Shunt Resistors?

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Could you possibly be a little less vague?

Certainly John, sorry about that...

Im working on a power meter residential 3-wire 120VAC. 30 AMPS MAX current. I want to use a shunt resistor as my current sensing transducer but I'm not sure what criteria should I use in selecting the shunt. What manufacturer or what material type should be the best for handling a current of 30A.

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Probably the single most important thing you\'ll be looking for is
the drop across the shunt with 30 amps going through it.  As for
materials and manufacturers, I believe the best shunts are made from
manganin, and Google will help you locate who makes them.

If you need isolation, you would be better served using a current transformer.

The easiest shunt to get hold of would be the Hobut ammeter shunts stocked by Farnell/Newark. The smallest on offer are two 60A shunts with 60mV and 75mV voltage drops at rated current. Both cost close to $50.

Farnell also stock a fair range of low resistance resistors which are quite a lot cheaper - the Tyco parts mostly specify temperature coefficients of resistance, which puts them ahead of the Vishay parts that Farnel stocks, but the specifications range from +/-250ppm/C down to +/-100pp/C for the thick film parts down to one 75ppm/c part.

None of them are remotely competitive wuth manganin.

-- Bill Sloman, Nijmegen

Need isolation? Cheap single-element meters don't, but 2-element US

120/240 volt meters do.

If you don't need isolation, or can get it somehow, punching or chemically milling shunts out of sheet manganin is the way to go. You can even use wire manganin, soldered to a pc board, at 30 amps. IRC sells some nice croquet-hoop manganin shunts. Beware magnetic pickup... zero is a surprisingly small number.

I did a meter a few years ago, designed to be manufactured in India, where it's 240 volt, single-phase for residences. The parts cost was $9 or something like that. We used a simple manganin shunt in the low leg and a MC68HC05 uP with the built-in 8-bit mux'd ADC and a lot of number crunching. It had a bunch of anti-tamper provisions, since stealing power seems to be a national pastime in India. Worked great but didn't sell.

John

Why not use an off the shelf clamp type sensor/transducer? eg.

All the hard work is already done for you.

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snipped-for-privacy@nowhere.com wrote:

Thanks Tony. I'd missed the "Low Ohmic Value" section in the Farnell "resistor, potentiometer and thermistor" section. The OAR and OARS parts from TT Electronics/Welwyn are indeed very attractive and cheap, and I should have found them myself.

20ppm/C is definitely close to what you can get out of manganin, although manganin's resistance doesn't vary linearly with temperature around room temperature - see the data sheet (in English!) at

The curve in Graph 2 at the bottom of the first page is informative.

Rayner and Kibble, in "Coaxial AC Bridges" (ISBN 0-85274-389-0

prefer Evanohm

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Bill Sloman, Nijmegen