small, very accurate shunt resistors?

Hi - I'm looking for some small (physically), very low resistance, very
accurate resistors to use as shunts for current sensing. Ideally I'd like
something like a .05 ohm resistor in a 0805 package with maybe .1% or
better accuracy. I've been able to find plenty of larger resistors that
match these specs, and among 0805s I've been able to find ones of the right
resistance but that are less accurate (lots of 1% parts), and I've found
many that are the right accuracy but not the right resistance (normally 10-
100 ohms is the smallest I can find among the .1 and .05% parts).

Any ideas?

Thanks!

-MJ Noone

 

An 0.1% 50 milliohm resistor must be accurate to within 0.05 milliohms. If
you really need that kind of accuracy, then I think that trace resistance
will be a catastrophic problem. Is your circuit designed for a Kelvin
(four-wire) connection? I have very little knowledge in this area, but I am
sure that it would be necessary and I doubt that it would be sufficient.

Also, you implied in a previous posting that you were trying to use a
smaller current sense resistor so that your circuit would produce 0 to 2.048
volts instead of 0 to 4.096 volts. Clearly there is more to your circuit
than just the resistor; perhaps if you posted that then someone with
experience in this area could help you.

Jonathan

 

Keep in mind that when you get to such low value resistors, you need 4
connections to them to get an accurate voltage proportional to the
current through them. The best you may be able to do is to have the
current carrying traces going into two of the outside corners of the
resistor and the voltage connections coming out from the opposite
inside corners of the pads. Very nearly as good is current into one
side of the resistor, voltage from the other side. The idea is to
include as little trace and pad drop as possible in the measurement.

 

Try Isotek for their 4-wire parts; but your specs are tough.

John

 

You need a 4-terminal shunt. I happen to have one 0.1 ohm shunt that was
used for current measurement. The accuracy is .02%. I also have a matched
set of 0.1, 1.0, 10, 100.1 and 1010 ohms. They were designed to be loaded
by a 100K input impedance. Let me know by e-mail if this is what you want.

Norm Strong

 

Shunt resistors 0.1 Ohm and lower are mostly 4-terminal even in
through hole mount, and anything below 10 Ohms is rare in SMD.
The parts you think you want may be custom (interpret as $$$$).
You could make your own from manganin sheet cut a few percent
low calibrated with software or dremel...

 

www.isabellenhuette.de

 

And the problem with a paper clip is....?


--
Many thanks,

Don Lancaster
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
voice: (928)428-4073 email:

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

I can't tell from the description if these are anything near an 0805,
but Digikey does sell some 0.5%, 50 milliohm 4 terminal current sense
resistors. Part # RW1S0CKR005D

 

TC.

John

 

http://www.vishay.com/docs/63023/vcs2516.pdf

These are about 3 times the size you asked for and I don't know who
stocks them but they look like fine devices.

By the way, have you figured out how you are going to amplify and
digitize the signals from your shunts in the presence of trace drops,
while keeping the accuracy any where near 0.1%? I have managed 2% or
3% without heroic efforts, but not much better.

 

If you use Zeranin paperclips rather than the plated steel ones from
Wal*mart it would be better, but you still would have the tolerance
issue (unless you can handle the calibration issues). ;-)

Tempco of steel is about +0.5%/K, worse even than Cu, so the desired
0.1% accuracy is lost with tenths of a degree change in temperature.

Can you use a less-than-spec accurate (but stable) Kelvin-connection
shunts that other have mentioned and calibrate? As others have
mentioned, you really don't want hardly any of the +0.39%/K copper
traces in the measurement path.

Do you need a shunt made with four terminals? If the copper is to
contribute, say, 0.5%, then you need an equivalent length of about
3mils of 12 mil wide 1-oz traces on each end. Maybe this is achievable
by running traces off of either side of an SMT pad for a four-wire
connection, but it sounds a bit dodgy. I've done something similar for
an instrument that measured standard CT output...

Bare (low tempco alloy) shunts are made in high volume for multimeter
use (both a jumper style for low accuracy meters, and IIRC, some
4-terminal types) but generally not nearly that accurate.

Then there are possibilities like this:
http://www.precisionresistor.com/LVS2.htm
http://www.precisionresistor.com/LVS3.htm

For an LVS3 (0.5" x 0.25" SMT) 0.05 ohm 0.1% with 15ppm/K tempco they
want $8.30 each, which is a bit pricey, but maybe tolerable for you.
They're obviously a lot bigger than 0805, but it meets all your other
specs (and appear to be in stock).

Here are some in 2516 (0.25" x 0.16") which are offered in your specs
as well: http://gensemi.com/docs/63023/vcs2516.pdf (watch the tempco
and maximum current rating)

And some in 2512 (0.25" x 0.125") which also seem to be offered in
your specs: http://www.powertron.de/POWERTRON_RESISTORS_GROUP_SMD.PDF
(UHS 4-2512)


Best regards,
Spehro Pefhany

 

A major problem with all of the SMD resistors, is that there are *NO*
kelvin contacts!
So, assume you have a 50 milliohm SMD resistor that you determined
was within 0.1% of that value.
Now solder it to your PCB, and measure the resistance on the traces
close to the resistor.
Hmmm...seems to me that you will not be any where close to 50
milliohms + 500 MICRO-OHMS!!!
The damn solder joints will have far larger resistance, and one might
ask if you remembered to attempt kelvin connections???
And..where does one measure these low-R SM units? at the top, close
to the "J" bend? half way down from the top? at the bottom of the "J"
bend where it would connect to the PCB trace?
Furthermore, what about thermo-electric effects?
I presume one could go on about the problems...

 

Maybe the paper clip is still attached at the top of his Word
document, and will not motorcycle away...

 

I stand corrected about Kelvin SMD resistors.
I see that Vishay complains about tin/lead oxidation and about poor
reliability of conductive epoxy.
BUT in the same breath they seem to recommend !gold! which has a
history of seperating from solder at the interface...

 

Go have a look at:

http://www.koaspeer.com/koa/master.asp?URL=/koa/resistors.asp

or

http://www.isotekcorp.com/productDisplay.asp?CatID=1&SubCatID=

That ought to get you started.