Is there someting to improve in this design? - Milliohm meter - Page 1
Forum section: electronics / beginners

author: Arek_R
author_rank: Regular Contributor
timestamp: November 10, 2018, 10:33:20 pm
content: Hi I'm making Milliohm meter basend on the design from "Scullcom Hobby Electronics" youtube channel:And I was thinking if there's something that I could do better on my schematic, any suggestions?

author: spec
author_rank: Frequent Contributor
timestamp: November 11, 2018, 04:23:53 pm
content: + Arek_RHell, that is a nice piece of engineering: very professional.Attached is another approach, which I have had on the back burner for a while. Thought you may be interested. The schematic is hot off the drawing board, and has not been prototyped or tested, so it may have some errors, but it's the principle that I am trying to convey.

author: Arek_R
author_rank: Regular Contributor
timestamp: November 11, 2018, 04:36:32 pm
content: ThanksThat looks like a EEVblog electronic load xDIt's a good idea as well, but I will stick to my concept as I have all the parts and I like it, was just asking maybe swap one or two components and add something there and there.

author: spec
author_rank: Frequent Contributor
timestamp: November 11, 2018, 04:47:23 pm
content: Let us know how it goes.

author: Gyro
author_rank: Super Contributor
timestamp: November 11, 2018, 07:53:24 pm
content: Here's a Maxim app note (AN106) that shows a possible alternative current source. Deriving the test current from a 1.5V cell would offer a longer life than step-down from a 9V battery. Ref, opamp and mosfet can of course be substituted.http://www.wentztech.com/radio/resources/Projects/AN106.pdf

author: mvs
author_rank: Frequent Contributor
timestamp: November 11, 2018, 08:12:18 pm
content: Quote from: Arek_R on November 11, 2018, 04:36:32 pmADS1115 has pretty good PGA (differential input, zero drift, low gain error). How about throwing away ina106 and max680?

author: Arek_R
author_rank: Regular Contributor
timestamp: November 11, 2018, 09:24:25 pm
content: Hmm, I never was thinking about using ADS' internal amplifier, didn't think it would do any good, will put everything on a breadboard and will compare both concepts, but for that I will have to wait for so8 adapter from ebay.So I use maximum gain of x16(+/-0.256V)?What about input protection, some schottky diodes maybe?

author: mvs
author_rank: Frequent Contributor
timestamp: November 11, 2018, 10:10:10 pm
content: Quote from: Arek_R on November 11, 2018, 09:24:25 pmWith PGA you can implement ranges and switch gain on the fly. Current source will probably go up to 2V, so you can also use 2.048V range.Quote from: Arek_R on November 11, 2018, 09:24:25 pmInput multiplexer has already clamping diodes (10mA).PS On schematic you have 4.096V version of LT1634, it should be 1.25V.

author: MasterT
author_rank: Frequent Contributor
timestamp: November 11, 2018, 10:32:43 pm
content: In the original design they use panel meter, basically voltmeter they can't adjust, so all this precision 100 mA bla-bla, 4+ resistors in parallel makes sense. As you introduce uCPU into design, current doesn't have to be precise 100 mA anymore, and can be equal to any reasonable value 10 - 200 mA. Doing re-calibration on start-up by software would allow to correct all nuances.

author: Arek_R
author_rank: Regular Contributor
timestamp: November 11, 2018, 10:44:31 pm
content: Thanks!Oh yeah that should be 1.25V version, sorry eagle doesn't have much when it's about such a unusual components, very often I end up using different component just with the same footprint/symbol...Quote from: MasterT on November 11, 2018, 10:32:43 pmSo throw away current adjustment 100R pot and do it in software?That's a good idea, thanks.But because I'm a complete ultra noob in programming, I'm not sure If I will be able to implement all these great functions, was planning on using couple example arduino codes glued together with wet paper xD

author: MasterT
author_rank: Frequent Contributor
timestamp: November 11, 2018, 10:54:55 pm
content: Quote from: Arek_R on November 11, 2018, 10:44:31 pmIt's more than kick-off one pot. What I'm trying to say, design concept shown in the video was good in the past century, in 80-th & 90-th. Precise reference, precise current source, same with voltmeter. It cost a lot. Having uCPU, you don't need all this analog precision, as song as it's not drift much in the minutes time frame. Using only one precise resistor in the circuits and doing comparative measurements by dual channel ADC, ads1115 , ads1232 etc, you can greatly reduce the cost of this project.

author: Arek_R
author_rank: Regular Contributor
timestamp: November 11, 2018, 11:00:12 pm
content: Yes yes one resistor, that's what I mean :pWhat do you mean by "comparative measurements" ?

author: MasterT
author_rank: Frequent Contributor
timestamp: November 12, 2018, 02:12:09 am
content: This picture explains:https://www.quora.com/How-can-we-measure-the-value-of-resistance-with-CROMeasure V and Vm using dual channel adc resistance can be calculated w/o knowing a current, since it zeroed out of equation.

author: spec
author_rank: Frequent Contributor
timestamp: November 12, 2018, 03:17:59 am
content: Quote from: MasterT on November 11, 2018, 10:54:55 pmWhat a brilliant approach- thanks for posting

author: David Hess
author_rank: Super Contributor
timestamp: November 12, 2018, 10:07:11 pm
content: It would be a major design change but I would chop the excitation and synchronously demodulate to remove thermocouple errors. Essentially this means reversing the test current between positive and negative and doing the same on the sense leads. Otherwise it is not possible to take advantage of the accuracy (low drift) of even modern bipolar precision amplifiers never mind chopper stabilized amplifiers.This is commonly done in high end multimeters which provide milliohm resistance measurement.

author: Arek_R
author_rank: Regular Contributor
timestamp: November 12, 2018, 10:37:40 pm
content: And how do you switch the current without any losses and stability issues of switching components?It can't be just a H bridge out of N channel mosfets right? xD

author: bson
author_rank: Unknown
timestamp: November 12, 2018, 11:23:30 pm
content: Get rid of the pot - it's better to calibrate and correct the measurements before readout since you have a µC. No need for anything adjustable.

author: David Hess
author_rank: Super Contributor
timestamp: November 13, 2018, 12:28:34 am
content: Quote from: Arek_R on November 12, 2018, 10:37:40 pmSince it is a current being switched, the only losses in switching are limited to bias current errors which are trivial (or nonexistent if the design is clever) and voltage drops which are irrelevant. An h-bridge made out of MOSFETs could be used successfully although that might not be my first choice.

author: Arek_R
author_rank: Regular Contributor
timestamp: November 16, 2018, 11:43:53 pm
content: Actually you're right, 4-20mA exists for a reason :pSo what would be a better way to switch it if not h-bridge?

author: unitedatoms
author_rank: Frequent Contributor
timestamp: November 18, 2018, 02:08:00 am
content: may be add button for "set zero" routine. and may be 1 more button for good known cal standard ( say some independently measured specimen in range of 10 milliohm). i dont know what is this cpu board includes, i assume it has led and lcd.also a jumper for firmware to choose between 60 hz vs 50 Hz if you decide to measure with immunity to mains background in integer amount of cycles during acquisition and averaging

author: Arek_R
author_rank: Regular Contributor
timestamp: November 18, 2018, 11:26:30 pm
content: Well its arduino pro micro(mega32u4) + 1'3 OLED graphic display.But I also may use graphical "8x2" oled display instead.And now with the amount of functions I may add a 4 membrane buttons, plenty of them on ebay for dirt cheap.50/60Hz rejection haha, this primarly weekend project turns into quite advanced device xDNot much time recently tho :/

author: David Hess
author_rank: Super Contributor
timestamp: November 21, 2018, 06:09:08 pm
content: Quote from: Arek_R on November 16, 2018, 11:43:53 pmDiodes can be used for current switching with the advantage of lower capacitance and lower leakage. But an h-bridge is easier to understand and likely good enough.

author: Lukas
author_rank: Frequent Contributor
timestamp: November 21, 2018, 09:16:18 pm
content: Seems like you're all missing one trick: Instead of using a DC current, use AC current and all offset errors are gone. Seehttps://www.analog.com/media/en/technical-documentation/application-notes/an98f.pdfpage 10 for an example. Instead of using a synchronous demodulator, digitizing the signal using the MCUs builtin ADC might also be feasible.

author: kripton2035
author_rank: Super Contributor
timestamp: November 21, 2018, 09:26:46 pm
content: this one is using an ADS1115http://circuitcellar.com/cc-blog/build-an-accurate-milliohm-meter/