little' wire.

    Do you have an overriding need to monitor the starter current?

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  5. gearhead

    gearhead Guest

    Right, just measure the current after the starter.
    That's how they did it when they were making cars with ammeters in
    them, like my old IH Scout.
     
  6. z

    z Guest

    yeah, i never saw anybody try to measure starter current, other than
    maybe diagnostic purposes. it would be so out of scale with normal
    current draw as to either pin the needle, or make the normal readings
    infinitesimal; plus, you don't want to put a resistor in series with
    the starter, even a small one, thereby reducing the current.
     
  7. Ross Herbert

    Ross Herbert Guest

    :I was considering putting a current shunt in my car to monitor current
    :draw from the battery. I found a 200 A current shunt and was wondering
    :if that would be enough. Obviously during normal operation I'm not
    :going to be pulling near that. But during start I could draw
    :significantly more than 200 A. The shunt I am looking at is ....
    :
    :http://www.allelectronics.com/cgi-bin/item/SNT-200/search/200_AMP_SHUNT,_50MV=200_AMPS_.html
    :
    :I'm guessing since it is rated at 200A, it can handle 200A continuously.
    : If it can, it doesn't seem like short bursts of high current during
    :start would cause it to heat up too much to cause any problems. If the
    :200A shunt isn't going to handle the start current, I guess I could find
    :a way to have it measure current for everything but the starter motor.
    :Then I could get away with a 100 A shunt.


    I don't see that any useful information can be gained by trying to measure the
    starter current. The shunt would have to be installed in series with the main
    cable to the distribution fuse box.

    When you think about it what useful information can be had by installing an
    ammeter in a vehicle used for domestic journeys anyway? It's not as though you
    are going to be continually glancing at the dancing pointer to keep yourself
    ammused while driving, because it doesn't really tell you a lot.

    If your electrics develop a fault which drains the battery an ammeter is
    probably not going to help in finding the cause. At best it will tell you if
    your alternator is charging the battery or not, or if there is a short circuit
    to chassis which drains the battery - and you will probaly only get an
    indication of this if you take the time to look at the ammeter when switching
    off the ignition. If an alternator fault occurs while driving you probably won't
    be in any situation where it can be fixed instantly so knowing how many amps
    your battery is discharging is not really helpful. The possibility of either of
    these situations occurring during the lifetime of a vehicle is next to zero so
    vehicle manufacturers figured correctly that all the driver really needs is an
    alternator warning indicator.

    In all the cars I've owned since the mid 50's (only 5) only one has had an
    electrical problem or developed a situation where an ammeter may have helped,
    and that was caused by a body repairer. Somebody had pranged my rear end and the
    trunk lid needed repairing. The repairer had to slightly straighten one of the
    trunk lid support struts, which he apparently did in situ using an oxy-acetylene
    torch. He didn't wait for the strut to cool before closing the trunk lid to
    check the alignment before repainting the strut and the heat caused the trunk
    light switch plunger to melt thus causing the trunk light to remain permanently
    on. It took about a week for the battery to go flat but an ammeter (if
    installed) would probably not have indicated such a small current drain anyway,
    even if I had bothered to look at it when switching off the ignition. And even
    if it did, it wouldn't have told me where the fault was. I had to do that by
    analysis using a standard DMM.
     
  8. Tim Wescott

    Tim Wescott Guest

    That's a good point. Many folks feel that if you put a meter on your
    electrical system at all you should put in a volt meter. You can get a
    lot more information about the _state_ of the health of your electrical
    system, rather than the _trend_ (which is what an ammeter may or may not
    give you).

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  9. z

    z Guest

    good point. i had a car with a factory ammeter once, and most of the
    time the deflection was barely visible. it occurs to me that something
    with a logarithmic scale would probably be more useful
     
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