Current measurement

Randall Nortman · 2005-09-10T20:18:39+00:00

It's been quite a few years since my last physics class, but here I amtrying to measure a current without putting anything in the circuitbeing measured. I'm talking about measuring 60Hz AC currents in therange of 0-20A or so, preferrably with close to +/-1% accuracy throughthe whole range. I ultimately need to get the signal into a DCvoltage to feed into an ADC and then to a microcontroller (which canperform whatever calculations are necessary).Last time I messed with this sort of thing (many years ago), you useda simple coil and passed the current-carrying wire through the middleof it. If I remember correctly, this induces a current in the coilproportional to the current passing through the wire and the number ofturns in the coil, right? I don't remember the exactformulas... perhaps it is actually also dependent on the rate ofchange of the current, so that measuring AC is different than DC. I'mguessing that an AC primary current generates AC current in the coilas well, so I'll need to figure out how to turn that into a DC voltagesignal. (I'm thinking maybe just run it across a resistor with oneleg pinned to +Vcc/2, then I get a zero reading at +Vcc/2, withnegative currents below that and positive above? Need to choose theresistor value right to get the right range, of course.)But browsing through catalogs these days, I mostly see Hall Effectsensors, which is something I vaguely recall being new, nifty, andexpensive a few years ago. They seem to be down into the $20+ rangenow, but that still seems expensive for what I'm trying to do. Thenagain, looking at CTs (which seem to be what I described above), thoseare in the $10+ range -- why so expensive? It's just a coil of wirein a plastic housing, right? Am I missing something?Also, is there any reason to use a Hall Effect for the sort ofapplication I described? Perhaps the output will be easier tocondition for feeding into the ADC? Or maybe the concern is that thecoil would have an effect on the circuit...

Current measurement

Sep 10, 2005 43 Replies

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John Larkin 20 years ago

How about a Pearson 411,

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which has 3 dB points of 1 Hz and 20 MHz.

John

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JosephKK 20 years ago

Not at all true. I have used current transformers with good fidelity with

300 nS pulses and 10 nS rise times. you just have to construct them carefully. Ion Physics sells the with bandwidths to over 1 GHz.

JosephKK

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JosephKK 20 years ago

No Phil i think you are. Check the following web sites:

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0.1 Hz to 2 GHz from at least 2 vendors. at 1% accuracy no less.

The above are not solutions for OP but i did well from 10 Hz to 300 MHz winding my own on ferrite cores. About 2% accuracy. And a $4 core some wire and an accurate terminating resistor was all i needed.

JosephKK

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Phil Allison 20 years ago

"JosephKK" Phil Allison wrote: "Robert Baer"

** You appear to have no thinking capacity whatever.

RB's is "asinine" because it completely ignores the OP's stated need for a very low cost solution.

........ Phil

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