diy low ohm meter schematic - Page 1
Forum section: electronics / projects

author: uranium235
author_rank: Contributor
timestamp: November 16, 2010, 04:48:36 pm
content: does anyone have a diy low ohm meter schematic diagram??with the following features?1.easy calibration2.measures 1 -0.001 ohm 1% accuracy3.use minimal and easy to find components4. can be used as an adaptor for DMMi already searched the net still i cant find any good diagram

author: TheWelly888
author_rank: Frequent Contributor
timestamp: November 16, 2010, 04:55:56 pm
content: Will this do?http://baec.tripod.com/DEC90/ohmmeter.htmNo, I haven't tried it myself, all I did was google "low ohm meter" and found this in 10 seconds!

author: uranium235
author_rank: Contributor
timestamp: November 16, 2010, 05:06:19 pm
content: ive already seen the site before, im having problem on CA3140 i cant find it locally in our area.

author: TheWelly888
author_rank: Frequent Contributor
timestamp: November 16, 2010, 05:32:23 pm
content: You don't have to use a CA3140 op-amp in that circuit. What you can do is to work out why the CA3140 was chosen for this application and have a look at the data for it and find the nearest equivalent.

author: Balaur
author_rank: Unknown
timestamp: November 16, 2010, 06:24:25 pm
content: A 4-wire mili-ohmmeter is quite easy to design or to improvise.You just need a serious (10mA-100mA-1A depending on voltmeter sensitivity) constant current supply and then measure the voltage on the resistor with a multimeter. The presented schematic was implementing the same approach.Off course, you can use any serious opamp for a 10x, 100x or whatever gain to minimize the required current. Put all this in a cheap panel voltmeter or a PIC with a HD447880 display and presto you have a miliohmmeter.Cheers,Dan

author: Fragrance
author_rank: Newbie
timestamp: October 11, 2016, 08:20:04 am
content: Quote from: uranium235 on November 16, 2010, 04:48:36 pmhighly accurate and used any 4 1/2 digit DMM i made it using it with my DMMhttp://electronics-diy.com/electronic_schematic.php?id=951

author: kripton2035
author_rank: Super Contributor
timestamp: October 11, 2016, 10:09:25 am
content: plenty of examples here :http://kripton2035.free.fr/continuity-repos.htmlthe one I prefer is :http://kripton2035.free.fr/Continuity%20Meters/continuity-an220.html

author: KerryW
author_rank: Regular Contributor
timestamp: October 11, 2016, 01:47:54 pm
content: I use CA3140s as a drop in replacement for for TLO81. I don't know how close they are, but they are close enough for the circuits I build.Edit: After reading the circuit description, I see that the CA3140 gets a lot closer to the negative rail than the TLO81, so you can't use one. Sorry about that.

author: oldway
author_rank: Super Contributor
timestamp: October 11, 2016, 03:51:57 pm
content: To measure resistance as low as 1 mR with precision, I would use a 10A current source for the lowest range.(four wires measurement of course !)

author: Kalvin
author_rank: Super Contributor
timestamp: October 11, 2016, 04:13:46 pm
content: Quote from: oldway on October 11, 2016, 03:51:57 pmFew milliamperes is sufficient:https://www.eevblog.com/forum/projects/hx711-based-milliohm-meter/msg782192/#msg782192

author: Dbldutch
author_rank: Regular Contributor
timestamp: October 11, 2016, 05:07:18 pm
content: I have built the milli-volt meter from the following link as a well working prototype. In the meantime a user developed a pcb, and I am building that one. Parts are on order as we speak.Have a look here :http://www.scullcom.uk/category/projects/milliohm-meter/There is also a youtube series describing the design.You can also gave a look at my own blog where I am trying to complete a BOM :http://www.paulvdiyblogs.net/2016/09/building-milli-ohm-meter.htmlIt's a work in progress ahem process, but well worth it.Enjoy!

author: oldway
author_rank: Super Contributor
timestamp: October 12, 2016, 03:31:11 am
content: Quote from: Kalvin on October 11, 2016, 04:13:46 pmHow could you measure 1mR with 1% precision if your project has:- Initial drift was 3 milliohms- After initial drift, the variance was +/- 0.5 milliohms- Measurement current is in range 5mA - 10mA.

author: Kalvin
author_rank: Super Contributor
timestamp: October 12, 2016, 08:27:19 am
content: Quote from: oldway on October 12, 2016, 03:31:11 amThe idea of my posting was that 10A is not needed in order to be able to measure milliohm resolution, only few milliamperes is sufficient. Dannyf did some further development in the discussion thread and he was able to get the accuracy good enough for milliampere accuracy. The HX711 has some temperature drift issues what was found out, but the temperature effect was few milliohms at worst, and the error due to the temperature drift got less after the HX711 was kept in constant temperature.The OP wanted to have a milliohm resistor measurement device with a range 1 milliohm ... 1 ohm with 1% accuracy. If the accuracy of 1% means the accuracy over full measurement range of 1 ohm, then the required accuracy should be better than +/- 5 milliohms which can be achieved even with the cheap, non-compensated HX711. If one replaces the HX711 with a better ADC or builds a simple temperature control for the HX711, the measurement accuracy can be easily in range of +/- 0.1 milliohms, which is pretty good. However, the dannyf's idea to use a cheap HX711 ADC with ratiometric measurement and small measurement current with a high resolution A/D-converter was a clever solution.Edit: Some microcontrollers have pretty high resolution on-chip A/D-converters. For example, the Psoc5 LP devices have a 20-bit differential input delta-sigma ADC. Unfortunately I do not have time atm to give it a try how good milliohm meter could be built using the the Psoc5 LP device, using ratiometric measurement, chopped stimulus current and synchronous detection. If the accuracy of the on-chip reference voltage is not good enough, one can use an external reference voltage for improved performance.

author: oldway
author_rank: Super Contributor
timestamp: October 12, 2016, 12:00:18 pm
content: Quote from: uranium235 on November 16, 2010, 04:48:36 pmWithout using any op amp:Range 1mR to 10mR ...Current source 10A ...reading DMM 10 to 100 mVRange 10mR to 100mR ...Current source 1A...reading DMM 10 to 100 mVRange 100mR to 1R....Current source 0.1A....reading DMM 10 to 100 mV

author: Kalvin
author_rank: Super Contributor
timestamp: October 12, 2016, 01:00:29 pm
content: Quote from: oldway on October 12, 2016, 12:00:18 pmTake a UNI-T UT61E DMM, select the 220mV range and use a 10mA current source to generate the test current. Measure the voltage across the unknown resistance,and the UT61E will display the resistance directly in milliohmsand the display will show 10uV per milliohm. The measurement range is 1 milliohm ... 22 ohms with the resolution of 1 milliohms.Edit: Whoa, made some trivial mistakes here as I didn't have the UT61E at hand.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 12, 2016, 02:57:48 pm
content: To get a really good resolution at a moderate current, one should measure with both polarities of the current. So do a kind of low frequency (e.g. 1 Hz) AC measurement. With DC one might resolve something like 1 µV and even than can have trouble with thermal EMF of the measured resistance. As ther thermal EMF can very well be at the probed resistor there is no real point in improving the measurement part.With AC one can measure much smaller voltages. Something like 0.1 µV or even 0.01 µV gets reasonable with a low noise amplifier. So it can be a good idea to have a switch to reverse the polarity of the test current. An maybe an extra (low noise) amplifier, as the ADC inputs and even more the DMMs are usually not that low in noise. With an ADC a µC could aso directly do the difference calculation.

author: TimFox
author_rank: Super Contributor
timestamp: October 12, 2016, 03:01:58 pm
content: For an AC measurement, a lock-in amplifier (with long averaging time) can be useful to measure the voltage.

author: Gyro
author_rank: Super Contributor
timestamp: October 12, 2016, 03:05:26 pm
content: Quote from: oldway on October 12, 2016, 12:00:18 pmI still like the elegant little battery operated current source used in the Maxim AN106 milliometed design (attached). There's nothing particularly special about the reference or the opamp, use your own choice. You can trim it to be as accurate as you like. I'm still on the original batteries after long enough to be worrying about their shelf-life!It's DC of course, but as long as you have decent meter sensitivity and take into account any thermal emfs before and after measuring then it won't go far wrong.

author: rob42
author_rank: Newbie
timestamp: October 16, 2016, 08:47:21 am
content: C8051F350 costs about 2.2$, contains 24-bit ADC, PGA/opamp:https://ru.aliexpress.com/item/C8051F350-GQR-8-k-ISP-Flash-MCU-Family-C8051F350/32709262043.htmlHere is design of precision milli/micro ohm meter:http://bbs.38hot.net/thread-45486-1-1.html

author: laffreux
author_rank: Newbie
timestamp: August 09, 2023, 02:47:05 pm
content: Here is a simple way to measure sub-ohm...You just need a Lab power supply and a volt-meter.It is as accurate as the meter !!!

author: kripton2035
author_rank: Super Contributor
timestamp: August 09, 2023, 03:09:26 pm
content: shorty with display to build yourself :http://kripton2035.free.fr/Projects/shorty-display.html

author: Dave
author_rank: Super Contributor
timestamp: August 09, 2023, 04:00:39 pm
content: Quote from: laffreux on August 09, 2023, 02:47:05 pmNo?You either also need to measure the current with a second multimeter, or have averyaccurate power supply (SMU territory). The tolerances of both instruments multiply to get the measurement tolerance.

author: TimFox
author_rank: Super Contributor
timestamp: August 09, 2023, 04:11:28 pm
content: Or, put a relatively high resistance measured-value power resistor in series with the lab voltage supply.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 10, 2023, 07:29:26 am
content: The easiest DMM adapter is to use an LM317 with an (about) 1.25 Ohm resistor to use it as a 1A current source / sink.It's not so easy for calibration (you have to adjust the 1A capable power resistor), but otherwise it's quick, simple and effective.Then have some power source (lab supply?) and push that 1A current though your DUT, and measure the voltage drop with your DMM. This is also inherently a 4 wire measurement.

author: Marco
author_rank: Super Contributor
timestamp: August 10, 2023, 11:08:22 am
content: Low ohm meter is an ambiguous term, do you just want to measure small resistances or do you specifically want to measure low resistances at high current. Micro-ohm meters generally do the latter, not because high current is necessary for the resistance measurement but because it's part of the overall measurement which is not just a resistance measurement.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 10, 2023, 01:31:57 pm
content: It would probably help to use (very) low frequency AC to null out the effects of thermocouple offset.

author: Marco
author_rank: Super Contributor
timestamp: August 10, 2023, 07:26:59 pm
content: Quote from: NiHaoMike on August 10, 2023, 01:31:57 pmIf it's a sine or a repetitive 3 current bipolar measurement, say -x, +x and +2x Ampere, you could not only subtract out thermocouple offset but also measure Josephson voltage for your room temperature superconductor

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 10, 2023, 10:52:39 pm
content: Quote from: uranium235 on November 16, 2010, 04:48:36 pm1. is taken care of by using 4.You are unclear on spec, do you want 1% of full-scale, so 1 ohm measures 10mOhms, or do you want to measure 1mOhm to 10 microohms.1A into 1mOhm is 10uV at 1% scale.Since you already have a calibrated, decent? meter, the missing part is higher injection currents than a DMM usually uses on resistance.A good place to start, would be to create a precision, stable current source of 100mA and 1A and maybe 10mA.If you have a bench power supply, that gives you more choice to control the circuit power loss, which will be a significant issue.If you pick a low dropout regulator, with a low reference voltage, and low Vmin, you can slash the power dissipation needed.Wirewound parts at 1% and ±50ppm/°C are plentiful.Failing that, a 2-3A USB wallwart means you have 5W to spread over the (choose large) dropper elements, at the 1A test current.Depending on how good your DMM is, you could also add a fixed gain opamp, low noise and low offset. OpAmps keep getting better !eg a TLV387 has 10uV vos and 8.5nV/sqhz and 0.01uV/'C with 0.1% gain resistors gives you a solid foundation.With typical numbers of 1uV Vos, you may even find 100mA of test current is sufficient for your tolerance specs.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 11, 2023, 01:26:04 pm
content: There are indeed different ways of measuring small resistances.The easiest is to use a high current, as a higher current generates a bigger voltage drop and thus a larger test signal.There are also meters designed for measuring contact resistances, and these deliberately use a very small test current (micro amps or less?)But a micro amp at a milli ohm is a nano volt. And I'm not sure that is even measurable by normal folks. Small temperature differences can easily swap this due to thermocouple effects.The "Low Level Measurements Handbook" is a good read for what the real life limits are of what can be measured. When measuring small currents for example, at some point you are counting single electrons. Once you start measuring the time between single electrons passing, you're well into the VooDoo range.I also like the method of the article posted in circuitcellar #314 (Schematic attached). It uses simple hardware and smart software to get very usable results.

author: intabits
author_rank: Frequent Contributor
timestamp: August 29, 2023, 02:09:00 pm
content: Here's what I made to measure low resistances:-

author: Terry Bites
author_rank: Super Contributor
timestamp: August 30, 2023, 07:46:47 am
content: www.analog.com/en/design-notes/batterypowered-circuit-measures-milli-ohms-and-micro-ohms.htmlEasy

author: Shock
author_rank: Super Contributor
timestamp: August 31, 2023, 01:30:29 pm
content: Terry Bites, are you using that last design or see any video on it?

author: Gyro
author_rank: Super Contributor
timestamp: August 31, 2023, 05:40:47 pm
content: It's the AD abstract of the acquired old Maxim AN106. See reply #17.Here's the version of it that I built...https://www.eevblog.com/forum/projects/design-review-ac-milliohm-meter/msg4589653/#msg4589653

author: Shock
author_rank: Super Contributor
timestamp: September 01, 2023, 07:22:53 am
content: That enclosure looks the real deal.