Anybody know about a 4 wire Ohm handheld DMM? - Page 1
Forum section: electronics / testgear

author: ErikTheNorwegian
author_rank: Frequent Contributor
timestamp: November 09, 2017, 09:52:00 pm
content: Anybody know about a 4 wire Ohm handheld DMM?A good quality one?Anyone, Bueller..Bueller...`?

author: EEVblog
author_rank: Unknown
timestamp: November 09, 2017, 10:21:20 pm
content: That's what LCR meters are for. The decent ones are all 4 terminal and do 1mOhm resolution.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: November 10, 2017, 02:09:51 am
content: There is a GMC Multimeter with Milliohmmeter functionality, which uses 4-wire measurements in the lower resistance ranges. Besides, it is a full-fledged multimeter.

author: 001
author_rank: Super Contributor
timestamp: November 10, 2017, 07:33:09 am
content: Look for de5000Decent meter

author: Electro Detective
author_rank: Super Contributor
timestamp: November 10, 2017, 08:59:18 am
content: I had a small 4 wire black body meter from Jaycar a few years back that did the job ok, but got zombied by leaking batteriesThe 4 wire leadset survived, not sure if it's of any use for anything elseBeen looking to replace it either with a cheapie no frills bench meter or handheld that does 4 wire,but in the meantime my PEAK Atlas ESR-70 cap tester reads decent low ohms if I zero it first

author: HalFET
author_rank: Frequent Contributor
timestamp: November 10, 2017, 01:11:36 pm
content: I made a small black box temperature compensated current source with one of those LT 2 terminal regulators and then simply used a multimeter on volts. If you want I can try to trace out the schematic of what I dead bugged back in the day.

author: 001
author_rank: Super Contributor
timestamp: November 10, 2017, 04:51:56 pm
content: Quote from: HalFET on November 10, 2017, 01:11:36 pmIt is amasing idea. Can You share schematic etc.?

author: ErikTheNorwegian
author_rank: Frequent Contributor
timestamp: November 10, 2017, 08:03:44 pm
content: Has worn out 2 DE5000 in about 14 moths, due to moist, dust and not very resistant for salty enviroment. We use them on boats/platforms in a salty humid enviroment. They are not as good built as Fluke for wear and tear.Was hoping that there was a handheld that was more solid buildt.I`l get the last one back to the workshop and take som fotos and post. Whats happening is tht there are like corrosion "growt" in the traces on the pcb, right around where the solder joints are and around the joints. Its like that its a small copper stripe that does not have any masking around the solder pads.Erik

author: Wytnucls
author_rank: Super Contributor
timestamp: November 10, 2017, 08:12:49 pm
content: Gossen MetraHit 30M, if you can find one.

author: HalFET
author_rank: Frequent Contributor
timestamp: November 11, 2017, 10:51:24 am
content: Quote from: 001 on November 10, 2017, 04:51:56 pmI cracked the chip trying to pry it off the case to see which one it was. But I owe you a circuit! My main question is, which error are you willing to accept?These 2 terminal current regulators normally have a positive temperature coefficient, so the trick is to grab a couple of small signal diodes and find one with roughly the same or opposite behaviour and put this one in your feedback network in a way that counter-acts the current increase. The stereotype implementation of this can be found in the LM234 datasheet. But I was just looking over at the newer 2 terminal regulators, e.g. LT3082 and LT3092, and they seem to be a lot less temperature dependent, so one of those should do the trick. Between 0 and 50°C you don't seem to have more than 0.005 µA drift on the internal reference current source so that's more than sufficiently stable for most applications I think, and I doubt you're planning to do arctic exploration with this thing.So lets assume we run it of two 9V batteries, of which we lose 2V to the dropout. So we use a measurement current of 100 mA, because that makes for easy math. So we would then realistically be able to go from 150 Ohm down to about 10 mOhm (1 mV) on the average meter. So lets take the LT3092 (http://cds.linear.com/docs/en/datasheet/3092fc.pdf) with the suggested LT1634 shunt reference, as shown on page 18 of the LT3092 datasheet. The LT1634 has a change of about 0.5 mV over the desired temperature range (Page 6 of the LT1634 datasheet -http://cds.linear.com/docs/en/datasheet/1634ff.pdf), at 10 µA however the current has no real effect on the voltage, so we can throw that variation of 5 nA in the trash bin. The LT3092's output current in this configuration is simply Reference Voltage / Shunt resistor. So, neglecting the temperature increase of the 12.5 Ohm resistor we'd need, we could calculate the effect of the temperature variation now quite easily: 0.5 mV / 125 Ohm = 40 µA. So on the 150 Ohm measurement (15V) this would create an error of 6 mV, and on the 10 mOhm we're down to 0.4 µV, so I think we're safe if we modify that circuit slightly to take the temperature increase of the resistor into account. I'll draw something up after lunch.*Back after lunch*We could either come up with a complicated feedback circuit that acts and talks like a resistor based on the reference, but then we're stuck with using an OTA, and it's the equivalent of shooting a mosquito with an ICBM. (Though, as a side note, I do approve of that.) So the easier solution is to take it into account in the resistor and minimize the resistor heating by using multiple in parallel. Loads of 24.5 to 24.9 resistors available right here! e.g. if we takehttp://www.farnell.com/datasheets/2111203.pdfyou have about a 0.12 Ohm resistance change over the desired temperature range. Now since we're using it in parallel we have 12.45 Ohm + (0 - 0.062 Ohm) + +/- 1%. This means we go from 100.4 mA to 99.9 mA roughly, or an error of 0.5 mA, which in turn is about 5 µV error for 10 mOhm or 0.5%. And on a reading of 15V for 150 Ohm we get 0.075V error (also 0.5%). So I think we're quit safe in just taking this circuit with a small adjustment and a 1W thick film 24.9 Ohm 1% resistor with 100 ppm/°C.So I'd like to propose the following circuit, I took the 1N5817 from memory, so not sure if it'd be really suitable for this. But I think it was 1A with about a 0.45V forward drop. The trimmer is optional, but I highly recommend it, might have to take a slightly higher value (since the bias current is 100 µA). The 18V zener with a power NPN is as input protection against higher voltages. Think this is a simple enough approach that should be sufficient. If you care more about accuracy it's a good idea to invest in a good handheld LCR meter I think?Do tell if you see any mistakes, haven't done a quick ballpark calculation like this in quite a while.

author: noidea
author_rank: Unknown
timestamp: November 11, 2017, 02:14:53 pm
content: Quote from: Wytnucls on November 10, 2017, 08:12:49 pmGossen 27m and 27i also do kelvin resistance measurement. Maybe a bit more affordable than a 30m too.They would also meet your rugged requirements too.

author: nanofrog
author_rank: Super Contributor
timestamp: November 11, 2017, 02:55:50 pm
content: Given your environmental needs + portability, I'd second opting for one of the Gossen's mentioned.Not cheap though, so be prepared for some sticker shock.