Update to the Milliohm Meter Project
Updated V1.5 board assembly for the Milliohm Meter Project.
The Milliohm Meter board V1.41 is featured in Scullcom’s Milliohm Meter Udpdate YouTube video.
I had already started on the version 1.5 board update a week earlier and was about to send the prototype files out to OSH Park for manufacturing, when I noticed a new Scullcom video thru my YouTube subscription notice. I was pleasantly surprised to see that it was an update to the original Milliohm Meter and it was featuring the V1.41 board that I had designed. Louis had made some similar updates to components, so I quickly updated the V1.5 board file to match before sending it off.


The only major differences in components, are some even larger capacitors in the +/- 10 volt power supply section, and a PTC resettable fuse on the 9V battery supply section.
I had been putting together a BOM (bill Of Materials) for the 1.5 board and found that the 33uF capacitors were the same size and price as the 22uF capacitors that were originally used in the V1.41 board. Using a larger capacitor reduces the output ripple even further and also slightly increases efficiency for no additional cost in size or money, a win — win design change.
I have also been wanting to get some power protection added to the board for a while and should have included it in the 1.41 board. I tend not to include it in the earlier prototype’s until I am happy with the design, and can measure the current used under actual operating conditions. While prototyping I will often use a current limited power supply, or an external fuse.
The V1.5 board now has a PTC fuse to limit current, in case of a catastrophic fault on the board.

For those not familiar with PTC fuses, they are a resetable fuse, sometimes called polyfuse, polyswitch, or polymeric positive temperature coefficient device (PPTC). They protect a circuit by changing from a low resistance at room temperature when operated at or below their rated holding current, to a high resistance if the circuit exceeds the trip current. The excess current causes the device to heat up, greatly increasing it’s resistance under fault conditions and limiting the current. After the fault has been removed, and the PTC fuse has cooled down it will normally return to it’s low resistance state allowing the circuit to operate.
Other updates to the V1.5 board include larger traces in the 100 mA current source circuitry to reduce resistance and trace heating to less than 0.005°C. The small via’s are now masked to reduce the possibility of shorts, and I also included a two pin 5V header for those wanting to power the panel meter directly from the on-board 5 volt supply.
A word of caution, some panel meters can generate significant noise back thru the 5V supply. Additional inline filtering for the display may be necessary.

I created a new front panel for the V1.5 Milliohm Meter using “Front Panel Designer” to fit a standard Hammond 1455N1601 extruded box with metal end plates 6.299″ L x 4.055″ W x 2.087″ H. Link to design file
The panel is made from “Medium bronze” anodized aluminum and is 2.5 mm in thickness. It has a rectangular cutout for the panel meter, D‑holes for the four banana jacks, and countersunk holes for box mounting.


The front panel meter was again purchased from ColdfusionX on Ebay and is a 2 Volt full scale meter that operates from a 5 volt supply.
Bill of Materials for the Version 1.5 board including banana jacks and battery pack.
Part Number, Manufacturer, Manufacturer Part Number, Reference Quantity, Description A105944CT-ND, TE CONNECTIVITY, YR1B10RCC, R11, 1, RES 10.0 OHM 1/4W 0.1% AXIAL BH26AAW-ND, MEMORY PROTECTION DEVICES, BH26AAW, BATTHLDR9V, 1, HOLDER BATT 6-AA CELLS WIRE LDS 493-3717-ND, NICHICON, RR71C151MDN1, C5, 1, CAP ALUM POLY 150UF 20% 16V T/H 3296P-101LF-ND, BOURNS INC, 3296P-1-101LF, R13, 1, TRIMMER 100 OHM 0.5W PC PIN INA106U-ND, TEXAS INSTRUMENTS, INA106U, IC4, 1, IC OPAMP DIFFERENTIAL 1MHZ 8SOIC MAX680CSA+-ND, MAXIM INTEGRATED, MAX680CSA+, IC3, 1, IC REG SWTCHD CAP INV 10MA 8SOIC LT3092EST#PBF-ND, LINEAR TECHNOLOGY, LT3092EST#PBF, IC2, 1, IC CURRENT SOURCE 1% SOT223-3 LT1634BCS8-1.25#PBF-ND, LINEAR TECHNOLOGY, LT1634BCS8-1.25#PBF, IC5, 1, IC VREF SHUNT 1.25V 8SOIC 3266P-1-104LF-ND, BOURNS INC, 3266P-1-104LF, R12, 1, TRIMMER 100K OHM 0.25W PC PIN PPC56.2ZCT-ND, VISHAY, MRS25000C5629FRP00, R1 R3-5, 4, RES 56.2 OHM 0.6W 1% AXIAL PPC60.4ZCT-ND, VISHAY, MRS25000C6049FRP00, R2, 1, RES 60.4 OHM 0.6W 1% AXIAL A105891CT-ND, TE CONNECTIVITY, YR1B499KCC, R10, 1, RES 499K OHM 1/4W 0.1% AXIAL 100ADCT-ND, YAGEO, MFP-25BRD52-100R, R9, 1, RES 100 OHM 1/4W 0.1% AXIAL 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 BC1084CT-ND, VISHAY BC COMPONENTS, K104K15X7RF5TL2, C6-10, 5, CAP CER 0.1UF 50V X7R RADIAL 493-14231-ND, NICHICON, RNS1C330MDS1, C1-4, 4, CAP ALUM POLY 33UF 20% 16V T/H LM2940IMP-5.0CT-ND, TEXAS INSTRUMENTS, LM2940IMP-5.0, IC1, 1, IC REG LDO 5V 1A SOT223 BC1078CT-ND, VISHAY BC COMPONENTS, K103K15X7RF5TL2, C11, 1, CAP CER 10000PF 50V X7R RADIAL RXEF025HF-ND, LITTELFUSE INC, RF2628-000, F1, 1, POLYSWITCH PTC RESET 0.25A


I ordered 100 of the 56.2 Ω resistors, and 20 of the 60.4 Ω resistors and then picked the best of the group by measuring the resistance at two different temperatures 15° C apart for the lowest temperature coefficient. The board has an area for adding a copper shield around the dual charge-pump voltage converter +/- 10 volt supply section, which has a switching frequency of around 8 kHz. I used a 1/2″ strip of 26 Gauge copper sheet from Integrity Beads on Amazon to form the shield, and soldered it in place using the thru-hole grounds.

Front panel banana jacks are connected using silver tinned Teflon 22 AWG wire, with a ferrite bead on each wire. I also used ferrite beads on the panel meter connections, along with a 2 pin jack for the panel meter power connection.


Calibration was fairly easy with two trim potentiometer’s to adjust. To calibrate the zero reading, short the Sense (S) + and — jacks and adjust the zero trim pot to read 0.0000 on the panel meter. Then connect a high quality multimeter for measuring milliamps and connect the leads to the Current Source © + and — jacks and adjust the 100 mA trim pot to read 100.00 mA on the multimeter. Disconnect the calibration leads to prevent drain on the battery as soon as the 100 mA calibration is complete, and connect your 4‑wire kelvin leads to the meter and you should be ready to measure a known milliohm resistance as a check.
Be sure and watch Scullcom Hobby Electronics — Milliohm Meter Update on YouTube as he explains the theory of how the meter, and different devices in the circuit work.
Link to original Milliohm Meter Project page.
OSH Park printed circuit board Milliohm Meter V1.5 project page.
EagleCAD Milliohm Meter V1.5 board files ZIP

Hi Greg,
This project is coming together really nice. Thank you for improving on an already good project!
(I just missed your PCB update to 1.5, my order for the 1.41 version was already in process, but that is no major problem for me)
In any case, I found an alternative source for the precision resistors at Mouser that have the same price as the ones you got and selected. I added a note on the website from Louis and explained that there too.
The NEOHM YR1B series are only 0.18–0.25 Euro cents a piece, depending on the value. They are 0.1% and have a TC of 15PPM/C.
Enjoy!
Hi,
Just wanted to give you a heads up. We’ll be putting up a blog post on your build over at hackaday.com — should be published in a few days. Kudos, to you and Louis too, for a great project with good documentation.
Anool
Great blog post over at hackaday.com on the Milliohm Meter Version 1.5
http://hackaday.com/2017/01/24/milliohm-meter-version‑1–5/
You have a mistake in your BOM or silk screen. Is R10 499k as it says on the board, or 499 ohms as you have it in the BOM?
Russ
Upon closer inspection of the board photo, it appears to be 499 ohm and the P/N is correct also then.
if you build this, note the BOM lists an incorrect part for R10. It should be 499K ohm not 499 ohm. This is per the schematic and the ina106 datasheet application note. This is an acceptable part for R10. https://www.digikey.com/product-detail/en/te-connectivity-passive-product/YR1B499KCC/A105891CT-ND/
Bill, This has been corrected in BOM, and you are correct it is a 499K 0.1% resistor.
Thanks, Greg
About how much did that front panel cost?
Hi Grant,
Current pricing from Front Panel Express is $42.14 USD + shipping.
You can reduce the price by going to a thinner panel and removing the outside bevel though.
Greg (Barbouri)
Thanks for the quick response Greg!
Nice job on the project!!
Hello,
LCD display is not available anymore. Do you know other supplier?
Nice project!
Almost any 4–1/2 Digit 2 VDC LED or LCD meter display will work. A required feature is a separate power supply input for powering the meter (ie not input powered).
Also good linearity and accuracy and sized to fit the panel. An option that some have used is an external multimeter to view the output.
Greg (Barbouri)
The same LED display is available here (as of 4 Oct 2017): http://www.lightobject.com/4–12-Digital-Blue-LED-2V-Meter-P76.aspx
Good find Brian! I see it’s still available there for slightly cheaper than eBay over a year later. Thanks!
If someone would prefer buying it from eBay, the updated link is:
https://www.ebay.com/itm/4–1‑2-Digital-Blue-LED-2V-Meter/232659433603
I’m sorry but I’m confused!!!
what is the correct value of R10?
in your photo it appears to be 499 ohm
Great project, thanks
Hi Vincenzo,
The value of resistor R10 is 499,000 ohms (499K).
Greg (Barbouri)
I’m preparing to make this in about a month. Any updates? This is going to be so useful! Thank you for putting this together!
Hello,
Not sure if this would be the right place to troubleshoot pcb. I put together the components and adjusted the 100mA current but connecting the LED meter. I get ‑0.000 and does not change when the sense pins are shorted to adjust zero.
I even put 0.01R resistor to test and no luck on the reading.
Any help would be appreciated.
Hi George,
The first things I would check would be the 5 volt regulator IC1, and the dual charge-pump voltage converter for +/- 10 volts.
Greg (Barbouri)
Hi,
I’m trying to import the Eagle files into DipTrace, but it gives me an error.
“The number of layers in the manufacturing rules does not match the number of layers on the circuit board.”
Can you please check this, or provide Gerbers so I can get PCB’s made?
Hi NFM,
Checked the board file and it only included two layers. Top layer is #1, and bottom layer is #16.
I only have the EagleCAD files available.
Greg (Barbouri)
Greg,
I am looking for the 4.5 digit display for the Milliohm meter but can not find it on e‑Bay or else where. Do you have a source?
Thanks,
John
Hi John,
Try this link:
https://www.ebay.com/itm/232659433603
The seller also has a red LED version.
These are both similar to the one I used, but I don’t know the quality.
Greg (Barbouri)
Hi,
Thank you very much for sharing this. I made a set of these and they work flawlessly, also with a wall-wart power supply (batteries are impractical for my purpose). I was in need of a milliohm-meter with voltage output for DAQ-purposes. All the commercial meters only have a display (or their own expensive and shitty software). I was able to capture changes of 1 mOhm in a fatigue test. This file shows the (0,01Hz Low-pass filtered) signal over a few hours. Y‑axis is in Volts: https://www.barbouri.com/assets/A1filtered.pdf
Hello Greg
Getting ready to build the Milliohm project
I did see that you added Farrite beads
Was wondering if you found them necessary?
Also the original showed a 220 uf cap and yours
Has a 150 uf cap. I have both
Thanks
Hi Keith,
I usually use ferrite beads and cores on external DC connections to reduce EMI.
They are not absolutely necessary, but just another layer of protection. For me they are necessary.
I found that the 150 uF Aluminum-Polymer capacitor with an ESR of 7 mOhm, performed very well.
The 220 uF Aluminum electrolytic used in the original Scullcom project had a much higher ESR.
Greg (Barbouri)
Thanks for the information
And for all your great posts