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Ohms law brain freeze

Discussion in 'Boat Electronics' started by Steve, Nov 26, 2004.

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  1. Steve

    Steve Guest

    You could use a shunt but I have found that if the length of wire from
    the switch/distribution panel is sufficient and the wire is not totally
    overly thick you can get good results just measuring the drop across it.
    Typically it is not unusual to have a drop of 500mV from the battery
    to the load but this will include the cables to the distribution panel
    so the wire you are voltage drop you are working with will be less. On
    my boat the wire was so thin the drop was over 1V at the dis panel when
    the fresh water pump kicked in. Copper wire is not be as thermally
    stable as a proper shunt would be but copper is pretty good anyway
    (within a few percent for a 10 degree C temp change). Your electronics
    may not be this stable anyway. You will need to calibrate the wire using
    a known load or ammeter/volt meter and will still need the differential
    amplifier to boost to get your full 2.5V range.

    Obviously you would be best using the negative side of fridge circuit or
    you will need to bring the voltage down from 12ish volts to <2.5. As a
    warning I have found with some data logger circuits that when the
    voltages are close to 0 there can be a issues. Might be worth
    considering when you test. If your logger power supply is tapped from
    somewhere closer to the battery than the fridge you should at least
    avoid getting a relative negative on the logger input. Negatives
    voltages can be as bad as over voltage I have found.

  2. I should know this but I just can't get my brain around it.

    I have a data logger that will accept 4-20 mA or 0-2.5VDC. I want to track
    the current going to the refrigeration compressor which I figure will be in
    the range of 0 to 25 amps (to be safe for start ups).

    What is the best way to accomplish this?

    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at:
    Shameless Commercial Division:
  3. Doug Dotson

    Doug Dotson Guest

    Put a shunt in the negative power lead and measure the voltage
    across the shunt. A 30A/50mv shunt should work nicely. An
    Inst Amp with a gain of 50 can bring the voltage up to the range
    of your data recorder.

    s/v Callista
  4. I really need to mesure actual amps because the compressor has a "smart"
    variable speed control. I found a 25A 50 mv shunt for $10 so I think I will
    go that way.

    These little 4 chanel Hobo U12 data loggers are neat. What I plan to do is
    record ambient, freezer and fridge temps along with amps used for a week on
    a friends boats headed for the Bahamas next month. The interior size and
    mechanicals of his refrigeration system are the same as mine but
    traditionally foam insulated so that will give me a good baseline. This is
    a particularly good opportunity because he is a detail fanatic and plans to
    record everything he adds to the box.

    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at:
    Shameless Commercial Division:
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