Milliohmmeter measures internal resistance of batteries, too

Check the condition of your car battery by measuring its internal resistance, without discharging it. Also track shorts and measure with high accuracy all kinds of low resistances: shunts, PCB tracks, contact resistances, relays, chokes and various windings, RTD sensors, etc.

This instrument is based on an AC low current very low frequency 4-wire resistance+ inductance measurement technique, which solves the issues of DC measurement errors. And it has the ability to measure the internal resistance (+ voltage) of DC current sources, with optional DC bias.
There are 4 measuring ranges from 1 up to 1000 ohms (best resolution of 10 microohms) with manual or automatic selection.
Powered by a standard 9V alcaline battery.
All details in the attachments.

Project Elements

lrm19-6013-w.jpg (JPG, 680.06 KB)

mesure d'une résistance de précision

ohm-references-6834-w.jpg (JPG, 1.8 MB)

My ohmic references for calibration

lrm19-6771-ac.jpg (JPG, 1.42 MB)

vue de l'instrument

lrm19a-rear-view-6843-w.jpg (JPG, 1.74 MB)

LRM19A rear view (PCB top)

LRM19A_BOM.pdf (PDF)

LRM19_block_diagram.pdf (PDF)

LRM19_timing_diagram.pdf (PDF)

LRM19A_source_unit_list.pdf (PDF)

LRM19A_measure_unit_list.pdf (PDF)

LRM19A_gerber.zip (ZIP)

Gerber files

LRM19A_component_location_top.pdf (PDF)

LRM19A_pcb_top.pdf (PDF)

LRM19A_pcb_bottom.pdf (PDF)

LRM19A_component_location_bottom.pdf (PDF)

precision_milliohmmeter.pdf (PDF)

Milliohmmètre_de_précision.pdf (PDF)

LRM19A_HEX.zip (ZIP)

LRM19A coding HEX files

LRM19_schematic_diagram.pdf (PDF)

Discussion (13 comments)

RikV 2 years ago

Sourcing components really is a full time job these days. Delays up to 53(!!!) weeks. This will take a while....

henrip4 2 years ago

One year dreaming of a milliohmmeter...It's crazy!!!

RikV 2 years ago

The positive side: by that time I will know the device and it's components by heart....
I am desperately seeking for drop-in replacements. Hard thing.

1 Comment(s)

RikV 2 years ago

Henri, why 2 different chips? I see no meaningful difference in the datasheet? Could they be replaced by ADI ADG719BRTZ and/or ADG819BRTZ?

henrip4 2 years ago

The current source requires rather low "on" resistance switches (less than 10 ohms), , because of the limited voltage headroom. I chose the STG719 because it was much cheaper than the ADG719, with the same characteristics. And for the synchronous detector, I chose the MAX4544 because of its low charge injection (not specified for STG719) and slower switching times (for transients at the amplifier outputs). I must say I didn't quantize precisely all these error sources (conversely as I did for the DAC).

1 Comment(s)

RikV 2 years ago

I must apologize. I relied on my memory concerning the TC of the refVoltage in the AD5681. This is indeed a good chip with 2ppm/K. Other specs are as good as one could wish.
I am consolidating the bill of material towards Mouser. Parts availability is still a problem.
Regarding the PCB I have a problem with the drill file LRM19A.exl. I loads faulty. The coordinates are scaled down by 10 but the hole diameters are correct. This is with Flatcam as well as Gerbview. What software are your files extracted from? Did the PCB manufacturer manage to read the file correctly?
You know of a drop-in replacement for the ISL28633? It will be months before it comes available.

henrip4 2 years ago

I checked the Excellon file, and I found it ok. Coordinates are in microns with 6 digits as required. All my PCBs are manufactured by Eurocircuits, and they never asked me about that.
Unfortunately, I have no alternative part for these ISL28633 zero drift PGAs. Actually, today's availability of any active component is really an issue.

RikV 2 years ago

I made a quotation request to JLCPCB and to my surprise the excellon file was accepted whitout any complaints. So something must be wrong with the configuration of my Gerbview AND Flatcam since both react the same. What might that be??? I recently upgraded from Win8.xx to win 11, could this mean something??
By the way, JLCPCB comes to 8.95€ for 5 pieces of the board and 24.99€ SH cost.

henrip4 2 years ago

I use Gerbview (Kicad) with Windows10, and there is no problem when reading the Gerber drilling file (drill.gbr). However, Gerbview doesn't recognize the .exl format, I don't know why.
I didn't experiment JLCPCB, but 8.95€ seems to be a good price, I think it was more expensive with Eurocircuits, but for only 2 pieces.

1 Comment(s)

RikV 2 years ago

Are you willing to share the native schematic files? What ECAD soft do you use?
Do you have a detailed functional description available?

Stéphane, I do not pretend to be native English speaking, and by the way: Elektor (Elektuur) is not a French but a Dutch magazine.
Let's focus on the subject please.

henrip4 2 years ago

I work with an old CAD software (Winschema and Wintypon) which was mainly used for education, so I assume that native files won't help you.
I didn't write the detailed functional description yet. Such a documentation (in English, not my mother language) requires much time. However, you may ask me about any subject (like DL2-DL3).

1 Comment(s)

RikV 2 years ago

What is the function of DL2-DL3? Current/voltage limit/stabilisation?

henrip4 2 years ago

DL2/RT1 and DL3/RT2 act as a temperature compensated 1.030V voltage reference used for the voltage clamps.
And very important for the protection of the instrument, this configuration is functional even when the power is off.

1 Comment(s)

RikV 2 years ago

Indeed, precision and stability of measurement come at a (high) price.
But equipment should not be more expensive than really necessary. I understand tha many of the component choises have been made on a "I have this at hand so I don't have to buy it" basis.
However some of the componenet are excessively expensive since there are much cheaper alternatives available.
One example is the display. Over 16€ at Mouser (not counting the extra components) but an ordinary 1602 LCD only costs 4 or 6€! Add 2€ and you have I2C interface. I would prefer to spent that mony on an external RerVoltage for the DAC with significantly better tempco and drift specs (about €20 for 2ppm/K).
If ever you are planning to make a revision to the PCB please include these items as an option?

henrip4 2 years ago

Hi. You are right, I have a small stock of LCDs and microcontrollers. But actually, I've been using this LCD since almost ten years. And at that time, it was the only one able to work at 3.3V or 5V without any added component. Moreover, it is temperature compensated, does'nt require an external trimmer for contrast adjustment, is compatible with I2C and SPI interfaces, and offers an extended character set very useful. This is why I didn't want to change.
And about the DAC, thanks to the ratiometric configuration used, reference drift doesn't appear in the measurement result.

Stéphane PEREZ 2 years ago

understand tha many => that, the ?
of the componenet are => component ?
that mony on => money?
etc... Look at yourself !

2 Comment(s)

Vladimir I. Yershov 2 years ago

The same functionality, but much cheaper:
https://aliexpress.ru/item/1005001926986935.html?

0 Comment(s)

FR0130893ID 2 years ago

Bonjour
En Français ce serais bien
Cordialement

henrip4 2 years ago

Je vais poster la version française d'ici quelques jours.
Cordialement.

Renaat Wendrickx 2 years ago

Pour les francophones il existent des cours de langues!

Stéphane PEREZ 2 years ago

Pour un magasine en Français, on s'attend à des articles en Français....
A noté que je n'ai pas de soucis de lecture, mais ça me laisse moins de doute lors de la lecture d'un article.

RikV 2 years ago

Pour les francophones: "A noté" == à noter

Stéphane PEREZ 2 years ago

Enkor un proph de frenssai a la retréte?
Bonne journée monsieur Parfait!

https://www.youtube.com/watch?v=xKxTrHLSt1g

2 Comment(s)

Hevandyr 2 years ago

Very good information, which will be useful for me since in my system I work with 70 nobreaks spread around the city in automation stations. I always have to keep an eye on the batteries, replacing them when necessary. Thanks for the tip.

0 Comment(s)

Dj831 2 years ago

Hi Henri, nice to see at least a project that uses SO ICs and 0805 resistors, things that are hand solderable

henrip4 2 years ago

Hi. Unfortunately, there is an unavoidable IC which is not in a SO package: the DAC (U3) with a pitch of 0.5mm.
I added I think all the missing files needed to build the project.

Dj831 2 years ago

Hi Henry, thanks! As I can see, you have at least one of these famous AOIP standards resistors (I have some, too). And you made a good use of these ugly Teko P series boxes (ugly, but so useful!).

I think I will give a try to your very nice project next time I place an order to JLCPCB.

Out of curiosity, how did you make a such smart front panel? Did you use one of these chinese CNC?

Thanks and greetings from a place between Le Havre and Fécamp :)

henrip4 2 years ago

Bonjour Dj831. Je présume pouvoir répondre en français (c'est plus facile pour moi).
En fait, je fabrique moi-même la plupart de mes pièces mécaniques. J'ai taillé ma face avant dans une plaque d'aluminium anodisé de 2mm (de récupération), détourage à la scie et finition à la lime, les trous sont faits à la perceuse sensitive. Le marquage est une étiquette transparente autocollante imprimée sur laser (pas très facile à positionner, et attention aux bulles d'air). Le boîtier est de la tôle d'acier de 1mm (de récupération aussi) pliée sur les grands côtés, avec les 2 extrémités haute et basse soudées à l'étain-plomb. Finition avec apprêt plus peinture pour carrosserie. Comme je dispose d'un tour, je fabrique également sur mesure mes boutons, entretoises, etc.
Je fais fabriquer tous mes circuits imprimés chez Eurocircuits. Je connais JLCPCB de nom, mais je ne l'ai pas expérimenté, c'est à priori moins cher.
J'ai 2 résistances étalon AOIP: 0.001 et 0.01 ohm. J'aurai préféré en avoir une de 1 ohm, mais c'est pratiquement introuvable aujourd'hui. J'ai donc acheté des résistances de référence de chez Vishay que j'ai placées dans ces fameux boîtiers Teko P1 (j'ai racheté l'année dernière une dizaine de ces boîtiers chez Conrad, j'avais utilisé le dernier de mon stock pour mon projet "failsafe continuity tester").
Merci de votre intérêt à ce projet très marginal et salutations depuis la pointe Sud de l'Yonne!

1 Comment(s)

Kabron 2 years ago

Опираясь на эту статью и материалы, мог ли кто-нибудь собрать это устройство?
Где смысл

henrip4 2 years ago

Hi. With the missing files I just posted, I think now that anybody is able to build the project. Notice however that most of the mechanical parts (enclosure, front plate, switch buttons, spacers, ...) are all home made.
After your first question, you began a new sentence, but there is missing text. Could you complete so that I may answer ?

1 Comment(s)

ElektorLabs 2 years ago

Thanks for this excellent project. Would you like to share the software too? That would allow other people to build the instrument as well.

henrip4 2 years ago

Hi. I just posted the 2 firmware files, plus a timing diagram and the bill of materials.
I have a few other documents, but they are not English translated yet.

1 Comment(s)

Content Director, Elektor 3 years ago

Hi, In your documentation, you write " Because they use high grade components, the cost of precision instruments is unfortunately high, and this is the case here. Maybe it is possible to build a more economical version by sacrificing some features." Can tell us, ballpark, how much you spend on this project? Hundreds of euros? Just curious.

henrip4 3 years ago

Hi,I computed the cost of my project based on the billings I received in 2020: 198 euros. Add the PCB for 69.50 euros (VAT included for both) .
This amount doesn't include the home made enclosure and miscellaneous mechanical parts. However, components were bought at unity (or close to) prices. As you can see, this is actually not a cheap project. And today, with the shortage of components, the cost will still be higher, unfortunately.

1 Comment(s)

Milliohmmeter measures internal resistance of batteries, too

Project Elements

Discussion (13 comments)

RikV 2 years ago

henrip4 2 years ago

RikV 2 years ago

RikV 2 years ago

henrip4 2 years ago

RikV 2 years ago

henrip4 2 years ago

RikV 2 years ago

henrip4 2 years ago

RikV 2 years ago

henrip4 2 years ago

RikV 2 years ago

henrip4 2 years ago

RikV 2 years ago

henrip4 2 years ago

Stéphane PEREZ 2 years ago

Vladimir I. Yershov 2 years ago

FR0130893ID 2 years ago

henrip4 2 years ago

Renaat Wendrickx 2 years ago

Stéphane PEREZ 2 years ago

RikV 2 years ago

Stéphane PEREZ 2 years ago

Hevandyr 2 years ago

Dj831 2 years ago

henrip4 2 years ago

Dj831 2 years ago

henrip4 2 years ago

Kabron 2 years ago

henrip4 2 years ago

ElektorLabs 2 years ago

henrip4 2 years ago

Content Director, Elektor 3 years ago

henrip4 3 years ago

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