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  • Author Author: fmilburn
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Kelvin (4-Wire) Milliohm Meter:  Version 1.3

fmilburn

This post documents Version 1.3 of an inexpensive but reasonably accurate meter for measuring resistance in the milliohm range. The development to date is documented in the links at the bottom of this post.  Version 1.3 contains numerous upgrades and suggestions from Shabaz, Gene Breniman, John Wiltrout, and Jon Clift to which I am indebted.  I intend to send off for new PCBs tomorrow evening and any comments in the meantime are greatly appreciated!  NOTE: This post was revised on 24 Jan 2019 to update the schematic and PCB screenshots per upgrades and comments from Shabaz.

 

Summary of Changes

  • Added biasing for VREF of the instrument op amp which improves reading of resistances around 1 milliohm
  • Added ability to add capacitance and / or diodes in front of instrument op amp
  • Moved trim pots to trim gain of operational amplifier - current sources are not trimmed
  • Modified arrangement of range switch
  • Changed PCB footprint to fit enclosure used by Shabaz
  • Added through hole 5V alternate input
  • Relocated on / off switch
  • Numerous other small improvements

 

Development is a bit behind as I have been struggling to remove the one known bug - intermittent instability when measuring resistances below 20 milliohms.  I have been making modifications to the Version 1 PCB with flying leads to a breadboard.  This seems to introduce unwanted capacitance and perhaps other problems.  To address this and allow quick changing of components I have determined that I can bend the pins on duPont female sockets and solder them to an 0805 footprint.  Then insert passives as desired.  The black 2 pin socket is a bit hard to see but the concept is demonstrated in the mockup using the Version 1 PCB below.

image

 

The new schematic and PCBs are shown below.

 

Schematic

The original schematic posted here was in error and has been replaced with the corrected version.

image

PCB

The original PCB screenshots posted here were replaced with a version that include an upgraded layout based on comments posted by Shabaz below.

image

image

image

 

As always, comments and suggestions are greatly appreciated!

 

Related Links

Building Frank's Milliohm Meter

Testing Current Sources for a Kelvin (4-Wire) Milliohm Meter

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

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

Working Prototype of a Kelvin (4-Wire) Milliohm Meter

PCB for a Kelvin (4-Wire) Milliohm Meter

  • Hi Frank ,

     

     

    Fine project!

    Best Regards

    Gerald

    ---

  • I was away from home much of last week but when I returned last night the PCBs had finally arrived!  The quality is not as good as the ones from OSH Park - for example the solder mask is not present between the pins on the smallest SMD parts, the alignment isn't as good, etc. - but it is adequate and I have soldered the small parts and commissioned the power section.  I hope to finish soldering and test in the next day or so.  At that time I will make a new post and if everything is working will get a PCB in the post to those who expressed interest.

  • More comments on the BOM.

    • I am using the Vishay T93 parts for 10 turn pots.
    • Use low Tempco resistors in the current source where possible.  Low Temco resistors may also be desireable around the Op Amp U3 as well.  I have not seen much drift in either to date however.
    • You can use a multimeter to display results or a panel meter.  See this post for an inexpensive meter that works well but be sure to order the one that Shabaz describes.  I have since purchased one of these and it works well.

  • Update on status:  It has been so long that some may have started to give up but the PCBs shipped from China today.  I plan to put one together myself to make sure it is working before shipping to those who have expressed interest.  Here is the bill of materials:

    QtyReference(s)ValueLibPartFootprint2C1, C31uDevice:C_SmallCapacitors_SMD:C_0805_HandSoldering1C210nDevice:C_SmallCapacitors_SMD:C_0805_HandSoldering1C41uDevice:CP1_SmallCapacitors_SMD:C_0805_HandSoldering4C5, C7, C9, C10100nDevice:C_SmallCapacitors_SMD:C_0805_HandSoldering1C62.2uDevice:C_SmallCapacitors_SMD:C_0805_HandSoldering1C810uDevice:C_SmallCapacitors_SMD:C_0805_HandSoldering2D1, D21N5819HWDevice:D_SchottkyCapacitors_SMD:C_0805_HandSoldering1J1ConnConnector:Conn_01x01_FemaleSocket_Strips:Socket_Strip_Straight_1x01_Pitch2.54mm1J2USB_B_MiniConnector:USB_B_MinimyFootprints:USB_Mini-B_Hand1J35V InputConnector:Conn_01x02_FemaleSocket_Strips:Socket_Strip_Straight_1x02_Pitch2.54mm5J4, J5, J13, J14, J16Conn_01x02Connector:Conn_01x02_FemaleSocket_Strips:Socket_Strip_Straight_1x02_Pitch2.54mm1J6ProbesConnector:Conn_01x04_FemaleSocket_Strips:Socket_Strip_Straight_1x04_Pitch2.54mm2J15, J17Conn_01x01Connector:Conn_01x01_FemaleSocket_Strips:Socket_Strip_Straight_1x01_Pitch2.54mm1JP1JumperDevice:Jumper_NC_SmallSocket_Strips:Socket_Strip_Straight_1x02_Pitch2.54mm2MH2, MH3MountingHoleMechanical:MountingHoleMounting_Holes:MountingHole_3.2mm_M32Q1, Q32N3906Transistor_BJT:2N3906TO_SOT_Packages_THT:TO-92_Inline_Narrow_Oval2Q2, Q42N3904Transistor_BJT:2N3904TO_SOT_Packages_THT:TO-92_Inline_Narrow_Oval3R1, R18, R20DNPDevice:R_USResistors_SMD:R_0805_HandSoldering1R210RDevice:R_USResistors_SMD:R_0805_HandSoldering2R3, R6470RDevice:R_USResistors_SMD:R_0805_HandSoldering1R415KDevice:R_USResistors_SMD:R_0805_HandSoldering1R50RDevice:R_USResistors_SMD:R_0805_HandSoldering1R7100KDevice:R_USResistors_SMD:R_0805_HandSoldering4R8, R9, R10, R11RDevice:R_USResistors_SMD:R_0805_HandSoldering1R1212K4Device:R_USResistors_SMD:R_0805_HandSoldering1R1343KDevice:R_USResistors_SMD:R_0805_HandSoldering1R1456KDevice:R_USResistors_SMD:R_0805_HandSoldering1R1520KDevice:R_POT_TRIM_USPotentiometers:Potentiometer_Trimmer_Bourns_PV36W1R16500KDevice:R_POT_TRIM_USPotentiometers:Potentiometer_Trimmer_Bourns_PV36W2R17, R28100RDevice:R_USResistors_SMD:R_0805_HandSoldering1R1933RDevice:R_USResistors_SMD:R_0805_HandSoldering1R21180RDevice:R_USResistors_SMD:R_0805_HandSoldering1R226K1Device:R_USResistors_SMD:R_0805_HandSoldering1R23130RDevice:R_USResistors_SMD:R_0805_HandSoldering1R241K2Device:R_USResistors_SMD:R_0805_HandSoldering1R2533KDevice:R_USResistors_SMD:R_0805_HandSoldering1R26470KDevice:R_Small_USResistors_SMD:R_0805_HandSoldering1R27100RDevice:R_Small_USResistors_SMD:R_0805_HandSoldering1SW1SW_SPST_PWRSwitch:SW_SPST_PWRSocket_Strips:Socket_Strip_Straight_1x02_Pitch2.54mm2SW2, SW3SW_SPDTSwitch:SW_SPDTmyFootprints:SW_SPDT_Pitch2.54mm_SLIDE1U1LD3985G47RRegulator_Linear:LD3985G47R_TSOT23TO_SOT_Packages_SMD:SOT-23-5_HandSoldering1U2LMV7271MFAmplifier_Operational:LMV321TO_SOT_Packages_SMD:SOT-23-5_HandSoldering1U3MCP6N16Amplifier_Instrumentation:MCP6N16myFootprints:MSOP-8_3x3mm_Pitch0.65mm_Hand2U4, U5LM334ZReference_Current:LM334ZTO_SOT_Packages_THT:TO-92_Inline_Narrow_Oval

    If there is a question on the part being used feel free to post below.  Also note that if only the lower range is desired, the 1.1 mA Constant Current Source can be omitted as well as SW3, R14, and R16.  If out of range detection is not desired then U2 and associated passives can be omitted.  The resistors R8, R9, R10, and R11 along with D1, D2, C9, and C10 are there for experimentation to potentially remove an intermittent instability at very low resistances.  The resistors may end up being shorted and the capacitors and diodes omitted.

  • Update on status:  I did not get the PCB design submitted in sufficient time to get the boards finished before the Chinese New Year so it is going to be a couple of more weeks before the PCBs arrive....