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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 fmilburn

    Hi Frank,

     

    I don't know if this is of any interest but a while back I was doing some temperature testing and I got tired of fighting with getting the temps where I wanted them and then moving them slowly in one direction or another. I took and old hot air gun and hooked its fan up so that it would run at a constant speed. I then used a small variac and powered the heater with it. I was able to vary the temperature up and down from room temperature to 340 F and stop at any point and remain more or less stable. I did not build a thermocouple into it but it could be an option. Instead I just calibrated the variac against empirical temps. I have to use an external thermocouple if I want to know exactly where I am at. It isn't perfect but it was a big improvement on trying to get readings as the temperature changes under a conventional warming or cooling.

     

    John

  • in reply to fmilburn

    Hi Frank,

     

    Interesting stuff! I didn't try a hairdryer, but I noticed that even just lifting the bowl causes large jumps, and I concluded (concluded is a strong word, it was really just a guess) that different parts are heating/cooling at different speeds, since the TO92 are thick plastic, and the resistors are tiny) and so in the setup I had, I found it hard to make many conclusions in free air. But I guess since you are sustaining the hairdryer heat since you have been able to clamp it into position, this was good enough for you to get a reasonably steady measurement of 0.017 to 0.01mA/degC. Even with that level, if your ambient temperature changes by 10 degrees C in your lab, you're still in the ballpark to measure resistance with just +-0.5% error. But I reckon it will improve more once it is sealed from drafts, for me that played a huge difference, and of course any tweaking of resistance ratio - I may have got lucky that the selected values worked well enough with the breadboard contact resistances. Also, are the resistors carbon film or something? Because those can be 300 or 500ppm/degC, so that too could be having an impact.

    In your PCB layout, the PN-junction and the LM334 need to be close too, since they have to be at the same temperature, I'm wondering if unexpected drafts are flowing differently around the two devices when they are not enclosed (or maybe I'm overthinking this!).

  • in reply to jw0752

    Hi John,

    The other thing I was thinking is maybe use my hot air rework station as it can be set down to 100 C.  If I pursue this further I should probably figure out a way to do it in still air like Shabaz.  Maybe baffles and mixers to still and  cool the air or something like that.

  • in reply to shabaz

    Hi Shabaz,

    Your post confirms my suspicions.  You never sleep :-)   Those seem like good theories and suggestions to me.  The resistors are 1% thru  holes of no known provenance that I have picked up over time.  Good point on the PCB layout which I will keep in mind.  Which reminds me, I will post the PCB design when I get to it and if you or anyone else who is actively following this wants one let me know and I will be happy to send one.

  • in reply to fmilburn

    I agree, some way with thermal damping is needed, so I figured heating the room could do that, although that's not practical beyond a degree or two : ( I'll try to think of other ways, although nothing is springing to mind image

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