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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

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  • 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 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.

  • 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

    Hi Shabaz,

    Great idea to use a BJT.  TI writes some really good applications reports.  I like your test setup and eagerly await the results.  Really clever.  It would be fun to get intermediate points closer to room temperature. If the pot of water is large will it hold temperature as it slowly cools long enough to approximate steady state and get some readings?

  • in reply to fmilburn

    Hi Frank!

     

    Unfortunately I was in the kitchen, doing the experiment : ) I didn't see your message in time regarding intermediate readings : ( and I've dismantled it from the kitchen now.

    I did want to do that, but in the end I didn't because the pot I'd used wasn't so large and it was taking me 60-90 seconds to do the readings for the 6 components (5 BJT and the diode) since I was swapping wires manually and waiting for things to settle - so wasn't sure how fixed the temperature would remain. But I may do it again since it's a good idea. But I did do the measurements at 6 different currents, so we have some numbers to play with anyway : )

    Anyway, here are the raw results : ) I've not examined them yet : )

    image

    There was quite a lot of bubbles (I used UK's southern water company's finest tap-water: ) it's not very great : ) Full of minerals and "stuff" : )

    Anyway, the water temperature did fluctuate throughout the experiment, but only by about 0.2 degrees.. I measured things twice, so we can also see if the fluctuation in the measurement is significant too.

    I've got erroneous decimal places in the first few results, so the results are of course 0.6734V, 0.6916V, 0.7025V etc.. for the first column, and so on. So, first set of results are at room temperature at 1-6mA, and then the next two sets of results are at 99.82-100.0 degrees C, at 1-6mA again.

    I'm off to find some actual food in the kitchen : )

  • in reply to fmilburn

    The temperature coefficient results : )

     

    These were the components tested:

    image

     

    And the results (i.e. change in Vf, in V/degC):

    image

    The TI document (http://www.ti.com/lit/an/sboa277/sboa277.pdf ) had this chart for a typical transistor:

    image

    The results are extremely close to the TI doc. It doesn't matter which BJT or diode is used, it will just affect the ratios of the resistors in the circuit anyway. But at least now we can more precisely set the ratio. The LM334 datasheet had suggested the IN457 had a tempco of -2.5mV, but I see -1.6mV at 5mA as you can see in the results, so the ratio wouldn't be the nice 10:1 that it is in the datasheet example.

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  • in reply to fmilburn

    The temperature coefficient results : )

     

    These were the components tested:

    image

     

    And the results (i.e. change in Vf, in V/degC):

    image

    The TI document (http://www.ti.com/lit/an/sboa277/sboa277.pdf ) had this chart for a typical transistor:

    image

    The results are extremely close to the TI doc. It doesn't matter which BJT or diode is used, it will just affect the ratios of the resistors in the circuit anyway. But at least now we can more precisely set the ratio. The LM334 datasheet had suggested the IN457 had a tempco of -2.5mV, but I see -1.6mV at 5mA as you can see in the results, so the ratio wouldn't be the nice 10:1 that it is in the datasheet example.

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  • in reply to shabaz

    Nice... That is really good stuff, and as you point out the results are quite close to the TI doc for the BJTs tested. It seems like it would be good to provide for ratio adjustment by a trimmer pot.  When I was doing my tests above I put another resistor in parallel to bring it within 1%.

  • in reply to shabaz

    This is a really good experiment you've done here. I'd vote for you to have your transistor badge for this!

     

    Perhaps you might consider doing it as a separate blog as well (would make it easier for people to bookmark and find again).

     

    If you had asked me what I thought the coefficient was, I'd have said -2.5mV/K. That comes from me learning electronics theory back in the 1970s and never having done serious transistor-level analogue design later on (where, presumably, I would have discovered the difference myself). I've looked in a few old books from that period and several just quote variations of "approximately -2.5mV/K" (one doesn't even mention that it goes off at higher emitter currents).

     

    That figure wasn't wrong back then. One of the books I looked at was published by TI, back when they made transistors, and the author was a real expert on everything to do with the devices.

     

    From what I can make out the theoretical value (for silicon) should be -1.7mv/K but a real transistor doesn't match the simple model that comes from.

    There have been a couple of major changes in the way transistors get made since the 1960s that may account for the change [gold doping to improve recombination and planar construction to allow the use of IC fabrication techniques].

     

    From what you've shown us here, "approximately 2mV/K" would be a better rule of thumb now.

     

    The coefficient moves off towards zero at higher emitter currents (since we're talking about the Vbe, we probably should refer to the emitter current rather than the collector) because of the ohmic properties of the base and emitter (which have a positive coefficient).

  • in reply to jc2048

    Hi Jon!

    Hehe the day off was hopefully productive then : ) I started some DIY, but it was more interesting to do the experiment : )

    That's a good idea, I'll put together a separate blog with the tempco details.