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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.

Comment
  • 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.

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  • in reply to fmilburn

    Hi Frank,

     

    That's very interesting! Is it figure 15 in the datasheet? There is a modification suggested here: https://www.electronicdesign.com/analog/what-s-all-lm334-stuff-anyhow

    that looks like it could help, it reduces the current through the LM334 by approx 50-100 fold (depending on BJT gain), to almost eliminate self-heating. That should stabilise it more, fingers crossed.

    I will try to also build this first stage on breadboard or something using the values you're using, so we can hopefully compare if you like.

  • in reply to shabaz

    Shabaz,,

     

    Yes, using the circuit in Figure 15 with 1% resistors and the diode noted above. I am at the limit of the resolution of my multimeter and there was some bouncing back and forth so am not sure how meaningful this is.  The change was noticeable but I am around the last significant digit. The diode datasheet does not seem to give the temperature coefficient for the iN4148 so I used what was in the LM334 datasheet example.  Are there "precision" diodes where the forward voltage drop and temperature coefficient are given with tight tolerances?  For example, the Vishay datasheet gives the forward voltage as 1V max and does not give minimum or typical for the IN4148.  I measured mine at 0.71V.  It does not give the temperature coefficient at all.

     

    The article you link to makes a good point - I am running right up to the 10 mA limit of the LM334 and could probably help things by using a transistor.

  • in reply to fmilburn

    Hi Frank,

     

    Good point! I'm not sure to be honest. I've not seen ones with a more precise definition. Usually when they state (say) 1V, that's only likely at a certain current anyway, since it changes depending on current (lower current results in lower Vf to a limit). I do have the diode they mention in the datasheet since I ordered that at the same time as the LM334, but I will also try 1N4148 so we can replicate.

    Here's what I'm currently attempting (basically same circuit as yours, and including the BJT that was in the EDN article. Also the datasheet (at least the Linear Tech one - although I'm using a TI LM334, not LT) mentions a resistor and capacitor for stability, which I sketched, but I'll leave out for now until I've read a bit more.

    I've quickly prototyped it on a small breadboard, and tried it with voltages from 3-11V, and it seems stable-ish, I see a difference in current of about 10uA as the voltage is varied from 3-11V, but even moving the distance between the BJT and LM334 makes an improvement : ) so maybe it is worth spreading out the circuit by a few holes in the breadboard could make a difference : ) I didn't run the test very long, it's very experimental : (  I need better parts to be honest, and then test indoors and outdoors, to see what change there is with the ambient temperature changing.

    I'm using cheap 5% resistors (from a Velleman kit, so no detail like ppm/degC spec), and I used 12 ohm and 120 ohm (matched with a multimeter as best as I could manage, which wasn't all that well) instead of the values in the diagram below. Anyway even mW level they will get warm slightly, so better quality resistors are ideally needed. The current does creep up (by a few tens of uA) over the space of the few minutes that I tried, but I suspect that's due to the resistors warming up, because it did not seem to accelerate if I increased the voltage. So big (or several in parallel) and more stable resistors could help : )

     

    image

    EDIT: Just as an experiment I also tried 120 ohm and 12 kohm combination, for about 1.154mA output, and it's stable to within 3uA, for the range 3-11V, over the several minutes that I ran it. But I still have no idea if it is stable versus ambient temperature.

  • 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.

  • 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 : )