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Old 18th Feb 2018, 7:48 pm   #1
duncanlowe
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Default Robin M220 Miliohmeter?

OK, to start I know milliohms is spelled incorrectly, but that's how it's spelled on the instrument.

So I've purchased myself this low ohm, four wire meter. It's of an age, coming in a leather case and using two 4.5v batteries. I haven't tested it yet as I didn't have stock of these batteries and my order for two resulted in only one being delivered. The second is allegedly on its way.

But apart from ebay listings, most of which seem to be for the one I bought, I can't find much information, and less any kind of instructions. So has anyone come across this, or have any literature? I can add some photos if it would help.
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Old 19th Feb 2018, 4:49 am   #2
Ed_Dinning
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Default Re: Robin M220 Miliohmeter?

Hi Duncan, it sounds like a "Kelvin" type of instrument.
2 leads pass current through the part under test, the other 2 are voltage measurement leads. This removes the resistance of the test leads from the circuit.
Commonly used in the power industry.

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Old 19th Feb 2018, 8:28 pm   #3
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Default Re: Robin M220 Miliohmeter?

Yes, it is. I recognised it as such which is why I bought it, useful for checking earthing on cars.

It has the four wires, but oddly only one pair of croc clips and they don't have two sockets for the two sets of leads.

But try as I might I can't find any similar units or any instructions when searching, so I don't even know it's age.
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Old 26th Feb 2018, 8:43 am   #4
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Default Re: Robin M220 Miliohmeter?

Quote:
Originally Posted by duncanlowe View Post
...It has the four wires, but oddly only one pair of croc clips and they don't have two sockets for the two sets of leads...
Are the plugs and sockets two-pole? I can understand the single pair of croc clips, but without at least one separate voltage feedback wire I can't see how it would work.
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Old 26th Feb 2018, 10:49 am   #5
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Default Re: Robin M220 Miliohmeter?

There are special croc clips known as Kelvin claps. Each jaw of a pair is wired separately and the two parts are insulated from each other. Clip one of these onto something and one side of the croc makes the drive current connection, while the other makes the voltage sensing connection.

There are also 4-wire bridges that use these things to make AC Z measurements with test lead effects removed.

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Old 26th Feb 2018, 9:16 pm   #6
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Default Re: Robin M220 Miliohmeter?

It has two leads, each with two cores, a red and a black each. There are four terminals on the meter. I would have expected the croc clips to have two terminals, to be exactly kelvin clips. Maybe they got lost sometime and were replaced incorrectly with a standard clip. I'll go and take a photo in a few minutes.
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Old 26th Feb 2018, 9:56 pm   #7
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Default Re: Robin M220 Miliohmeter?

Good tip about the Kelvin clips, David, I hadn't heard of them.

Without the Kelvin clips, the meter will probably still work but its calibration will be suspect.
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Old 26th Feb 2018, 10:13 pm   #8
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Default Re: Robin M220 Miliohmeter?

Here is said beast...
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Old 26th Feb 2018, 10:45 pm   #9
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Default Re: Robin M220 Miliohmeter?

What a useful meter! Usable down to 0.05 milliohm. Nothing wrong with those probes/clips, they form a genuine four-wire ohmmeter. I think it would be worth checking, with a voltmeter, which of the four terminals are the 'force' pair and thus, by elimination, which are the 'sense' pair, and their polarity. I'm guessing that the larger clips are the force pair and the smaller clips are the sense pair, but it's not clear (to me!) how the leads plug into which socket.
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Last edited by Phil G4SPZ; 26th Feb 2018 at 10:51 pm. Reason: Addendum
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Old 27th Feb 2018, 12:20 am   #10
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Default Re: Robin M220 Miliohmeter?

No, they're not Kelvin clamps.

Just follow the diagram near the terminals. Stick the black clips on the component leads near the body, and the red clips further out.

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Old 27th Feb 2018, 3:35 am   #11
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Default Re: Robin M220 Miliohmeter?

Hi All,
these is a Kelvin contacted low-resistance meter...
You see, the red lines are the forces, send current out "in such resistance", and you can/will sense with the black lines "over them" an voltage drop: its Kelvins idea.
Other question is the realisation their "Kelvin-clips", wich are in my opinion not original...
Good example is Meggers Kelvin Test lead to...
Best rgds, Karl
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Old 2nd Mar 2018, 8:53 pm   #12
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Default Re: Robin M220 Miliohmeter?

Quote:
Originally Posted by Radio Wrangler View Post
No, they're not Kelvin clamps.
Probably changed when the original Kelvin clips got damaged and someone saw the cost of a new set in a catalogue.
Cheap Kelvin leads can be found from China, no idea if they are any good though, I may buy some when I get round to repairing the meters I have.

David
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Old 4th Mar 2018, 11:45 am   #13
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Default Re: Robin M220 Miliohmeter?

The Kelvin clips used on Solartron's 7081 would be fairly easy to duplicate- they consist of two suitably shaped small metal bars which pivot round a short cotton reel shaped insulator with the closing force provided by a rubber sleeve around the business ends of the bars. Force to one bar, sense to the other, wires routed via drillings in the insulator but could just go direct to the bars if neatness not a problem. See pic of something similar.
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Old 4th Mar 2018, 12:07 pm   #14
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Default Re: Robin M220 Miliohmeter?

I purchased two sets from china (identical to images below) and they appear to be of good quality.

Difficult to fully assess them electrically (would need a set of high quality ones to compare against I guess) but well made.

I had to modify my Wayne Kerr B605 inputs though, as they used "triaxial" bnc coax inputs, which are VERY expensive to find plugs for. Best price I saw was over £100 each, plus the nightmare of fitting them to triaxial coax cable!!

Anyway worked very well, and now using another set on a HP Bench meter which uses banana plug inputs.
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Old 4th Mar 2018, 12:18 pm   #15
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Default Re: Robin M220 Miliohmeter?

Quote:
Originally Posted by Radio Wrangler View Post
No, they're not Kelvin clamps.

Just follow the diagram near the terminals. Stick the black clips on the component leads near the body, and the red clips further out.

David
David

I do agree that they are not "Kelvin clips" as such, but they are acting in the same mode.

Effectively they are shorted as pairs, when clipped to the component at each end, and so acting electrically in the same way as kelvin clips!


Admittedly the drawing on the meter terminals does not indicate how the leads are driven/read.
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Old 4th Mar 2018, 1:55 pm   #16
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Default Re: Robin M220 Miliohmeter?

Looks like you can buy the Kelvin clips on their own if you want to make custom leads or replace damaged clips, a lot of them look very similar to the genuine ones that cost a lot more.
Also some of the original clips are rewire-able. But I can't find any that are like the older ones at work with sockets for banana plug leads, those have a paxolin/tufnol insulator.

David

Edit, found a picture, used with Cropico meters?
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Old 6th Mar 2018, 10:57 am   #17
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Default Re: Robin M220 Miliohmeter?

Example of 2 or 4 wire measurement

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Old 9th Oct 2019, 9:37 pm   #18
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Default Robin M220 Miliohmeter update.

I posted some time ago about this meter, and trying to work out how it worked. Spent a little time on it today, and figured out the basic reason for my confusion: it has a sticky meter movement so the needle did different things according to how it felt.

Looking inside it must be fairly low volume. Two single sided PCBs. I need to also check and clean the PCB switches. There's a couple of Tant caps to check out too. But as I say, the first thing has to be figuring out why the movement mechanically sticks down around the zero point.

Last edited by Station X; 9th Oct 2019 at 9:41 pm. Reason: Threads merged.
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Old 10th Oct 2019, 8:49 am   #19
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Default Re: Robin M220 Miliohmeter update.

Do check the caps first - I had a fet multimeter whose needle appeared to stick at one point all the time but would go back to zero when tapped. However when I'd replaced a cap (small electrolytic in this case) it was fine. Presume the cap was leaky or had a crack and tapping let it make contact again.
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Old 12th Oct 2019, 9:06 am   #20
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Default Re: Robin M220 Miliohmeter update.

Quote:
Originally Posted by avocollector View Post
Do check the caps first - I had a fet multimeter whose needle appeared to stick at one point all the time but would go back to zero when tapped. However when I'd replaced a cap (small electrolytic in this case) it was fine. Presume the cap was leaky or had a crack and tapping let it make contact again.
Good tip, but this is physically sticking. It does it with the power off and batteries removed. I can feel the sticky point. Using the mechanical zero set mechanism, I can change it and the needle doesn't move at all until it is given a hand. Then it will settle back to its new zero position.
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