[volt-nuts] Best way to measure micro Ohms

John Phillips john.phillips0 at gmail.com
Thu Sep 28 07:56:24 EDT 2017

Any technology that does not DC must have some other overriding redeeming
factors given the AC measurements are not nearly as accurate DC and AC
currents are just as inaccurate. Best bridges I have used take several
reading with half of them opposite polarity and average them together.
Sometimes called the reversing DC method. Given what you are trying to
measure your temperature and skin effects  (not RF skin) induce more error
than a good instrumentation system. The current duty cycle can make a big
difference when you are looking this deep into the mud. The current source
should be a fast settling DC supply of about 10 amps. More than 10 amps and
stability and accurate usually degrades. The DC voltage measurement system
should look for a stable DC level before taking a usable current is then
turned off then reversed for the next reading. Several reading are filtered
and averaged together.

Use something like a Fluke 5520A. I believe the 488 bus control can reverse
the polarity, if not use an external relay to reverse the DC current.
Different current levels may be used to see where you get the most stable
readings. Use a standard resistor in the same range as the UUT.  2 HP
3458As can simultaneously read the standard restore and the UUT. 1 3458 may
work in ratio mode but probably will not give good readings at these
voltage levels. In any case, each input should be switched between the
standard and UUT. All of the hardware would be controlled by a basic
program running in excel to load the data into a spreadsheet for filtering
and analysis. In the end, the ratio between the standard resistor and the
UUT is all the counts. Well not quite. The stability of each instrument
will make a difference. The absolute current and voltage calibration kindof
cancel out as long as the meters are close to calibration stable and
linear. Make sure the voltage measurements are all taken in the same range.

If you do not want to get this elaborate there are several 10
amp micro-ohm metes to be had. The good ones do the reversing DC with math
internally and are quick enough to do minimal sample heating. Any that put
out a constant 10 amps while measuring are inferior.

On Wed, Sep 27, 2017 at 3:51 PM, Dr. David Kirkby (Kirkby Microwave Ltd) <
drkirkby at kirkbymicrowave.co.uk> wrote:

> On 27 September 2017 at 22:22, ed breya <eb at telight.com> wrote:
> > I just noticed this discussion recently, so I'm late to the party, but
> > that never stops me from adding my one-cent's worth.
> >
> > David, regardless of the aluminum and other material issues, I think your
> > initial idea of using a lock-in analyzer is definitely the way to go. I'm
> > very fond of LIAs, although I seldom need or use them, so my opinion is
> > somewhat biased. I have five - two Ithaco 391A orange-band, a PAR 5204,
> an
> > SR830 and SR850.
> >
> Yes, me too. I have used a number over the years, the nicest of which was
> the Stanford Research SR830. The EG&G 7260 I own has rather user-hostile
> interface. There's not even a power switch on the front, and I'm not sure
> if there's even on on the rear.
> The following link might interest others who don't know about these
> instruments
> http://www.thinksrs.com/downloads/PDFs/ApplicationNotes/AboutLIAs.pdf
> >
> > If you use an audio power amplifier for driving the experiment, you can
> > rig it up so that the LIA can be used to measure the drive current as
> well
> > as the resulting voltage drop. Let's say the amplifier is for 8 ohms, so
> > you put a few ohms in series with the output, then from there into a
> > precision one-ohm sampling resistor, then into the RUT, forming a voltage
> > divider. The RUT is expected to be in the micro-ohm region, which is many
> > thousands of times smaller than the sampling R, so its tiny voltage drop
> > will be negligible, allowing the sample voltage to be a good
> representation
> > of the test current. You could also just treat the whole thing as a
> voltage
> > divider and calculate the "exact" results.
> >
> I did purchase a Radio Shack 100 W Public address  I will set this up
> later.
> >
> > This could be fun and interesting. There are plenty of pieces and
> > variables involved to experiment with to optimize the measurement, and
> lots
> > of other tricks available to enhance it if necessary.
> >
> > Ed
> Yes, with lock-in amplifiers there are an almost infinite set of
> possibilities of how to do the actual measurement. I don't know if the
> reference output is a sine wave or square wave on this unit. I have the
> option of using a Stanford Research DS345 30 MHz function generator to
> generate a sine wave if needed. There are almost a million things that one
> can change.
> Dave
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*John Phillips*

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