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Re: DSE ESR meter



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fabulous.

You are not going to get that cheaply I'm afraid.

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Well you do realise that the *resolution* of the Keithley 6.5 DVM model 2000
is
100 microhm on the lowest 100 ohm range.
OTOH the voltmeter ranges will get you a reading with a lot less than 1000A.
What
your measurement uncertainty will be is another matter though. But then
you have no idea what it is at the moment do you? This is where the problem
of calibration comes in.

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But you mentioned a coil? I usually find that the resistance will be close
enough to the calculated figure not to matter, if the measured inductance is
also correct.
If you know what the resistance is per 100 metre for your wire, and you know
what the length is, you should be able to calculate within 10% IMO. That's
why I wondered why you need a better figure, and if so, you ARE going to
have to pay for it.

MrT.



Re: DSE ESR meter


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all very relevant if I turned it to Ohms and measured R directly.

not so much use if I turn it to extremely low volts, and continue
measuring current and voltage. I can measure my current to much better
than 1%.... with a uVolt meter I can wind the current down, avoid
self-heating problems and still get a great measurement.


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?!

There isnt much of a correlation between inductance and resistance,
unless the core fill factor remains constant. and even then I can still
get widely varying R for the same L - eg using TIW instead of magnet
wire gives much higher R for the same L, as TIW has much, much thicker
insulation. ditto for using Litz

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Im not using wire, and one of the things I dont know is the actual
resistivity of the material, although I do know the geometry.

Cheers
Terry

Re: DSE ESR meter



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2000
1000A.
problem

Which is what I said already, but doesn't address the calibration issue.

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close
inductance is

And you don't know what wire you are using or what the meanufacturers spec
for resitsance is?
Would be a lot easier to find that out and measure the length, than to try
to measure the resistance IMO.

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know
That's


I see, (well not exactly since it's a bit vague), good luck then.

MrT.



Re: DSE ESR meter


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    There's a few micro-ohm meters around. Dunno the price range though.
Have a look at:

http://www.vanguard-instruments.com/products/lrmeters/index.php
http://www.spotweldingconsultants.com/mm_ohm.htm
http://www.ndbtech.com/en/micro-ohmmeter-resistance-measure.php

    and other Google results on a search for -> measure micro ohms <-
and similar.

Bob






Re: DSE ESR meter


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Hi Bob,

thanks for that, some of those are pretty neat gadgets. 5A and 0.01uOhm
resolution up to 20uOhm is pretty good - 10 bits, 1LSB = 50nV.

Cheers
Terry

Re: DSE ESR meter



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If you can periodically reverse the current through your winding (e.g. with
a relay or MOSFET bridge, and not too fast, since the inductance will
prevent you from doing it fast and accurately), then you do not need a
low-offset voltmeter.  For example, turn the current on through the winding
forwards for 10 seconds and then reversed for 10 seconds and then forward
and then reversed, repeat... Average all readings of the voltage across the
winding with the current forward, and average all of the voltage readings
with the current reversed.  Find the difference between these two average
voltages, and then divide by twice the magnitude of your current source
(since the delta in current is twice the magnitude of the current).  You
can thereby remove any fixed offsets in the voltmeter, and any thermal EMFs
should also be removed because the heating would be the same for forward
and reversed current.  If your voltmeter does not have enough resolution
then you can build a pre-amplifier, and as long as the drift and offset are
within reasonable limits, they will be cancelled out by the measurement
technique.  You will have to determine the gain of you preamplifier
accurately however.  The easiest way to do this measurement might be to
hook up a PC to the GPIB or RS232 port of your voltmeter, if it has one,
and also to the control signal to the big relay that reverses your current
source.  Take appropriate precautions against back-EMF from the winding
lest it blow out your DVM.  I bet you could get a reasonable reading with
just a few amps this way.

Chris


Re: DSE ESR meter



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Having watched the responses emerge to the above, I felt compelled to add my 2c
worth.

Firstly, metering connection resistances will no longer be something you can
ignore.  I found this out decades back using a Gossen geohm bridge device for
low (milli-)ohms work.

Secondly, a bridge approach may be better than trying to simply invoke ohms law.

Finally, as one other poster alluded to - if you are winding the transformers
and know the length of wire you used, you have the answer to better than 10%
already.  And you should be able to keep track of the length of wire used.

FWIW

Re: DSE ESR meter


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law.
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4-wire (Kelvin) measurement.

Cheers
Terry

Re: DSE ESR meter



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2c
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law.
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I expected you would, but there is still the connection (aka junction)
resistance where you somehow make connection with the winding.

Re: DSE ESR meter



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Well in theory it doesn't matter as long as the total measured current flows
through the resistance under test, and the impedance of the voltmeter is
high enough, and the voltmeter connection is positioned where you want the
reading. ie not including current leads.


MrT.



Re: DSE ESR meter



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Sort of.  Even with a four-wire connection, you will need to ensure that the
sensing connections are good.

Re: DSE ESR meter



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Yes, if they are "bad" you might not get a reading at all :-)

(for the purpose of this limited discussion I will ignore things like
thermally induced EMF's etc.)

MrT.



Re: DSE ESR meter



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... and the customer is willing to pay enough to cover the price and delivery.
Tom

Re: DSE ESR meter


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Some Farnell/RS prices are actually as low or lower than DSE/Jaycar/
Altronics et.al, and they both have excellent trade counters you can
pick the parts up at.
But the fact that they have in the order of 50,000 parts that the
hobby players don't have, it kinda makes up for any price difference
IMHO.

Dave :)


Re: DSE ESR meter


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    Yeah, ain't that the truth? :-) That's why I avoid them even when
I'm ordering for the company I work for.
    Whatever happened to Radiospares (RS). Are they still around?
Farnell seems to have pulled the rug out from under them since they
started in Aust.

Bob


Re: DSE ESR meter


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Still around of course:
http://www.rsaustralia.com
I was of the understanding that RS Australia still pulls in (or used
to a few years back at least) more income than Farnell Australia,
although if that's true I'm not sure how, most engineers I know prefer
Farnell as the catalog is much better, and they generally have a
better range of semis.

Dave :)


Re: DSE ESR meter


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    When I think back (as W. Neville Williams used to say), RS used to
be  in exactly the same position in the industry where Farnell is now.
Then Farnell gradually became more and more popular while RS sort-of
faded into the background.
    Probably the better catalog and component range had a lot to do with
it, as you said.


Bob




Re: DSE ESR meter


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I remember reading an article a few years back that listed the
turnover for all the major electronics companies in Australia, and if
I remember rightly RS had about 3 times the turnover of Farnell at the
time. Anyone know who the current winner is?

I always reach for the Farnell catalog first, and only go to RS if
Farnell don't have it, or I want to price match for a larger QTY.

Dave :)


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Usually safe to assume they don't. You won't be wrong very often :-)

MrT.



Re: DSE ESR meter



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for
either.
etc
away in

Hell, the last few times I've been there (only in desperation 'cause Jaycar
didn't have it), they've glanced at the bunch of components I've dumped on
the counter, entered the most significant items and ignored the rest, making
the transaction rather cheap. Bless the attitude of Gen X or whoever they
are :)




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