help calibrating A/C shunt with known load

Discussion in 'The Projects Forum' started by bfarmer54, Mar 25, 2015.

  1. bfarmer54

    Thread Starter New Member

    Mar 25, 2015
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    If I apply a known load (100 watt incandescent light bulb or multiples of same) across an unknown shunt, will the circuit still draw 100 watts if the line voltage varies (120V A/C nominal). I do not have (that I know of) a way to vary the line voltage to determine this experimentally. I am trying to calibrate the shunt (Basically a length of wire with two taps)
     
  2. wmodavis

    Well-Known Member

    Oct 23, 2010
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    Calibration requires some ability to control and/or measure with the required precision in order to obtain suitable results.
    Yes power will vary if applied voltage varies.

    But do NOT apply your known??? load ACROSS the shunt.
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,993
    745
    Wattage = Voltage x Current
    or
    Voltage x Voltage /Resistance

    so increasing the voltage increases the current,
    best to use an heater from a kettle than a bulb.
     
  4. DickCappels

    Moderator

    Aug 21, 2008
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    Yes, sort of. Metals, in this case tungsten, when heated to a critical temperature, in this case a few to several thousand degrees kelvin tend to change resistances so as to maintain and almost constant current (in the case of AC it is the absolute value of current averaged over some time envelope. The result is that a 100 light bulb if given enough voltage would act very much like a constant current source. The problem, in line with what wmodavis's comment comes in calibration because a 100 watt incandescent bulb can easily be off by 20% or or more.
     
  5. bfarmer54

    Thread Starter New Member

    Mar 25, 2015
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    My bad. The load would be downstream from the shunt, not across it.
     
  6. bfarmer54

    Thread Starter New Member

    Mar 25, 2015
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    Ok, I was not precise enough with my question and I failed to disclose that the light bulb had been calibrated against a known precision shunt. It has been my limited experience, however, that incandescent bulbs are remarkably repeatable from bulb to bulb (1-2%) variation. You do, however, need to let them settle for 10 seconds or so. So the rephrased question is "if the circuit can supply enough current (we are talking a normal 20amp circuit here), would the same bulb draw 100 watts at 115V and at 120V? I understand that an amp reading would go up at the lower voltage. My goal is to be able to calibrate multiple shunts with a known load across each during calibration.
     
  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
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    If the shunt has an effective resistance of less than a few tens of mΩ, then it will not effect what is going on in the measured circuit, and it will not heat up enough so that changing the resistance of the shunt material will be a problem. I would make the shunt such that the voltage drop across it would be less than 100mV at the highest current you are interested in.

    My Fluke DVM has a 10Aac range, so I would put your load(s) (light bulbs), the Fluke, and the shunt in series. Vary the wattage of the bulbs to get your desired max. current. Then I would measure the ac voltage across the shunt with that same load current to compute the shunt resistance.

    Or buy a precision shunt ready made.
     
  8. DickCappels

    Moderator

    Aug 21, 2008
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    I can't tell you how much change in current you will see, but extensive experiments with low power incandescent bulbs (8 to 20 mw for critical temperature) shows that they keep the current nearly constant from the time there is a dull glow until the voltage is high enough to melt the tungsten (abnormal catastrophic mode of failure). That means that you should get nearly the same current at 120 as you at 115 if it is a bulb designed for 115V operation.

    I found huge deviations between the labeled wattage and the measured dissipation for several incandescent bulbs that I tested a few years ago, including large variations between bulbs of the same brand and part number. I can believe 2% variation but for a bulb used for indoor lighting, it is a very tight result.

    The best way to determine how much the current will change is of course, by doing the experiment.
     
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