Even More on Current Sources and a Kelvin (4-Wire) Milliohm Meter

RE: Even More on Current Sources and a Kelvin (4-Wire) Milliohm Meter

mahmood.hassan — Tue, 13 Nov 2018 21:02:17 GMT

U can also built constant current source using lm317 and its pretty decent.

RE: Even More on Current Sources and a Kelvin (4-Wire) Milliohm Meter

fmilburn — Sun, 07 Oct 2018 05:24:53 GMT

BTW, I moved the load high side because of issues with the rails on some of the other instrument amps. I also added the 47 ohm resistor to get approximately 0.5 V move off of Vdd but I don't think this is really necessary for the MCP6N16. Resistors are going to be 0805 SMD 1% with lowest reasonable tempco I find at Farnell / Newark.

RE: Even More on Current Sources and a Kelvin (4-Wire) Milliohm Meter

fmilburn — Sun, 07 Oct 2018 04:51:11 GMT

For those who are still following, here is one last update before I start a new blog post...

I have played around with the instrument amps I have and settled on the MCP6N16 with 100x gain. This has the advantage with a 10 mA current source that the voltmeter displays the resistance directly in ohms. Here is the schematic:

[View:/resized-image/__size/620x469/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-e4dcf46d-a363-406f-bc90-a68f9991c156/0312.contentimage_5F00_188661.jpg:620:469]

7 October EDIT: The schematic above has an error - emitter and collector backwards on Q2 - will be corrected in the next version...

Comments are very much welcome - including the way it is drawn if I have deviated too far from accepted norms.

I have it on a breadboard right now and here are the results of the tests run so far:

[View:/resized-image/__size/620x404/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-e4dcf46d-a363-406f-bc90-a68f9991c156/4544.contentimage_5F00_188662.jpg:620:404]

The two known resistors are right on. Great!

The "full coil" is the same coil of 26 AWG wire I have used for testing all along and the results are just a bit more (94 Vs. 92 milliohms) than I got with the 100 mA test reported in the main blog above but within margin of error since it is sensitive to how well it is seated in the breadboard and such. I was pretty excited - the agreement would be expected if everything was done right - it is just that I don't always do everything right :-). The thing to note is that the results start to deviate from expectation as the resistance drops - e.g. we expect the 1/4 coil to be 94 / 4 = 23.5 milliohms - not 28.1 milliohms. There are problems with the test setup being on a breadboard of course so perhaps to be expected. Along with the resistance in the board and jumpers, the contact with the 26 AWG wire isn't always great. I have wiggled the wire to get the best contact. None of the resistors on the instrument amp or current source are precision. Comments on getting better accuracy for lower resistances are also appreciated.

I received the parts for the Kelvin clips today and the clips are kind of cheap but better than anything I have so I will find a way to start using them.

RE: Even More on Current Sources and a Kelvin (4-Wire) Milliohm Meter

fmilburn — Sat, 06 Oct 2018 00:18:14 GMT

This is a summary of experiments with the LM334 Adjustable Current Source as the comments are getting pretty long and possibly difficult to follow. See the running dialog above for greater detail. Shabaz has provided links to interesting information on an improved variation of the LM344 circuit and the advantages of using BJT transistors as a PN junction for temperature compensation. He also ran a very interesting series of experiments to better determine the temperature coefficient of various common transistors as well as the diode recommended in the LM344 datasheet. The forward voltage was also measured. My experiments to date are consistent with cruder than his although and I have not run detailed experiments on temperature influence.

Here is a circuit for a milliohm meter, modified using the information above, and using the LM334 to set current.

[View:/resized-image/__size/620x878/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-e4dcf46d-a363-406f-bc90-a68f9991c156/5127.contentimage_5F00_188657.jpg:620:878]

The forward voltage drop across the 2N3904 transistor and the values of R1 and R2 in series with R3 are used to set the current through the load. The tempco of the 2N3904 must also be known in order to properly offset temperature effects from the LM334. The trimming potentiometer R3 allows error in the measured current to be corrected. Spreadsheets were developed to make the calculations which are described in the LM334 datasheet easier as shown below:

[View:/resized-image/__size/620x340/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-e4dcf46d-a363-406f-bc90-a68f9991c156/1817.contentimage_5F00_188658.jpg:620:340]

My current experimental setup on a breadboard looks like this:

[View:/resized-image/__size/620x620/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-e4dcf46d-a363-406f-bc90-a68f9991c156/5040.contentimage_5F00_188659.jpg:620:620]

During the test the following were relatively constant:

Input Voltage from Lab Power Supply: 5 V
Ambient Temperature: 19 deg C
Vf = Vd = 0.7183 V
R1 = 17.66 Ohms
R2 = 123.54 Ohms (includes trim pot, R3)

The small meter at left is measuring the voltage in millivolts across the DUT which is a 1 ohm 1% resistor. The larger bench meter behind is measuring current in milliamps. Since the current is very nearly 10 mA and the voltage drop is very nearly 10 mV the setup is performing properly for a 1 ohm resistor. The current was remarkably steady over the desired input voltage range of 3 to 6 Volts:

[View:/resized-image/__size/620x299/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-e4dcf46d-a363-406f-bc90-a68f9991c156/4544.contentimage_5F00_188660.jpg:620:299]

The test setup was then left to run for one hour exposed to ambient with no observed change in the current. Tests by Shabaz in the comments above better illustrate the temperature sensitivity.

RE: Even More on Current Sources and a Kelvin (4-Wire) Milliohm Meter

jw0752 — Thu, 04 Oct 2018 04:30:33 GMT

Hi Frank and Shabaz,

Really enjoyed listening in on your discourse. The popcorn may get stale as I was so interested that I didn't have time to eat any of it.

John

RE: Even More on Current Sources and a Kelvin (4-Wire) Milliohm Meter

shabaz — Wed, 03 Oct 2018 11:58:04 GMT

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).