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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

  • Hi Frank!

     

    I'm wondering if it is possible to use a dual-stage design, i.e. separate the current source from the final signal output stage, i.e. use an op-amp for the voltage amplification across the load, but perhaps with a lower-current source, since 100mA might make it more difficult, because the reference resistor will get warm and maybe drift. That op-amp you mention (in a min gain of 10 version) could also be used for the second stage.

     

    Any of the current source ideas could be used for the first stage, but I think the LM334 would be attractive (but means your first stage would not be an op-amp, so depends if this is a hard limitation or not : ) because the power in the resistor is really small (since the internal reference inside it is 68mV), i.e. if a 6.8ohm resistor is used, power dissipated in the resistor is 0.68mW.  Then for the desired 1mohm to 10 ohm range, if the meter is 0-2V (as an example) then a gain of 20 is needed (i.e. compatible with the min. gain of 10 version). Or for an op-amp version, it could be the same as the circuit you have now of course (with the reference resistor value changed). These are just some ideas, maybe unnecessary if the resistor won't drift much (or if you find in the end that less than 100mA is fine too, for the particular multimeter you use).

  • in reply to shabaz

    Hi Shabaz,

     

    An update... I read the datasheet thoroughly and set up the LM334 as a temperature compensated current source as described in the datasheet.  A IN4148 was substituted for the diode they used.  I came up with nonstandard resistor values so I put the closest values I had in.  The initial test was about 2% off the desired 10 mA current (not unexpected based on what was in the datasheet) so R1 was adjusted by adding another resistor in parallel until within 1%.  Voltage was varied over the range of interest and here are the results for the full length of test wire:

    image

    Things look good above 2.5V input (the planned instrument will have at least 3V) but notice there is a slight rise in the measured current at 5V.  This may be due to temperature from increased voltage which the datasheet warns of or just increase of temperature as it heats up in time in still air.  I left it on for an hour at 5V and from that point on it was stable although I am unable to make measurements with the resolution I would like.

     

    I added another tap in my test wire at quarter length.  The results are:

    Full Length:  0.092 Ohms

    Half Length: 0.045 Ohms

    Qtr Length:  0.022 Ohms

     

    The accuracy appears to fall off as it nears the bottom of the multimeter range but all is as expected.  I will look into adding amplification next.

  • in reply to shabaz

    I have the 2N3904 and will get started with that.

  • in reply to shabaz

    Hi Shabaz,

     

    I set up a spreadsheet to calculate R1 and R2 and input the values you determined empirically (nice tests by the way).  Here is an example of input and output:

    image

    Note that the calculated R2 / R1 is 7.8 and not the 10 from the datasheet.  I got pretty close to these values by using two resistors in series:

    R1 = 15 + 2.2 = 17.4 measured

    R2 = 100 + 33 = 134 measured

     

    I started with the diode I already had in the circuit, IN4148.  I measured the current through it to be 4.23 mA and the voltage drop to be 687 mV which is lower than the Vf your diodes are showing.  The  current out Iset (which ideally would be 10 mA) was 9.83 mA.  I plan to redo the calculations for the actual Vf and / or change out the diode but thought I would post this in case I don't complete it tonight.

     

    Since it is sensitive to Vf and maybe other stuff, I will try a trimmer pot to see how well that works also.

  • in reply to shabaz

    Hi Shabaz,

    Just noticed something....  The tempco in your test drops as the current goes up, but in the TI chart it goes up.  Am I misunderstanding?

  • in reply to shabaz

    One final post for the night...  I put the numbers into the spreadsheet for the IN4148 - Here is the resulting calculation:

    image

    For R1 I am still using 17.4 Ohms so that is off a bit.  For R2 I took out the 33 Ohm resistor and replaced it with a 100 Ohm trim pot as the datasheet says to vary R2.  Then I fired up the circuit and adjusted R2 until the current was measuring exactly 10.00 mA.  The measured resistance for R2 was 130.5 Ohms and above it was 134 Ohms.  So a 3.5% change in resistance was needed to correct a 1.7% original error in current.  My feeling is that a trim pot is needed to insure better than 1% accuracy.  I am using a cheap trim pot but it seemed fairly stable.  The Newark site is down for me but it looks like you can get a single turn 50 ohm version with 100 ppm / deg C temperature coefficient for less than a dollar which I think would be acceptable.

     

    An observation on flexibility:  With the LM334  it would be difficult to change current settings to measure larger resistors.  It would be more straight forward to switch sensing resistors with a microcontroller if using the op amp approach.

     

    I spoke to my friend today who has all the nice equipment.  He is going to let me borrow his Keithley current meter.  At least it will look good in the pictures :-)

  • in reply to fmilburn

    Hi! The TI chart rises as does the data in the table, I think maybe the chart I drew may be confusing things, it was just showing Vf (not tempco) just so we could see how the selected Vf would also change depending on what current setting was used.

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