# 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.

3. ### Dodgydave AAC Fanatic!

Jun 22, 2012
6,424
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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
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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.