There are special transformers called current transformers. They look like toroids where you put the wire whose current is to be measured through the hole and the pair of terminals on the side of the toroid have 1/N short circuit current as is in the through wire. You put this to your measuring circuit.
There are also special probes called clamp on current probe. These are made by Fluke and Tektronix.
You can use the current transformers with an LM311 to make a zero cross detector. Compare the output of the transformer with a zero reference on the other LM311 input.
I'm actually working on something that incorporates power measurement. Current can range from near 0 to 15 amps. The current transformer idea might work but was hoping to incorporate the components onto a PC board. I can probably do that with the transformer but I recall seeing somewhere the idea of using triacs. Problem is, I'm not that familiar with triacs. I am about to become more familiar but before going to far down the wrong path I thought I would seek some advice. Ideas are welcome
I think I'm getting closer and thanks for the suggestions but I can't see how this helps for measuring current zeros. My TRIAC thoughts were to somehow determine current thru the device - again, I'm a TRIAC newby so this may/may not be possible.
Most likely this will work in your design. Use Motorola zero cross over optically isolated triac. I am not sure but MOC3041 and others are the IC numbers. They come in different voltage ratings and they are very cheap.
You can use a shunt resistor to measure the current and with the shunt you can use the analog devices chip ADE7753 by this IC you can measure the current and voltage.
Why waisting time? the professional way is to use a current transformer.
Using a copper trace on the PCB is wrong (temp drift) you have to compensate!!!!
Using a sense resistor is wrong (temp drift and power derating) you
have to compensate as well.
Current transformer and a opamp or LM311, LM393 comparators is
the solution.
The cost for the current transformer is high than the cost of the shunt resistor. Also the resistance of the shunt is in the range of uohm and no problem found in using it.
