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Water Heater Monitoring

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vtsteam:
Okay, my turn to be confused (or perhaps, confusing).

Stuart, isn't he actually showing a few miliamps across a shorted secondary of a very, very inefficient 110 V step up transformer?

Inefficient as the transformer is, meter resistance must be the big factor in limiting current. A true short would theoretically pass infinite current no matter how low the voltage, unless there was some resistance in the circuit. That resistance is significant no matter how low, if the voltage is correspondingly low.

And I don't think the meter resistance in this case is likely to be a constant for two different primary voltages.

BaronJ:
Hi Guys,

A number of posts have touched on, but not actually said that a burden resistor is needed on the output of a current transformer used to measure AC current.  The burden resistor is needed to firstly prevent dangerous voltages being produced by the large turns ratio and secondly to provide a reference for the current being measured.  The meter should be set to read voltage and should have, ideally, infinite impedance.  Once these conditions are met the calculations for power are straight forward.

DavidA:
As promised above,  here is the result of my water raising experiment.

Equipment.

2,200 Watt electric kettle.
Workzone watt meter.
Therma 3 thermocouple thermometer.

Procedure.

1 litre of tap water added to kettle and left to stand for a while to allow temperature to settle.
Thermo couple placed in kettle.
Kettle plugged into powermeter.

The powermeter also displays voltage and current as well as wattage. It has a second counter on it.

Power supply to meter/kettle turned on. turned off after 120 second.

water in kettle stirred around.
Temperature taken

Result.

2,200 cc of water raised from 10 C to 37 C in 120 second.  = 27 degree raise.

Dave.

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