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  • Author Author: fmilburn
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Even More on Current Sources and a Kelvin (4-Wire) Milliohm Meter

fmilburn

Introduction

This is the third post on the development of an inexpensive but reasonably accurate meter for measuring resistance in the milliohm range. In the first post a simple current source was described that created a 10 mA current  across a resistor that allowed the voltage drop to be measured using a multimeter and the resistance calculated.  A number of helpful suggestions were received and I ordered additional components based on that feedback.  In the second post a block diagram for the instrument was introduced and initial measurements were made with a microcontroller using the built-in ADC.  Some, but not all of the ordered parts have been received now and this post will update progress as I don't want John's popcorn to get stale.

 

A Change to the Design Objectives

I originally specified that the current to the DUT would not be greater than 10 mA.  Testing to date has indicated that meeting the desired accuracy will be difficult without amplification of the voltage difference across the DUT which adds some complexity and cost.  Accordingly, the specification is being changed to 100 mA across the DUT.

 

Component Status

First, I have to admit to making a mistake in the orders.  The MCP6N16 instrument amp comes in three versions with different minimum gain.  I wanted the version with minimum gain of 1 and ordered the version that has a minimum gain of 100.  Doh!  Always read the datasheet carefully.  For now I am substituting the MAX9619.  I also ordered a precision LDO voltage source from the TI store which has not been shipped yet.  Usually they are pretty quick. The volt meter I plan to use is still in shipment from China.

 

100 mA Current Source

This is the revised circuit, the only real changes being the addition of a MOSFET to handle the increased current and a new precision 0.1% 10 ohm resistor to set the current.  I am using an inexpensive ANENG multimeter to measure voltage but it does agree well with my bench meter.

image

And here are the results:

image

The tests are being performed the same way as previously using a coil of wire that has been center tapped.  The measured resistance of the full length of wire is 0.092 ohms as seen on top while the measured resistance of half the length is 0.046 ohms - exactly half.

 

Next Steps

The inexpensive voltmeter needs at least 4.5 V to operate so I will probably use either 4 x 1.5 V AAA batteries or USB power and a precision voltage source to set the current.  If I decide to use a microcontroller instead of a voltmeter then a 3V3 LDO will be used to power that.  The parts for Kelvin probes are on order.  Progress depends on the postal service now...

 

Past Posts on this Topic

More on Current Sources and a Kelvin (4-Wire) Milliohm Meter

Testing Current Sources for a Kelvin (4-Wire) Milliohm Meter

Parents

  • For those who are still following, here is one last update before I start a new blog post...

     

    I have played around with the instrument amps I have and settled on the MCP6N16 with 100x gain.  This has the advantage with a 10 mA current source that the voltmeter displays the resistance directly in ohms.  Here is the schematic:

    image

    7 October EDIT:  The schematic above has an error - emitter and collector backwards on Q2 - will be corrected in the next version...

     

    Comments are very much welcome - including the way it is drawn if I have deviated too far from accepted norms.

     

    I have it on a breadboard right now and here are the results of the tests run so far:

    image

    The two known resistors are right on.  Great!

     

    The "full coil" is the same coil of 26 AWG wire I have used for testing all along and the results are just a bit more (94 Vs. 92 milliohms) than I got with the 100 mA test reported in the main blog above but within margin of error since it is sensitive to how well it is seated in the breadboard and such.  I was pretty excited - the agreement would be expected if everything was done right - it is just that I don't always do everything right :-).  The thing to note is that the results start to deviate from expectation as the resistance drops - e.g. we expect the 1/4 coil to be 94 / 4 = 23.5 milliohms - not 28.1 milliohms.  There are problems with the test setup being on a breadboard of course so perhaps to be expected.  Along with the resistance in the board and jumpers, the contact with the 26 AWG wire isn't always great.  I have wiggled the wire to get the best contact.  None of the resistors on the instrument amp or current source are precision.  Comments on getting better accuracy for lower resistances are also appreciated.

     

    I received the parts for the Kelvin clips today and the clips are kind of cheap but better than anything I have so I will find a way to start using them.

  • in reply to fmilburn

    Hi Frank,

     

    That's a very clear, neatly laid out diagram! I'm excited that it's all coming together.

    I just had a non-essential suggestion (there are many ways to do this), maybe instead of the 100R pot in series, it could be in parallel, like this as one example:

    image

    The reasoning was that the 130 ohm resistance is an E24 value, so easy to get in a low ppm version, but is close enough that the trimming can then be easier. Then the other resistors and the trimmer do not need to have a low ppm rating, and (if I've done the sums right) would still allow for trimming to 127.1 ohm at about one-third position of the potentiometer, and a total adjustment span of around 5 ohms from near-fully anti-clockwise to near-fully clockwise. The 'do not fit' position could be useful if a 130 ohm value is not available, and then two different resistors could be used in parallel to achieve that. Anyway, this is a non-essential change, but it might make the trimming a little easier, especially with a 1-turn pot, although a multi-turn would be better maybe, e.g. the bourns ones are fairly low-cost.

  • in reply to shabaz

    Thanks Shabaz - this is exactly the kind of feedback I was looking for.

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