Sub-ohm digital Ohmmeter

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#3
You want to build one, or post for everybody to try?

If the need is occasional, use a Wheatstone Bridge, and galvanometer arrangement. The galvanometer can be replaced with a decent resistance meter, the bridge works.

Op Amp designs may exist.

If regular, there are suppliers like Masibus in India, a friend uses those to check the ferrite coils he produces at his factory, so it is a production level unit.
I recall that AC is sometimes used, a pulse and its fading are used to measure coil resistance.
With cheap units like the Arduino and Raspberry Pi, that is also a handy way to achieve the result.
 
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#5
First step is the kelvin clips!

https://www.amazon.com/Dahszhi-Gold...ocphy=1025202&hvtargid=pla-593534746263&psc=1
I would then use one of the 200 millivolt LCD digital panel meters run by a 9 volt battery on on side of each clip. Be sure the meter can handle overload voltage! You might want to put a pair of inverse parallel diodes across the input. Maybe even a series resistor to limit current. As the input impedance of the meter is very high a current limiting resistor of a few thousand ohms should not affect accuracy.

You might try the 9 volt version of this one. (PM-1028A)

https://www.circuitspecialists.com/...MI3authLuV9gIVh4XICh1Efg1DEAQYASABEgI4Z_D_BwE
On the other side of the clips I would use a 100 milliamp current supply. This could be as simple as a 12 volt battery and 120 ohm resistor. You could also use a transistor, diode and resistor as a lower voltage current source.

With 100 milliamps and a 200 millivolt meter you could measure resistance below 2 ohms.

I think the trick is kelvin clamps and independent measurement and test current sources. If AC mains powered, two isolated power supplies. As the meter current is trivial, you could still use a battery just for that.
 
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#6
I just use a lab supply with current regulation to drive a known current through the DUT and measure voltage between the points of interest with the multimeter. then R=V/I
this for example worked fine for resistance of PCB traces.
 
#7
I designed and built the attached about 2 years ago (on Veroboard 😉 ) but the LCD display went faulty. I probably should resurrect it and do a proper board etc.

Although the A-D is only 10 bits, by arranging the front end amplifier as shown you can still get pretty respectable results. Each reading is taken about 1000 times (the A-D is about 5-10us for a reading) and averages so the result is very stable. Originally I took about 200 readings per cycle on each of the measurement terminals but ended increasing this to 1000 times.
 

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#10
TonyTecson said:
i am looking for designs, please post your design here...
Click to expand...
I built this simple constant current source circa 1970 to measure the low resistances in OPT secondaries & similar problems.
It is also useful while measuring non-linear devices such as SS diodes & incandescent lamps. All done with a separate VOM,
analogue or digital.

Some examples of common diodes are included in the note. Incremental resistance can be calculated from the test results.

HP marketed a MilliOhmeter circa 1970. It was built at YHP, Yokogawa HP in Japan.
We sold quite a few. But don't recall the model number off hand. There was a companion MegOhm Meter as well.
 

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#11
#13
stv said:
I just use a lab supply with current regulation to drive a known current through the DUT and measure voltage between the points of interest with the multimeter. then R=V/I
this for example worked fine for resistance of PCB traces.
Click to expand...
I do the same.

I was going to suggest a known resistor, a bench supply, a couple DVM voltage measurements, and some algebra gets you cheap answers. But you guys are way ahead of me. 🙂
 
#16
I use JAY_DIDDY_B's ESR meter with an instrumentation amp output - super for measuring down to and below 1 milliohm, and as accurate as any full-scale resistor you can find (I have used a 10R 0.05% for full scale). It is a Kelvin probe setup, and zero and FS are calibration points. As accurate as two double Kelvin bridges (L&N and Pontavi) that I have for milliohm measurement, and no need to pump lots of current through the DUT as one bridge does.
 
#17
TonyTecson said:
i am looking for designs, please post your design here...
Click to expand...


In both cases, never built a dedicated unit which I would use infrequently, just pass constant current 100mA through and measure voltage drop with any cheap multimeter

20mV scale= 0.2 ohm full scale.

200mV scale= 2 ohm full scale

You can use 1A if winding can stand it so increasing sensitivity 10X

No Arduino needed 😉

In any case, no matter what you build, you will still need to build the CC supply, so that is a given.

I have a small box full of fake unreliable 2N3055 which I amassed along years , unusable in an amp but perfect for these lowly chores.
 
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#18
Constant current resistance bridge.

Forget Wheatstone Bridge, noone uses this for serious measurements any longer.

Generally it is advised that for measuring 1s, 10s of milliOhms, 10A constant current is applied.

There are very good reasons for this, as detailed in the relevant BS/EN standard (60034.2 if I recall)

100mOhm, and the current should be 1 A

I could show you a schematic, if you aren't afraid to use 741, and 3 transistors and you can make a accurate shunt with temp.co. less than 30ppm
 

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#19
if you are dumping 1A through a resistor it will be heating up and then you are measuring its resistance at that elevated temperature. That may not be what you are looking for.

I have both of the HP instruments mentioned above 4328A and 4329A. The 4328 uses AC for the measurement so thermocouple effects won't degrade the measurements. Its quite sophisticated looking only at the resistive part of the impedance. Not simple to make.

Measuring low resistances into microOhms can also be done with a current source and a sensitive voltmeter. The current source is a critical element and its accuracy will define the performance. For low values 100 mA should be fine except- 100 mA across 1 milliOhm is 100 microvolts. You need a meter that can read 100 microvolts accurately. And adding an opamp to boost the DC gain is challenging since the opamp will have offset which translates to an error term. You can use an autozero/chopper opamp which will help, but back to accuracy the Seebeck-thermocouple effect will be another big challenge. Copper-solder is 3 uV/degree C. so a 5 degree differential will cause a 15 mV or 15% error in the above example assuming everything else is perfect. This explains the AC solution HP used.
 
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#20
1audio said:
if you are dumping 1A through a resistor it will be heating up and then you are measuring its resistance at that elevated temperature. That may not be what you are looking for.

I have both of the HP instruments mentioned above 4328A and 4329A. The 4328 uses AC for the measurement so thermocouple effects won't degrade the measurements. Its quite sophisticated looking only at the resistive part of the impedance. Not simple to make.

Measuring low resistances into microOhms can also be done with a current source and a sensitive voltmeter. The current source is a critical element and its accuracy will define the performance. For low values 100 mA should be fine except- 100 mA across 1 milliOhm is 100 microvolts. You need a meter that can read 100 microvolts accurately. And adding an opamp to boost the DC gain is challenging since the opamp will have offset which translates to an error term. You can use an autozero/chopper opamp which will help, but back to accuracy the Seebeck-thermocouple effect will be another big challenge. Copper-solder is 3 uV/degree C. so a 5 degree differential will cause a 15 mV or 15% error in the above example assuming everything else is perfect. This explains the AC solution HP used.
Click to expand...
You miss the point.

I am not dumping 1A into any load, but an appropriate load.

Hence 10A for unit of 10s of milliOhms.

This is how it is done in industry, complying with agreed standards of measurement.

The res bridge I show, is accurate to uOhms, has a direct 200mV FSD (as JMFahey does also), making it fit standard metering.

Trust me, if you are measuring 10 mOhm, and then unless you wish to measure a resistor at 10mOhm and very very low wattage, it is rhe best solution.

Using a microOhmeter, with a tiny excitation current, is vulnerable to all sorts of 'other ' effects, interference, and inductive loads should be measured in both polarities.

The typical use for a microohmeter of this accuracy is for large conductors, where 10A won't have any significant effect (think 100A capable cabling, earth bonding etc)

The best part is, you don't seem to realise that this is what we are already doing: supplying controlled current to a resistance, to measure voltage drop.

But it works better if you think outside the lab, and lab grade equipment, where uOhms are in the noise at the bottom of their capability

Lastly, the difficulty of obtaining unsullied measurements in the uV or lower level is precisely the reason that in industry there are standard for the current sourced into various load resistances, such as 100mA for less than 2 Ohm, 1A for 20 to 200mOhm and 10A for loads less than 20mOhm.

In fact, we regularly test the armature reistance of megawatt scale DC machines, at circa 500A or more, in exactly the same way, dictated by BS EN standards 🤔

in that case, there is a real heating that can occur, so speed of measurement is key
 
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