Shunt ? (was Re: Ideas for inexpensive True-RMS Metering?)

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Only thing I've found for accurate shunts better than 1% is lots of 0.1% resistors in parallel.

But I use axial lead or SMD big enough to hand solder. .1, .2R is high value for a shunt anyway?

Grant.

I did some meters using this:

MFR2 DIGIKEY KRL32C.005CT-ND MFR3 MOUSER 66-ULRB1R005LFSLT

which is a Susumu 0.005 ohms, 1%, 2512 package. There are lots of similar low-ohm resistors around these days. There are also 4-lead resistors and manganin shunts, lower TC, but maybe hard to find as high as 0.1 ohm.

A serious problem with electric metering is magnetic field pickup, causing crosstalk from adjacent channels or nearby transformers, fans, whatever. Layout matters a lot, as does pcb copper-foil heat sinking of small parts like this. We make our own shunts out of sheet manganin with exotic geometries to minimize hum pickup and eddy-current effects. But we get a few per cent initial accuracy and software calibrate around that.

Vishay makes 4-lead laser-trimmed metal-foil TO-3 can power resistors that meet all your specs, but they aren't cheap, and mag field pickup could be a problem. They have horrible transient response due to eddy effects in the can, but that wouldn't matter at 60 Hz.

Energy meters are fun, especially the processing algorithms.

John

Precision current shunts are readily available at 0.1% ratings. They have to be trimmed to that level of precision.

It is also very easy to perform said trimming for accuracy, so why would they not make them that accurate?

Answer: They don't.

They, in fact do make precision shunts and using paralleled banks is pretty damned silly.

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lots of stuff in this:

-Lasse

That's odd. Why would you want to do the awkward transition of a high current cable to a printed wiring board, then do it again to get off the printed wiring board? Instead of surface mount, I'd think in terms of welding/soldering the shunt to (cable clamps/connectors).

The whole meter has to have a low tempco; making each component so is not the easiest way to do it.

ep

To get high initial accuracy, someone ELSE did that calibration step. There's no hope of bypassing that (but maybe you can do it once and only once).

Hopefully, this is where one would keep the environment which the meter sees, and that which may be proximal to the shunt, separated.

Shunts usually self-heat, which adds a nonlinearity to the power measurement. So it's worth getting ones with a low TC.

Manganin has a parabolic TC that can be made to peak at room temp or about at 50C, depending on the alloy. It will have a TC of, say, +-5 PPM/K within 10-15 K of the peak. I have a bunch of data.

John

Hah, I'd love to see some of that data.

Hey, John, check your email. :-)

ftp://jjlarkin.lmi.net/Manganin.zip

We have developed some really weird shapes for planar shunts that can be epoxied to anodized heat sinks and have essentially no hum or eddy current problems. We punch or photoetch them from sheet manganin.

A lot of power resistors and shunts have hum and eddy-current problems. Some of the Vishay heat-sunk foil resistors have vicious step responses.

OK, got it.

John

Why not use a 4 wire shunt?

Not on my budget ;) But I'm not designing for production, prototyping.

Grant.

There are military surplus stores around and many times industrial liquidators.

You could find what is needed to make a bank at such a place. Ebay too has parts sections and people run whole stores there now.

Sure. But that doesn't help the self-heating/TC problem. If you're working with DC, you can have thermocouple potential problems, too.

John

Use the "Z" alloy.

It's expensive and hard to get, and I don't think we could photo-etch it.

John

This stuff looks pretty nice:

That's what lasers are for. Worst case, send them out to be custom trimmed.

My reference was to the previously posted foil resistor vendor specs.

One would have to get pretty fancy to keep inductance low with wire solutions. Flowery fan folds and such... it works though.

We used to make planar manganin shunts using dies and a punch press, but that was expensive. We tried CO2 laser machining, but the laser mostly bounced off (manganin is mostly copper, an excellent IR reflector) and the edges were awful anyhow. Lately we have them photo-etched, which works beautifully. I doubt that zeranin can be laser cut any better; it's very similar to manganin, just uses tin instead of nickel.

We don't trim our shunts, we just trim gain electronically downstream.

John