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  • Author Author: fmilburn
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Testing Current Sources for a Kelvin (4-Wire) Milliohm Meter

fmilburn

Introduction

I was inspired by a recent post from on Building Kelvin (4-Wire) Test Leads.  Shabaz explains in detail and with clarity why measurement of small resistances with the two leads on a multimeter is difficult.  This got me to thinking about how I might build my own 4-wire Kelvin instrument.  This post will describe initial tests of a simple current source that can be used with a digital multimeter to more accurately measure small resistances.

 

Method and Objectives

For accurate measurement a precise current source using two wires to connect to the device under test (DUT) must be created. Separate wires are used for voltage measurement.  I have a good bench multimeter (thank you again element14 and community for selecting my project and enabling me to buy this instrument with the prize money) which will serve as the voltage measurement instrument in this test.  I am considering building a self contained battery powered instrument and if so will replace the multimeter with either a small panel meter or a microcontroller and display.

 

Here are the design objectives:

  • Inexpensive
  • Can run off batteries
  • No more than 10 mA to DUT
  • milliohm level accuracy
  • Range 1 milliohm to 10 ohms

 

Design

To start, I chose a simple circuit using a single op-amp to create as a current source to see how far it would get me.

image

The op-am is a  dual rail to rail TLV2462a I had on hand with 500 uV typical input offset voltage.  The supply voltage can be from 2.7 to 6 V single supply so it will work well with batteries and a microcontroller.  The temperature coefficient of the input voltage is 2 uV per degree C so it should be fairly stable.  The second op-amp on the IC may come in handy later.

 

The remaining circuit components and values were chosen as follows:

  1. 3V3 power rail - this needs to be precise since along with a voltage divider it will set current.  I am using my bench power supply for this test but will use a voltage regulator in a final design.
  2. 0.3 V common mode op-amp input voltage - This was selected as it is well above the lower level for input and output voltage and is also at a convenient level for setting the current.  It provides room between the lower and upper voltage output levels for a range of resistance measurements.
  3. 10K and 1K values for the voltage divider - arbitrary 1% values I had on hand which set the voltage to the non-inverting input of the op-amp at 0.3 V.
  4. 30 ohm current setting resistor - since the non-inverting input is 0.3 V, the op-amp will do it's best to keep the inverting input the same due to op-amp action with negative feedback.  Through Ohms Law the current is 0.3 V / 30 ohms = 10 mA.

 

Construction

Here is the circuit on a breadboard:

image

Power comes in from the left out of my bench power supply and is supplied to the rails of the op-amp.  The resistors for the voltage divider can be seen lower left on the breadboard and feed the non-inverting input through the yellow wire.  I measured the voltage at 300.3 mV.  The salmon / pink colored wire is the output and goes to the DUT - a coil of orange wire.  The blue wire leads to the inverting input.  I did not have a 30 ohm 1% resistor and used 33 ohm and 330 ohm in parallel instead (measured resistance of 30.04 ohms).  Since the measured voltage across the current setting resistors was 300.5 mV and the measured resistance was 30.04 ohms the current is as close to 10 mA as I can get with my equipment.  The multimeter readings are steady.

 

Testing

I tested a range of 1% resistor values I had on hand from 30 ohms down to 1 ohm.  All were within the accuracy of the resistors and matched my multimeter well.  I followed the example of Shabaz and used wire for the final test.  I believe it to be 26 AWG telephone wire and it is approximately 1 meter in length.  I "center tapped" it so that I could measure from end point to end point or from the center to either end point.  The following photo shows the results:

image

The left photo reads 0.90 millivolts across the entire coil of wire.  Since the current is 10 mA the measured resistance is 0.090 ohms.  In the picture on the right the center tap to right end is being measured at 0.44 millivolts which is a resistance of 0.044 ohms where we expect 0.045 ohms.  I got identical results using the other end of the wire and the center tap.

 

I also measured the resistance of the wire using the multimeter.  When the leads are shorted together the meter reads 0.09 ohms.  On the center tap to end measurement the meter reads 0.20 ohms so if I back out the leads I get 0.11 ohms which is more than twice what was measured using the Kelvin method and the current source.  The spec sheet states that the meter has 0.01 ohm resolution with accuracy +/-(0.3%+40)+test leads open circuit value when the range is 400 ohms.

 

Summary

I was surprised at how well this simple current source performed.  I believe that this breadboard setup is already performing better than my multimeter on its own.  Should I decide to continue developing it I will consider doing the following:

  • Use a microcontroller with built in op-amps
  • Second stage amplification to increase the range
  • Select an enclosure with panel and design a PCB

 

Thanks for reading.  Suggestions for improving this project and a way to calibrate would be greatly appreciated.

  • in reply to jc2048

    Thanks Jon,

     

    I looked up this circuit in Horowitz and Hill.  They also have a modification (Figure 4.15) that adds a sensing resistor and buffering op amp to reduce current error due to mismatching of resistors.

     

    I have been thinking about it and feel it will simplify things if I stick with the floating load.  If I use an ADC to measure the voltage across the resistor being measured I will have to do it in differential mode but have successfully done that now with the 14 bit ADC in the MSP432P401 although I have to boost the current up to 100 mA to get reasonable results down to 1 milliohm.  I plan to post that soon and hopefully get some feedback on the direction I am headed.

     

    Frank

  • in reply to fmilburn

    This is the Howland circuit that Michael referred to [it's from a very old book, so I don't suppose it will matter if I reproduce it here]. The load current is proportional to the difference between the two inputs (and obviously limited by what the op amp can manage in terms of output current and output voltage swing). It's bidirectional as well as being ground referenced on the load.

     

    image

     

    One cool thing about it is that hiding inside it is a disguised NIC (negative impedance converter) - that's another interesting circuit you might want to look at sometime.

     

    To understand it in a simple, hand-wavy way, start by considering the case where the inputs are shorted together and at a positive voltage. To get the op amp inputs to be the same, the output will go negative until there's no voltage across the load so that it's not unbalancing things. As the inputs move apart, the voltage at the op amp inputs will also try to move apart - the output counters that by using the load to develop a current to sum into the non-inverting input to pull that input back to match the inverting input.

  • in reply to jc2048

    Thanks...  Very interesting and I will look into this.  It would definitely simplify and output looks great.  I have access to a copy of Horowitz and Hill and have been struggling through that on current sources and diff amplifiers / instrument amps - has my head swimming.

  • in reply to fmilburn

    Another alternative is to use a TL431 programmable-reference in a circuit something like this:

     

    image

     

    Nothing original - I've taken it straight from the datasheet. Only differences are that I substituted a MOSFET for the bipolar pass

    transistor and, since I didn't have a 250R resistor to give the 10mA, I used four 1k 0.1% resistors in parallel. Here is a

    graph of the measured output current for various supply voltages. It's reasonably accurate and it regulates quite nicely.

    image

     

    It drops out below about 6V on the supply because of the gate threshold and because, by that stage, the TL431 is being starved of current.

     

    Although it's a bit hidden, this is still the generic circuit with an op-amp to servo the loop - the TL431 contains both a

    voltage reference (that gets compared to the current-sense resistor) and the op amp itself.

     

    I know you want to do the discrete version, but thought I'd throw it into the ring as something else to experiment with (it's very simple to prototype on a breadboard).

  • Update on accuracy and precision....

     

    I decided to do another test before moving on to the improved design.  I took the current source out of the loop and connected the bench power supply through my good meter to measure current through the wire.  The current was set to 0.974 amps and the voltage drop measured with a separate meter.  This moves the measurements into a better range for my meters.

    image

    The meter on the left gives the voltage drop in millivolts for the full length of wire in the left hand picture and for the half length in the right hand picture.  Thus with a fixed current of 0.974 amps the full length of wire is determined to be 0.0941 ohms and the half length 0.0476 ohms (this compares to 0.090 ohms and 0.044 ohms measured with the 10 mA current source). 

     

    At least part of the difference is due to the fact that I was down to the last two digits of resolution before and I am encouraged to continue.