The Milliohm Meter Project
I built this milliohm meter several months ago which is based on a project by Scullcom Hobby Electronics and presented on YouTube.
Louis at Scullcom Hobby Electronics has a new video updating the milliohm meter, and features the board I produced in this post.
NEW Link to updated video on YouTube
NEW Version 1.5 board available at OSH Park requires additional 0.01 uF ceramic capacitor, 2 pin header, and 250 mA PTC fuse.
NEW Updated project post with Milliohm Meter version 1.5 board.
It uses a 4 wire kelvin measurement system that supplies a precise 100 mA current to the device under test using one pair of leads, and then measures the voltage drop across the device under test using a second pair of leads.
As designed it can be used to measure low resistances of under 2 ohms (Ω). Some of the design changes I made to fit my needs were:
- Change the number of resistors needed in the current source circuit
- Use higher precision resistors with a low temperature coefficient
- Add separation and shielding between input, output, and supply circuits
- Use lower ESR and higher value capacitors for the MAX680 power supply
- Add additional decoupling capacitors on the INA106, and MAX680 IC’s
- Change the voltage regulator from a TO220 package to a SOT-223 package
OSH Park generated render of top circuit layout of the V 1.4 board
OSH Park link to shared project page for V1.4 Milliohm meter
Components installed in version 1.1 board
Angled view of Milliohm meter circuit board
Front panel with switch, 4.5 digit display, and dual banana jacks for 4 wire connection to device under test.
Meter turned on with blue LED display.
The LED display was purchased on Ebay from seller coldfusionx and is a 4 1/2 digit blue LED digital 2V Volt meter. The presented specifications are:
Accuracy: +/- 0.1% «
Power: 5V
Range: 0–1.9999V
Resolution: 0.0001V
I was very happy with the performance of the display meter, and it looks much better than is shown in the photos. The battery assembly is comprised of 6 AA 1.5 volt Energizer Ultimate Lithium cells making a nominal 9 volt supply to the board. The lithium cells do very well with the + 130 mA load and had a very low voltage drop in use.Component placement inside case.
To power the display meter I added a separate buck converter to supply a dedicated 5 volts from the 9 volt battery.
On the V1.1 board I ended up with a bad footprint for the LM2940 5 volt regulator and bodged a TO220 version to the board. This has been corrected in versions after V1.1.
25 milliohm 1% resistor connected with 4 wire leads.
The completed meter works well for my purposes and I have found that it is used much more than expected for things that I would normally use my Fluke 189 multimeter for, and I don’t have to put up with the low resolution.
Link to EagleCAD Milliohm V1.41 schematic and board files ZIP
A big Thanks! to Louis from Scullcom Hobby Electronics for designing and presenting this project in an informative and professional video.
There is an updated project post with Milliohm Meter version 1.5 board.
Great work
Thanks Jonathan
Wow, that looks impressively nice. Is this purely a one-off personal project, or are you considering making them to sell? I for one could be interested in buying one if they were available…
Hi LeoNerd,
It was intended as a personal project to fill a need, but I wanted to document it to make it easier for the next person to build.
I do have two un-populated version 1.1 boards available, and the version 1.41 boards are available from OSHpark https://oshpark.com/shared_projects/UWvehNO2
Thanks
I’d be interested in buying one of those unpopulated boards then if it’s going spare. I don’t see an email address published we could discuss that on — maybe you could email me?
Thanks.
Cool!!!
Im also building one myself but your circuit is looking far way more simpler than my so far.
Great job, I gonna use your as a reference.
Thanks for share.
Hey Barbouri,
It seems we have very similar interests, and visit the same sites for some of our projects.
I was already planning to build the milli-ohm meter, and the 6.5 digit DMM, when I found the link to your boards.
Since you’ve made some interesting and worthwhile changes to both designs, one would need to have the BOM to be able to work with the boards that are available through OSH Park.
Do you by any chance have a BOM for both projects to share?
Mant thanks,
Paul
Barbouri,
In case you don’t have a BOM that will help ordering the right parts, I went through the exercise myself but could use your help in verifying that I picked the right parts to match the PCB layout.
If you can send me a PM with your email address, I can send you the Excel sheet, and can start on the Milli Volt/6.5 digit DMM.
Many thanks,
Paul
Part Value Information Supplier Partnumber
C1 22uF 16V Tantalum? Or low ESR Mouser / DigiKey Nichicon UMA1C220MDD1TP
C2 22uF 16V Tantalum? Or low ESR Mouser / DigiKey Nichicon UMA1C220MDD1TP
C3 22uF 16V Tantalum? Or low ESR Mouser / DigiKey Nichicon UMA1C220MDD1TP
C4 22uF 16V Tantalum? Or low ESR Mouser / DigiKey Nichicon UMA1C220MDD1TP
C5 220uF 10V Electrolyte LOW ESR Mouser / DigiKey Panasonic EEU-FR1A221B or EEU-FR1A151B
C6 0.1uF MLCC X7R Mouser / DigiKey Vishay K104K20X7RH5TL2
C7 0.1uF MLCC X7R Mouser / DigiKey Vishay K104K20X7RH5TL2
C8 0.1uF MLCC X7R Mouser / DigiKey Vishay K104K20X7RH5TL2
C9 0.1uF MLCC X7R Mouser / DigiKey Vishay K104K20X7RH5TL2
C10 0.1uF MLCC X7R Mouser / DigiKey Vishay K104K20X7RH5TL2
G1 AB9V
IC1 LM2940‑5 LM2940IMP‑5.0/NOPB Mouser / DigiKey TI — LM2940IMP‑5.0/NOPB
IC2 LT3092 IST is better than the EST DigiKey LT3092EST#TRPBF or LT3092IST#TRPBF
IC3 MAX680CSA Mouser / DigiKey MAX680CSA (end-of-life) or MAX680CSA+
IC4 INA106U Mouser / DigiKey TI — INA106U-2K5
IC5 LT1634ACS8‑1.25 DigiKey LT1634ACS8‑1.25#PBF
R1 56R .1% 250mW 15PPM/C Mouser / DigiKey H856RBYA or YR2B56R
R2 56R .1% Mouser / DigiKey H856RBYA or YR2B56R
R3 56R .1% Mouser / DigiKey H856RBYA or YR2B56R
R4 56R .1% Mouser / DigiKey H856RBYA or YR2B56R
R5 56R .1% Mouser / DigiKey H856RBYA or YR2B56R
R9 100R .1% Mouser / DigiKey YR1B100RCC or H8100RBYA or RC55Y-100RBI
R10 499K .1% Mouser / DigiKey YR1B499KCC or 279-H8499KBYA
R11 10R .1% Mouser / DigiKey YR1B10RCC or H810RBYA
R12 100K 3266 Bourns Mouser / DigiKey 3266P‑1–104LF
R13 100R 3266 Bourns Mouser / DigiKey 3266P‑1–101LF
I just got an email today that the BOM has been made available and is published on scullcom.uk
Hi Barbouri,
Great work on this and your other projects.
Can you tell me where you got your test clips from? Did they come with the 2 leads per clip or did you make them yourself?
Thanks,
Tim
Hi Tim, Thanks!
The Kelvin 4 wire test leads were purchased from Amazon.com. Search for “kelvin test”.
They were an inexpensive set purchased at under $20 USD, but will build my own set next time as their quality is not the greatest.
Greg (Barbouri)
Paul Hi,
Very impressive work, mind sharing links for the test leads sockets and the enclosure ?
I thinking of building one based on your latest v1.5 PCB — many many thanks for that.
Eddi
Hi Eddi,
The V1.5 enclosure is a Hammond 1455N1601 extruded box with metal end plates 6.299″ L x 4.055″ W x 2.087″.
Sockets are:
501‑1081-ND, POMONA ELECTRONICS, 1581–3 PJS+, 1, JACK BANA PANEL MT TIN ORG
501‑1079-ND, POMONA ELECTRONICS, 1581–1, PJS- 1, JACK BANA PANEL MT TIN BRN
2269–0‑ND, POMONA ELECTRONICS, 2269–0 PJV+-, 1, BANANA JACK DOUBLE BLACK
Greg (Barbouri)
Thank you for all the info and help on this page .
One more thing i cannot find it.
The alligator clips where you plug the four wire.
Any suggestion ?
Digikey would be nice.
Hi ion,
In Digikey search for kelvin, and on Amazon search for Kelvin in electrical.
Greg (Barbouri)
Milliohm Meter prize
Hi,
Do I absolutely have to buy the PCB from OSH Park or can I have it made anywhere?
Thanks
Hi Andre,
No you do not need to order from OSH Park.
You can download the Eagle CAD files and order from any PCB manufacturing service.
Greg (Barbouri)
Hi Greg,
Where we can found version 1.5 schematic ?
Thank
Here is the link to the Eagle CAD ZIP file for V1.5
https://www.barbouri.com/assets/MilliOhm15.zip
i search a miliohm meter module (maybe own build) with a link function RS232 to read out the value remote?
any help would be very nice.
kind regards
benoit