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  • Author Author: fmilburn
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New Improved DIY Milliohm Meter V2.0

fmilburn

The milliohm meter has been one of the most popular projects I've posted on element14 judged by views, comments, and stated interest.  Thanks to a modification by (documented way down in the comments here) and much help over time it appears to have reached or is very close to the design objectives:

  • Low cost
  • Dual range covering from 1 milliohm to 40 ohms
  • Choice of power source - USB or batteries
  • Kelvin (4-wire) probes
  • Low current (1 to 10 mA) through the Device Under Test (DUT)
  • Provision for "out of range" indication
  • 1 milliohm accuracy down to 1 milliohm

 

The current Version 1.3 PCB requires bodging in some parts and the documentation, posted over many blogs with endless comments, is difficult to follow.  So I've set a goal to clean it up and post a new version before yearend.  I'd also like to add some features like microcontroller auto-ranging and output, otherwise known as unnecessary wishes and feature creep.  But before adding the feature creep there will be an interim step documented here.

 

Schematic

 

The revised schematic has these features:

  • Added resistors R29 and R30 to remove the intermittent instability that occurred when measuring resistance below 20 milliohms or so
  • Took out diodes D1 and D2 which did not seem to help
  • Moved the connections on the PCB for probes and the User Interface to a single 14 x 2 header

 

The revised Version 2.0 schematic is shown below:

image

One thing I'd like thoughts on is the addition of input protection as there is none currently.  There is not much headroom with the lower limit of USB voltage and the 4.7 V LDO voltage regulator.  There is an alternate source of voltage (labelled battery input - it could be anything) that could be hooked up backwards by the user or power applied with the USB source simultaneously.  Is it worth dropping USB and designing for a higher voltage (could be USB-C or just a jack) with more input protection?

 

Enclosure

 

One of the concessions I will make to accommodate feature creep is a larger enclosure and PCB.  The enclosure selected is the 5.287" L x 5.323" W (134.29mm x 135.20mm) X 2.008" (51.00mm) Hammond 1598BHammond 1598B.  There are PCB standoffs on Eurocard centers so the PCB may well fit other enclosures of a similar size.  It can be purchased with brushed aluminum or plastic end panels.  The slots for the end panels will also take a PCB.  Of course 3D printed end panels will also work.

image

credit:  Hammond Manufacturing

 

PCB

 

The new layout and design decisions are described below.

image

The noisy stuff and future digital stuff is on the bottom and analog is on top.  Power input and regulation is lower left.  All tracks in and out of the power section are widely separated with ground in between tracks and a clean ground plane beneath.  The constant current section is located just above it.  The ICs are spaced out in order minimize temperature influence.  The panel behind the constant current section and the power section could be vented.

 

The amplification of the differential signal from the Kelvin sensing clamps attached to the device under test are upper center.  I am curious about thoughts on the spacing of the feedback resistors and the trimmer pots.

 

All connections to Kelvin clamps, panel meter, and other user interfaces are on the 28 pin header to the far right.  A comparator to determine when the instrument goes out of range is sandwiched between it and the amplification.  In future it would be replaced an ADC for the microcontroller located bottom right.  A separate ground plane with star connection could be set up for the digital section.

 

Front view with silk screen:

image

Back View with Silk Screen:

image

All Tracks and Pads:

image

Front Copper

image

Back Copper:

image

 

Conclusion:

 

I plan to send off for a new set of PCBs by mid-week and given that it works will then start working on Version 3.0.  All thoughts and suggestions are appreciated.  And if anyone is interested in building one and has an idea let me know.  KiCad files are available as well.

 

 

Parents

  • Hi Frank,

     

    That looks nice! The different areas on the board (power, current source, amplifier, processor) make it easy for people to tweak and enhance the design I think. Also, it seems a great idea to have the single 28-way connector, because it could reduce wiring, if there is a front panel PCB in future.

    Regarding the USB connector, I have lots of DC barrel plug supplies so I prefer socket for those, but equally others may prefer USB power, it's hard to know what is more popular.

    I was thinking, maybe the blank area on the PCB could be populated with 0.1" spaced pads, and perhaps even a land pattern for the ADC you wish to use (maybe the same one as in the panel meter, which is MCP3421, since it worked well), so you have space to experiment.

    If you're ready to send off your PCB though maybe it's not worth the bother.

    I only mention it because by coincidence I just finished a PCB design to fit a Hammond case too (love those because of the front panel), and I had some space to fill too.

     

    If you're looking for more feature creep, maybe the power LED could be replaced with a low-battery indicator. I'll write up an example implementation in a short blog post tonight, in case you wish to use it, but if you're planning on having a microcontroller eventually then low-bat functionality is better implemented in the microcontroller.

Comment

  • Hi Frank,

     

    That looks nice! The different areas on the board (power, current source, amplifier, processor) make it easy for people to tweak and enhance the design I think. Also, it seems a great idea to have the single 28-way connector, because it could reduce wiring, if there is a front panel PCB in future.

    Regarding the USB connector, I have lots of DC barrel plug supplies so I prefer socket for those, but equally others may prefer USB power, it's hard to know what is more popular.

    I was thinking, maybe the blank area on the PCB could be populated with 0.1" spaced pads, and perhaps even a land pattern for the ADC you wish to use (maybe the same one as in the panel meter, which is MCP3421, since it worked well), so you have space to experiment.

    If you're ready to send off your PCB though maybe it's not worth the bother.

    I only mention it because by coincidence I just finished a PCB design to fit a Hammond case too (love those because of the front panel), and I had some space to fill too.

     

    If you're looking for more feature creep, maybe the power LED could be replaced with a low-battery indicator. I'll write up an example implementation in a short blog post tonight, in case you wish to use it, but if you're planning on having a microcontroller eventually then low-bat functionality is better implemented in the microcontroller.

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