Kelvin (4-Wire) Milliohm Meter: Version 1.3

RE: Kelvin (4-Wire) Milliohm Meter: Version 1.3

geralds — Thu, 07 Mar 2019 09:56:26 GMT

Hi Frank [mention:cff82ed12b714001a1ba5205f2cb553c:e9ed411860ed4f2ba0265705b8793d05] ,

Fine project!

Best Regards

Gerald

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RE: Kelvin (4-Wire) Milliohm Meter: Version 1.3

fmilburn — Wed, 06 Mar 2019 04:37:38 GMT

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.

RE: Kelvin (4-Wire) Milliohm Meter: Version 1.3

fmilburn — Mon, 18 Feb 2019 21:59:03 GMT

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.

RE: Kelvin (4-Wire) Milliohm Meter: Version 1.3

fmilburn — Mon, 18 Feb 2019 21:37:13 GMT

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.

RE: Kelvin (4-Wire) Milliohm Meter: Version 1.3

fmilburn — Thu, 07 Feb 2019 01:04:47 GMT

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....

RE: Kelvin (4-Wire) Milliohm Meter: Version 1.3

fmilburn — Fri, 25 Jan 2019 05:56:50 GMT

I made the order this evening since Chinese New Year is approaching and I wanted to get the boards fabricated and shipped before things shut down. Changes were minimal compared to what is shown above with no changes to the schematic. The layout was tidied up a bit and the mask clearance modified so that hopefully there will be solder mask in between the smaller SMD parts which has been a problem in the past from the Chinese board house. HASL lead free was used instead of ENIG to save a little money. Should get the ten boards in 2 to 3 weeks.

Hopefully we can hunt down that pesky intermittent instability with the new pads on this board. If not, it should be useable but it is sometimes necessary to reclip the probes with resistances below 20 milliohms.

The board was revised up to Version 1.3 instead of 2.0 since it has no new active components over the original prototype or Version 1.0. A major upgrade to Version 2.0 would include auto-ranging and possibly a microcontroller with SCPI. This started out as a very modest project using an op amp on a bread board to control current and has morphed into this. Fun! I have learned a lot.

RE: Kelvin (4-Wire) Milliohm Meter: Version 1.3

DAB — Thu, 24 Jan 2019 20:44:30 GMT

Great update Frank.

DAB

RE: Kelvin (4-Wire) Milliohm Meter: Version 1.3

three-phase — Thu, 24 Jan 2019 20:24:26 GMT

Nice to see the good progress being made, look forward to seeing how the new version tests out.

Kind regards

RE: Kelvin (4-Wire) Milliohm Meter: Version 1.3

shabaz — Thu, 24 Jan 2019 13:00:34 GMT

Hi Frank,

Wow. I don't know how you manage keeping everything so neat on such a tiny PCB!

I'll print it all off and do a review, in case I can spot anything. Just initially the red connection needs removing:

[View:/resized-image/__size/403x364/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-fdd51393-d457-4a62-9b69-e8a14dda9c58/5417.contentimage_5F00_189968.png:403:364]

I had one suggestion though, I don't know if it will have any effect, but just in case any instability is caused by signal paths, it could be worth attempting to bring the components closer together around the amplifier (it will still be hand-solderable), especially to reduce the feedback path (although that's hard with the trimmers). It would mean the switch connections would have to be moved off the edge of the board but I think it's worth doing.

What do you think about some layout like this?

[View:/resized-image/__size/620x721/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-fdd51393-d457-4a62-9b69-e8a14dda9c58/6087.contentimage_5F00_189969.png:620:721]