I have a three-phase variable supply which is 0-50v. I want to read the voltageand current into a PC vai my a/d interface which is working fine. What would the best way of interfacing to get the line voltage between two phases? I was thinking of a differential amplifier scaling down from 50-5v and something similar for the current using a few windings of a home built current transformer.
You might give us a hint about that three phase source, its frequency, VA capability, and current range. Generally you will want to isolate it from the T&M stuff.
A potential transformer would be the safest. A differential amp would likely not have an input range up to 50 V, but if you connect matched pairs of resistors at the input, that should work. Current transformers will work fine for current, but also using the matched pairs of resistive voltage divider with something like the OP620 instrumentation op amp to sense drop across current sense resistors will work.
Well, three phase delta will require two voltage-difference measurements (the third is a combination of the other two), while three phase wye might require three voltage measurements. Then there's current, you can meter two or three AC currents with two (delta) or three (wye) regular-old current transformers.
The current transformers do the requisite isolation from the high power side, you can get 'isolation amplifier' components (for relatively modest money) that do the same for the voltage-difference measure (probably you'll want an accurate voltage divider on the inputs, 50V is more than the typical common-mode range allowed).
Usually, measurements of instantaneous currents and voltages have to be more finely time-resolved than the AC frequency by a six to thousand factor, and have to be synchronous to each other in order to be meaningful. It might be easier to use non-isolated preamplifiers and converters, and just squirt digital data to your PC with an isolated (optical or transformer RS485 or maybe even Ethernet on twisted pair) interface, rather than let the PC take on the timing task. There's gotta be prebuilt systems for this out there in industrial-control land. interface of some sort
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