Need advice on current measurement

Hi, all:
I need your advice on current measurement. My problem is that I need to measure the current VERY VERY precisely. This is AC current whose magnitude will be between 50-100A. I thought that a clip-on ammeter would be sufficient but its accuracy is about +-20% due to poor calibration. I have well calibrated shunt resistor but its rating is only 5A. But I need to have at least 50 Amps in the circuit. I do not have the means to recalibrate the clip-on ammeter and frankly speaking an not sure how since I do not have a well-calibrated current source. At this point of time, I am running out ideas and was wondering if anybody can steer me in the right direction or inform me of a usual techniques that may be employed in the cases such as mine.
Thank you,
Edgar Lobachevskiy Physics Dept University of Hawaii PH: 808-956-2949 FAX: 808-956-7107

Hi Edgar,
For 100 Amps AC you'll need a current transformer with a ratio of 100:5. Then use a good quality ammeter rated for 5 amps. With quality instruments you can get better than 1% accuracy. If you need isolation and/or a permanent setup use a current transducer and a digital pannel meter with the current transformer. One word of warning - never disconnect the ammeter (or other load) from the current transformer when power is applied. Otherwise, the current transformer will act like a large ratio step-up transformer for the voltage and POW!
If money is tight just rent some portable equipment from a instrument rental place for a day or two. As to vendors try Transdata, Inc. for current transducers and such. For portable stuff (possibly to rent) try Dranetz.
Hope this helps, Big John
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You can use a 1% 0.01 ohm power resistor and read the voltage with a 3% multimeter. That is easy. They are gold colored, metal cased, flat on one side to heat sink. 100 watt.
(if you like this idea, please let me stay at UH on my vacation!)

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BG Micro (http://www.bgmicro.com ) has some 0.005 Ohm 10W resistors for under $1 each. (p/n RES1385) A handful of these and a simple DMM should do the trick.
Norm

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There's an eBay item right now
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&categoryX278&item836759032 which is a 1000Amp 50 mV current shunt and "matching" analog panel meter. It's currently going for $5.50 with 4 bids on it and 1 day to go as of 11AM EDT this morning. With a good DMM with a 200 mV scale it sounds exactly like what you need.
Norm

in article chb18f$bks$ snipped-for-privacy@news.hawaii.edu, Edgar Y. Lobachevskiy at snipped-for-privacy@hawaii.edu wrote on 9/2/04 6:10 PM:
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You do not say how precise or accurate a measurement you require. Do you need to trace back to NIST? For Government work, equipment must be clibrated. There certainly are laboratories around the country and even in Hawaii that make a business of calibration and even measurement. But until you specify what you need, nothing will make sense.
Bikll

wrote on 9/2/04 6:10 PM:
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one method: meter shunts http://www.fairradio.com/panelm.htm here is a new 200A for $4 meant to be used with a 50 mV @ full scale meter movement.
the low voltage drop minimizes heating and consequently minimizes drift in accuracy due to temp coefficient.
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makes it a challange dont it?... and sometimes we guess wrong

Hi, all:
I would like to thank you all for your help regarding my question. I would like to thank Big John for suggesting to use the current transformer. I located such item and it will do the job to the precision of about -+1%. This is sufficient for now. Also I would like to thank Norm and Tim for giving me leads on where to get some parts for my measurements. I think I will get some shunt resistors for future use, since the current transformer is not mine and I will have to return it.

make a 10:1 current transformer? you didn't mention the frequency. is it constant? sam
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