Measuring high AC current

[BorisKontorovich]

I need to measure current going to an 1800W AC heater (its a heater used in common household coffee maker.)  Its powered form 110V 15A line.  I am looking at using a shunt resistor, but all shunt resistor are rated for DC.  I know that resistor behavior is the same for AC and DC but I want to check if there are other considerations here.

Can anyone think of any caveats?

Please, let me know.

Thank you.

[ejeffrey]

Easiest/best is to use is a current transformer.  This gives you galvanic isolation, common mode rejection, and low power dissipation.  If you don't need these advantages a shunt resistors work fine for 15 amp as long as they are rated for the peak of the AC voltage waveform (~170 V).

Hall effect sensors can be used for DC current measurement with galvanic isolation, but they are not needed for 50/60 Hz AC.

[BorisKontorovich]

I did not know of the current transformer option, so even if I dont use it its still good to know.

Thanks for your help!

[james_s]

Current transformer is definitely the way to go for something like this. They're readily available and inexpensive, you just have to split the wires somewhere so that only one side of the circuit passes through the CT, otherwise it will cancel out and read zero.

[IanB]

For one off measurements a clamp meter would be the easiest way to go.

[ed_reardon]

Current transformer is definitely the way to go for something like this. They're readily available and inexpensive, you just have to split the wires somewhere so that only one side of the circuit passes through the CT, otherwise it will cancel out and read zero.

You know, I work with an electrician who on more than one occasion has placed a clamp ammeter over traditional mains flex and reported a current of 0A.... 

A basic clamp meter would be my weapon of choice, but there's something nice about using a CT!

[rstofer]

If you use a physical current transformer, as opposed to a clamp-on meter, DO NOT open the secondary circuit.  Make certain the output is connnected at all times.  BTW, the output of a current transformer will be a current, not a voltage.  You still have to measure the current.

Imagine a 100:5A current transformer with, say, 15 Amps in the primary.  The output current will be 15/20A and the transformer will try to produce that current into whatever resistance is connected across the secondary.  If the resistance is infinite  (open circuit) then Ohm's law says the voltage will also be infinite.  E = I * <a really big number> results in E being <a really big number>.

Current transformers come with serious safety implications.  See the Caution on the first page:
https://www.omega.com/manuals/manualpdf/M1858.pdf

[Kleinstein]

For just a quick measure of the current consumption of a appliance from a normal outlet one could use one of those relatively cheap energy measuring devices. In addition to the power they usually also measure the current and line voltage. Not very accurate, but save and easy to use.

The current transformer is very similar to the AC clamp on meters. Due to the closed core they tend to be more accurate and extend down to smaller currents, though high currents are possible too. For high currents one turn for the primary, just like the clamp on meters can be enough. However as the core is fixed it needs the wire to be opened to put it through. They work for AC only - a superimposed DC current could saturate them and lead to wrong readings.

[BorisKontorovich]

Thank you everyone for the information.  Much appreciated!

[larsdenmark]

You can use a ACS712. It has high degree of isolation and outputs a voltage that is proprtional to the current.

[james_s]

For just a quick measure of the current consumption of a appliance from a normal outlet one could use one of those relatively cheap energy measuring devices. In addition to the power they usually also measure the current and line voltage. Not very accurate, but save and easy to use.

Those Kill-A-Watt devices are actually surprisingly accurate, years ago when they first showed up on the market I compared one against a proper benchtop power analyzer and discovered it was accurate to within about 1% across most of the range. I was astonished, I expected it to be much worse.

[Zero999]

If you use a physical current transformer, as opposed to a clamp-on meter, DO NOT open the secondary circuit.  Make certain the output is connnected at all times.  BTW, the output of a current transformer will be a current, not a voltage.  You still have to measure the current.

Imagine a 100:5A current transformer with, say, 15 Amps in the primary.  The output current will be 15/20A and the transformer will try to produce that current into whatever resistance is connected across the secondary.  If the resistance is infinite  (open circuit) then Ohm's law says the voltage will also be infinite.  E = I * <a really big number> results in E being <a really big number>.

Current transformers come with serious safety implications.  See the Caution on the first page:
https://www.omega.com/manuals/manualpdf/M1858.pdf

And make sure the current sense resistor is can handle the power dissipation, otherwise it will overheat and quite likely fail open circuit. Someone where I work damaged an oscilloscope, because they used a current sense resistor with an inadequate power rating, which failed open circuit and injected many kV into the 'scope input.