Milliohm Meter Version 1.5

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.

Milliohm Meter Version 1.5 circuit board bare component side view
Milliohm Meter Version 1.5 cir­cuit board bare com­po­nent side view
Milliohm Meter Version 1.5 circuit board bare back side view
Milliohm Meter Version 1.5 cir­cuit board bare back side view

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.

Polyswitch PTC fuse. photo from Wikimedia Commons
Polyswitch PTC fuse. pho­to from Wikimedia Commons

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.

Milliohm Meter V1.5 front panel from Front Panel Express in shipping pack
Milliohm Meter V1.5 front pan­el from Front Panel Express in ship­ping pack

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.

4.5 digit front panel meter 1.9999 volts full scale, back component view
4.5 dig­it front pan­el meter 1.9999 volts full scale, back com­po­nent view
4.5 digit front panel meter 1.9999 volts full scale, front view
4.5 dig­it front pan­el meter 1.9999 volts full scale, front view

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
Milliohm Meter V1.5 board with surface mount components reflow soldered
Milliohm Meter V1.5 board with sur­face mount com­po­nents reflow soldered
Milliohm Meter Version 1.5 circuit board ready to install in enclosure
Milliohm Meter Version 1.5 cir­cuit board with cop­per shield, ready to install in enclosure

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.

Preparing to connect the wires to the front panel of the Milliohm Meter
Preparing to con­nect the wires to the front pan­el of the Milliohm Meter

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.

All front panel connection made on the Milliohm Meter, and ready for testing
All front pan­el con­nec­tion made on the Milliohm Meter, and ready for testing
First milliohm resistance check of a 0.01 ohm 1% resistor after calibration
First mil­liohm resis­tance check of a 0.01 ohm 1% resis­tor after calibration

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

Open Hardware

 

30 Replies to “Milliohm Meter Version 1.5”

  1. 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!

  2. 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

  3. 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

    1. 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)

    1. 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)

  4. 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

  5. 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!

  6. 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.

    1. 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)

  7. 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?

    1. 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)

  8. 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

  9. 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

  10. 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

    1. 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)

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