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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

  • in reply to fmilburn

    Hi Frank,

     

    I was reading last night that using a couple of legs of a BJT as a PN junction could be more consistent than using a diode (PDF doc here:   http://www.ti.com/lit/an/sboa277/sboa277.pdf  )

    I'm going to try measuring the Vf at 5mA (since that is approx what goes through the diode in that circuit) of a few popular TO92 BJTs (I'm using five, two are 2N3904 (from different manufacturers) and three are BC547 from different sources too, and I've recorded the exact part numbers. I'm not too experienced with this, I've waterproofed them with epoxy glue (photo below shows them without the epoxy) and will put them in boiling water (and measure that with a temp probe too), i.e. just two datapoints for now, room temp and ~100 degrees C. I'll also compare with the diode they mention in that datasheet, that's on the left side of the array in the photo. It's a bit of a simple test : ( and different batches may be different : ( but for now I'll see what happens with just these components, and report back : )

    image

  • in reply to shabaz

    I haven't looked at the LT datasheet.  Will do that...  When I ordered the LM334 I ordered a through hole and SMD version so I have an extra to play with.

  • Hi Frank and Shabaz,

    Really enjoyed listening in on your discourse. The popcorn may get stale as I was so interested that I didn't have time to eat any of it.

     

    John

  • in reply to fmilburn

    Great idea. I'll try sourcing a few slightly better resistors meanwhile, although it may have to wait until there are enough things to order : ( since I already ordered some stuff yesterday.

     

    By the way, if you've only so far read the TI datasheet, it could be worth checking the LT one too, I just noticed it says to locate the resistors close to the LM334, and also to avoid a socket on that particular part. It's all quite interesting how much care is needed for each component and location : )

  • in reply to shabaz

    Hi Shabaz,

    I have been wondering what kind of mischief the use of a breadboard might be causing.  I might solder something up and use sockets for the expensive components.  Will also try adding a transistor and redo the math on the resistors and probably change them.  I hooked up the MCP6N16 with calculated gain of 101 and it introduced about a 4% additional error but I know I am too close to the output rail.  I need to go back see what I can do to fix that.