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

Andrew J — Thu, 30 May 2019 08:09:31 GMT

Nice project, I’m bookmarking to come back to later. I like making these tools rather than buying them.

Andrew

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

three-phase — Tue, 26 Mar 2019 15:27:31 GMT

This has been a fantastic project to keep following and appears to be getting better and better.

Many thanks foresting this.

Kind regards.

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

fmilburn — Fri, 08 Mar 2019 02:19:17 GMT

Edit: Replace updated Version 1.3 schematic with higher resolution copy 8 Mar 2019

[View:/resized-image/__size/620x469/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-041026c6-8ea3-461f-91d1-9a2d7b350e78/5102.contentimage_5F00_190333.jpg:620:469]

The only change is that values for R18 and R20 have been put on the sheet. In truth, you could probably just use a 180 ohm resistor for R19 and a 160 ohm resistor for R21, then leave R18 and R20 unpopulated. This is because the current sources have a fair tolerance around the calculated values and the trimming is done on the amplification of the instrument op amp.

I am using different values for the trimming pots R15 and R16 since I only have 100K pots on hand. This means that I had to use a larger value for R14 but I recommend the values on the spread sheet.

Today I quickly checked both ranges and values from around 2 milliohms up to 4 ohms. Everything seems to be working and I am not seeing the instability often and only down at very low values. It is intermittent as before although possibly less frequent. I have not had time to check whether capacitance / diodes can remove it completely. The non-linearity that occurred at very low values now appears gone completely, fixed by setting Vref a bit above ground. I am using standard resistors as opposed to low tempco and have not seen issues but also am not using an enclosure and have not quantified...

I have too many projects open at the moment and will put this one aside for now since it is at an easy stopping place. Too be continued...