A D.I.Y. Milli-Ohm-Meter Called the Walsmho

About: Technologist (40th year)

I had three Keithley 580 micro-Ohm-meters in my Shop. I sold two. I kept one and recently pulled that meter out of storage. I did not realize the weight of the top of the pile, and the meter was crushed with a cracked LCD.

Alright, so...,

Can I make one?

I have a 0.0 mV to 199.9 mV LCD module, with famous Intersil 7106 IC driver. 9Volt power. No case. 200mV in.

I (think I) have some e-skill, and WWWeb-searched this topic of DIY milli-Ohm Meters.

I call this project 'Walsmho" in honor of Georg Ohm. {anagram; OhmsLaw.}

I know Kelvin types of meters use a very accurate constant current source, and apply that direct current to the unknown resistance. The voltmeter I chose has a sensitivity that results in a direct reading on a millivolt meter of the low resistance from 0 to 1 Ohm, when a certain value of constant current is applied.

The cost issue is that I have no spare time nor funds, to create ultra-sensitive super-precision instrumentation to measure 1000th's or 100th's Ohms.

This tool will help me determine wire length on a spool from wire gauge diameter, measuring Inductors, and Transformer windings. My objective is to locate shorts between (non-powered) printed circuit board traces. I have to be careful because high current at elevated voltage can destroy active components.

I looked at a number of designs, and their methodology, and then created this mash-up.

The current source power and the LCD module meter power sources must be separated, but in my design, I have one three position power switch, with OFF, 10mA, and 100mA ranges.

The 10 mA range will be good for unknown circuit boards, and the 100 mA range will be good when I know I will not destroy active components, and determine the direction towards a low resistance to another PCB rail or trace.

I have one legacy tester, and I use it occasionally. The test device uses an audible interrupted oscillator and is quite sensitive ; it was sold in Radio Shack (now known as The Source) in Canada in the late 1970's.

Step 1: What: Circuit Explanation

A quick look at my schematic; There are two attached PDF's to this instructy. Version one is similar to many designs found on the WWWeb. Version two is my own mash up.

The constant current source is a three terminal regulator with two precision 10-turn potentiometers and a switch to 'select' the Constant Current ranges of 10mA and 100mA .

The overload protection is a 12V 3W Auto Dash Lamp Bulb in series, before the CC regulator stage. I found this helps to mitigate drift and improve stability over longer bench sessions, in the 100mA range.

A C&K three position toggle was used to select range and switch power to the constant current reference, and to turn off the LCD Meter module, I used a 5V SPST reed relay to switch a 9V J1604 battery source to power the meter. Using the same power for both stages is not possible with LCD 710x based Voltmeters unless a separate linear comparator is used, and I did not want that complication. As an alternative, the builder could use two separate switches.

I limit the voltage coming into the milli-voltmeter by using back to back signal diodes ( 0.7V for bidirectional clamping ). I expect a measurement range of < 200 mV, so I could have used two Shottky 250mV forward drop (2 x 1N5817) diodes instead.

I designed a 10mA range for relatively higher resistance up to 10 Ohms and 100mA range for 0 through 1 Ohm where my shop bench DMM's cannot follow. The reader can also just build the constant current reference and use a Bench DMM to measure the unknown R(uut) mV drop.

I discovered the 10mA range has a bunch of bonus applications; I am able to test L.E.D.'s and I can test small Zeners (to about 8Vdc).

I glued some known reference resistors to the top of my makeshift case (nice-to-have). I used an L-shape Aluminium Heat Sink as the base for this project so I could mount five Banana type bulkhead mounted Jacks; RED for VCC, ORANGE for Meter +, YELLOW for Meter -, GREEN for I-SOURCE and BLACK for ground Return.

I powered the current source from a 3.8V Li-Ion battery 800mAh and used a step up LM25xx module to source 9.0 Volts DC (adjustable). I also deploy a 1S USB battery charger module to keep that Li-Ion charged up.

I tried using a J1604 standard 9V battery for the constant current regulation in the 100mA mode, however, the current varied after 30 minutes of short hunting, and I need longer lasting run time operation.

The LCD draws a low 5-10 mA for the LCD, and a (1604A/6LR61) 9V standard Zinc or Alkaline cell will power that meter for a long time, given that the relay shuts down the meter when I switch the unit OFF.

Parts list

  1. Battery for Constant Current
  2. pre-load like 12V 225mA Auto Dash Illuminator bulb
  3. AMS1117 type or LM317 TO-220 type 3-terminal regulator
  4. two precision multiturn potentiometers
  5. C&K on-on-on DPDT series 72xx
  6. 9V (6LR61) battery and clip for LCD meter
  7. 5V reed relay for switching 9V power slaved to above source and switch
  8. LCD 7106 type LCD module with 0-200mV input sensitivity, 3.5 digit type 0.000 to 199.9 (+ and -)
  9. two 1N4148 or 1N5817 diodes in parallel / reverse configuration as 'clamps'
  10. 1x 100uF 50V capacitor for 3terminal regulator output
  11. case, banana jacks, wires, TO-220 case mounting to heatsink and so on

There is a device called the "Half-Ohm" on Tindie and I will order this $20 item soon. Powered by a coin battery, this will be the item that goes into my portable tool box.

Step 2: The Build

I built my meter on an L-shaped bracket of Heat Shink Aluminium .

The LCD meter module is from the 2000's but it fits right on the front of that heatsink.

I used bits of CDROM cases for Top, where I glued three selected low resistance references.

I used some Alligator clips from busted clip to clip cables , soldered to Banana Plugs.

I am using long tip DMM probes for M+ and M-. I have modified another custom set of probes with sharp sewing needle tips for use in PCB sniffery. The entire purpose of this project was to regain PCB short hunting capability on my bench.

I re-used some C&K multi-way toggle DP3T switch with 3 positions, but any selector will do. The reader can also use multiple switches for mode and power. I may even have 73xx series with 3P3T to switch the "decimal point indicator" in storage.

I found three (pulled) ceramic resistors to use as test references , and glued them to the top of my meter.

I built this dead-bug style, point to point, which is a mess, but with a reliable soldering methods.

Step 3: How: Circuit Explanation

In Versions 1 and 2, a Battery provides a steady current to the Step Up module XL6009 through the combined function of a range/power switch.

For Version 2, in the 10mA and 100mA positions, the C&K 72xx series switch power branch also enables a reed relay that closes the separate 9V power to the LCD VoltMeter ITL7106 module. The module has a basic range of 0 to 199.9 millivolts. The test current is passed through an Automotive Dashboard 12V Lamp, the teardrop kind, that draws 180 - 240 mA at 12 V. This lamp acts as a load during prolonged 100mA short sniffing operations, and will only glow "dimly" when current is passing in the 100mA range. The test current then passes to the constant current three terminal regulator with two switch selected precision 10-turn potentiometers set precisely to 10mA or 100mA. A capacitor helps to maintain stability with various inductive loads. The output is at a Terminal marked "A". The trace or resistor or winding is clipped between "A" and "GROUND". The LCD meter probes then "hunt" up and down the low resistance path. As the M+ and M- probes get closer to the source "A" and "GND" test current, the millivolt drop measurement will change.

The idea for settings 10 and 100 milliampere was to directly read milliOhms on the digits of the meter, but I had not switched the "decimal point" . I will find a C&K 73xx series switch later, and "switch" the decimal point, but for now, I do the decimal point in my brain.

Depending upon shorted calibration of the current (10.0mA and 100.2mA in my bench build) the accuracy can be quite good, in the order of 0.1% or lower with just a three terminal regulator. AMS1117-3.3 devices tend to give a better temperature stability than the LM317T in TO-220 package if the device is tied to a heatsink. I have noticed that when the device was case dissipated, the heating would create a drift in the constant current at 100 mA and therefore, a small calibration drift in the milliOhm measure.

First Image: theory in simpler form. Second and Third images: 0.47 Ohm Ceramic Power Resistors reads 0.5015 Ohms using a borrowed Keithley (one of my Sold Meter's). In the 10mA range, LCD indicates "50" or "51". 3rd image indicates "503" or "504" in the 100mA range. The Keithley meter is really good and only sources like 1mA.

The Alligator clips are connected to the Constant Current Output and Ground Return banana Jacks. The M+ and M- mV meter input are in parallel as shown.


NEVER use another mOhm tester in parallel with 10mA or 100mA! FOR USE WITH NON-POWERED CIRCUITS!

Step 4: Acknowledgements and Enhancements


Georg Simon Ohm and his 1827 Paper on Galvanic Circuit Mathematics

An original idea from A.M. Hunt, April 2004 (image attached), and

several Internet posts by various Authors including G. Lacompte in 2010.

The objective is Constant Current regulation and a integrated MilliVoltMeter in low power LCD format, with simple dual ranging, a banana jack cable format for the bench, and long lasting run time current, for sniffing (newly shipped, bare) PCBoard shorts.


Arduino control of both Variable Constant Current and of rail-to-rail Bridge Metering , range shifted to 1mV resolution with 1.1 VRef feature of ATMega328, etc. and Hands-Free "Audible output" so I do not need to shift my gaze from probe pointing -> to digit meter readings. The first image is a schematic of a concept found on the WWWeb however, that LM324 is *horrible* for rail to rail and has bad distortions when uni-rail powered off the 5Volt. I will be using a R-R CMOS OpAmp soon to accomplish this concept to a NANO in INTERNAL1V1 mode.

I may create a PCB layout 2 sided for this build, based on Bournes precision multi-turn potentiometers.

I will also purchase "HALF-OHM" because it may be the best for my portable kit.



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