Zero Ohm DIY 4-wire standard

[VintageNut]

This is extremely cool. Nice work! My guess is that no more than microvolts will develop across the driven pair of contacts. The resistance there is probably micro-ohms. That would be extremely useful information as to what exactly is the resistance between vertices of the device. Another exercise!

Something I had not thought of that your FEA points out is that while the potential diff may be 0, the value of each point is somewhere between the two driven vertices. Very interesting info!

You can download a free and surprisingly capable FEA program called FEMM. I usually use it for magnetics but it will also do current flow problems. I've never done it before, so may have some horrible error, but here's the result setting the upper hole at 1V and the lower left hole at 0V. You can see the other two holes do come close to equal potentials.

[Assafl]

What about the sense terminals? Don't they take some (tiny) bias current where the sense terminals are?

[VintageNut]

The sense portion of a 4-wire device consists of a low (hopefully) impedance source and a high impedance voltmeter.

For the tetrajunction, the device sense terminal are some micro-ohms or less and the voltmeter is 1 megohm or greater.

How can adding a parallel resistance help this in any way?

The tetrajunction is not a resistor. It is a 4-terminal device that changes current on two terminals into voltage on the other two terminals. You have to look at it as a trans-resistance amplifier with a gain of something on the order of 10^-9. The only way to improve it is to change the gain. That is the question. How to adjust the gain?

[Conrad Hoffman]

I'm still digesting this, and Thanksgiving's turkey, but it seems the tetrajunction is mathematically elegant, and the choice of terminals doesn't matter, but if all you want is a zero ohm standard for a 4-terminal meter, a cross device would be fine. In fact, it seems like a couple bars, connected by a small wire to the midpoints might even work, IOW, a cross with a thinned section on opposite legs. It's just a matter of making sure the sense lead voltage is identical, not much different than the thought process for a single point ground system in an amplifier.

Also thinking of practical implementation, you probably shouldn't screw terminals to a tetrajunction with ordinary nuts, as the contact points are uncertain. I'd make up some old fashioned machine tool nuts, with a perfectly circular area turned on one end. That way you preserve symmetry of the connection point. Or use some small diameter tight fitting flat washers like split rings without the split. It might be even better to press fit some small copper rods in the holes, and solder to the top of those. It's all about preserving electro-mechanical symmetry.

[3roomlab]

i might convince myself to do a PCB version (triangle version) to replace my shorting plug, no wait ... i DO NOT HAVE A REAL SHORTING PLUG  . so in reality, this could be a zero ohm and a shorting plug as well 

zlymex did it hand soldered, so i figured it shouldnt be worse off with some precise mucking around in eagle? using a "fake" pad as a ruler, adjusting for 1000mil or so, it can be used as "arms" referenced from zero/zero XY to mark the tips of the equilateral triangle, the angle can be set very easy. then fine tuned using 0.2mil grid, actual connection pads can be fitted at the 3 tips and the center. then the entire setup shifted into a PCB "blank frame". the result in mycase is somewhat 999.978mil each way, but i think the accuracy of measurement is subject to +/- 0.2mil, it is the smallest grid i could see.

assuming PCBway, seeed, OSHpark, etc all have 1:1 perfect ratios in XY dimensions, things should turn out quite well i guess? the only variances would be the leads connected onto the pads, should they be all strictly of the same length?

then to take things further, a single unit is not just 1 PCB by itself, but could be a sandwich of 2-10 pieces, @ 0.6mm FR4, 10 pc = total of 20x 1oz copper (6mm++). soldering could be a huge challenge, since there cannot be any thermals to preserve the continuous copper pour.

this is getting interesting, vs an actual expensive professional calibrated tetra-junction, esp for peeps who do not need extensive calibrated zero ohms. short thick lead soldered on with Z-serated 4mm banana = it could plug into DMM for shorting or other banana could plug into it.

is this too good to be true? lol

[SeanB]

Would not solder, you then have another uncertain thermoelectric junction. Rather use a large copper block and machine the junction out of it, and also machine the pins ( with a interference fit in the holes) out of the same block. You turn 90% of the bar into scrap shavings, but it will all be the same composition of Beryllium copper throughout so almost no thermal effects.

Know a toolmaker who started with a 40kg block of brass, and the finished bush out of it was under 1kg. Took him a week to machine that out of it.

[Conrad Hoffman]

How about a round or triangular copper piece with holes drilled "banana sized", then just plug in your low thermal emf plugs? I can't see any advantage between round or triangular.

[3roomlab]

i know machined blocks are great, it be nice to hit those nano numbers, but the interesting bit here after seeing the simulation is that, it may not really need a huge block of copper to get those numbers (maybe more layer? i dont know), and it could be done just by accurate PCB fab. and correction could be done by shifting the leads on the solder pad (if allowance is there)
i think problem of thermal EMF already have many ways to mitigate its effects, mainly by thermal insulation to force parts to reach equilibrium quickly, so i wont worry about that. what is out of reach of many hobbyists is ability to access milling equipment and do precision milling, which in this case is mitigated by just using good PCB layout. it is really killing 2 stones with 1 tetra (a short and a zero ohm). and 1 PCB is about 13 grams, but of course, nobody have anything in tetra PCB format to say it works yet 

update

I can't see any advantage between round or triangular.

i was itching to find out too.
here are 2 similar layout. in both cases, i shifted 1 of the nodes off by 0.02 inch to see how much of the field lines are affected.
im not sure how much of this is real in actual, but it seems by suffering a shift off a main axis, the symmetry will suffer badly. but in my case of the simulation scale (=1uV full scale) badly = 20-30nV or so it seems (99 lines, change of about 2-3 lines). so if we look at zlymex's experiment, and compare with this, with a "fabrication" accuracy better than 0.02 inch, anyone should be able to attain very good accuracy effortlessly in theory by placing the circuit entry points smack on the dot.

so i guess this in theory shows the amount of deviation if the circuit pads are in-accurately placed.
A1-2, vertical shift (WRT +ve)
A3-4, vertical shift(WRT +ve)
A5 and 6 horizontal shift

added a7 = crude representation of normal shorting plug. sense side is out of symmetry by 9 -10 lines (90-100nV per uV)

[Conrad Hoffman]

I suspect the symmetry of the part is all that matters, not the resistance. It could be a block of copper or a thin PCB trace, so long as the connection points are reasonably accurate. Armchair thought (my best kind) just take a piece of copper-clad and use a circle cutter in the drill press to create a circle of copper. Don't even cut all the way through. Now lay out the holes and solder in some wires. The resistance of the solder is higher than the copper so even the joints don't need to be identical. If you can measure it well enough, the copper can be corrected a bit with a Dremel or similar to find those last attoohms!

[Gyro]

Just thinking, do these standards suffer from the Hall effect? I know it's not a very strong phenomenon in copper, but if you're looking down at such low levels then might throw the zero reading off. I don't know if the Earth's magnetic field would be enough though.

[Conrad Hoffman]

Ha! Any kind of DIY Hall Effect demo would be really cool. I tried a quick PCB with the circle cutter on the drill press and learned a couple things. It can work well, but the wire attachment is an issue. The solder joints turn out to be critical, in fact solder can be used to fine tune the thing. That explains the lumps on my quick proto. I got about 5 uV errors in any direction with 5 amps of current. Better, I think, to get everything as accurate as possible and use identical screws and washers. I did learn that the zero of my HP3478A is off by about 0.002 ohms. Fun stuff for a Sunday afternoon.

[3roomlab]

i try to simulate a blob of solder (pink corner). could it be the 5uV is more of PSU noise? than a difference in the tetrajunction?

@ conrad *edit : lets see how accurate is this FEMM. i try to guesstimate how much noise is coming out of your PSU, and estimate = 33mV. is there anyway to see if this value sticks on your power supply setup? the value is obtained from scaling up the difference in symmetry.

[Kleinstein]

Measuring the 4 wire resistance is not really sensitive to the quality of the current source or current source noise. Noise in the current would enter linear to the resistance - so only a problem if you need the resistance to 3 or more significant digits - not if it is to approach zero. However thermal effects at plugs could be a problem. Also induction due to moving cables are a possible problem.

The Hall effect is quite small, I doubt on would notice is. I have once done a test / demo for the hall effect in copper. It was quite hard to measure. I used a 17 µm copper PCB (thus half the normal thickness), about 5-10 A of current and a Nd magnet (though small) directly on top of the PCB. Even with a layout more suitable for hall effect, the voltage was really small, hardly a few µV.

For the zero the PCB is not that well suited anyway, as one usually also wants a low 2 wire resistance and a higher sheet resistance increases the requirements for symmetry and the sensitivity to solder blobs.

Having directly holes in the sheet is also not a good idea, as the contact around may not be uniform. The same is true for threaded connections. A thin layer of solder is hard to beat for a good stable contact. No thermal EMF as long as temperature in uniform. The thick copper helps with this. One needs the thin part (wires) to make sure the current enters well defined, independent for a not so well defined plug.

AFAIK the resistance is symmetric to interchanging current and voltage contacts: thus the same value for current between contacts 1 and 2 and current form 3 to 4. So there are only 3 independent resistances to tune. This may help to find good tuning points and simplifies the measurements.

[Conrad Hoffman]

That all makes sense. I did the PCB version using no precision tools, though I have a full shop, just to see what could be accomplished with simple things like a ruler and scribe. I measured the error using a Kepco supply in constant current mode, and a Fluke 845 null meter. The noise level is low, or at least the meter averages a lot of it out. I briefly applied current and watched meter deflection, then made sure it returned to near zero. Based on the comments, a slice off a copper rod, or heavy sheet, precisely drilled, with press-in studs seems like the most direct way to extreme accuracy, though the PCB version was plenty good enough to check my low precision meter.

[VintageNut]

I tried a cross using ROMEX solid core bare copper wire before I fabricated the solid copper disc version. The cross was no better than mico-ohms. Its similar goodness to a Fluke and Keithley PCB zero plug.

When you measure a good tetrajunction, reverse current and then average the two voltage measurements. This should zero out the micovolt offsets that are stable. Otherwise you are just measuring microvolt circuit junctions that are unavoidable.

I don't think that a PCB version will work if you plan to just plug it into a DMM. No DMM has enough current to drive it...and....a tetrajunction needs to be mounted in a magnetic and electrically shielded box. Normal air circulation in a room will prevent the junction microvolts from becoming stable enough to remove via offset compensated current reversal.

[3roomlab]

comparison of excess in material around the 4 points. more material seem to suggest a spread of the density lines around the sense pin making for more/less allowance in construction error (or maybe im interpreting it in reverse? less interference lines? or more?). so bulk/solid copper = more advantage than thin PCB, but it would also suggest mechanical positioning need to be better than PCB fab accuracy in some way.
i do suppose with solder mask, it is possible to control the uniformity of the solder interference with the copper, but then the thickness of solder = uncontrolled (which could become a form of fine adjustment?)
maybe instead of screw bolts with threads, solid pins put in place can be smooth pins set in by friction fit? like a tapered peg in a fitting hole?
if we are looking at this for what it is, then wouldnt the perfect shape for this "device" be a solid sphere?

[acbern]

Guys, 4 short wires drilled together and soldered/crimped well in a single spot will be superior to any triangle or so. The triangle solves a mechanical problem (e.g. banana plugs required in certain positions such as e.g. with the SR1010). If you just want to have 0 ohms, then there is a easier and better way to do it. But from a ohms-nut perspecitive using all kinds of geometries makes perfect sense of course...

[quarks]

4 short wires drilled together and soldered/crimped well in a single spot will be superior to any triangle...

before I build my "Zero Ohm Standard" I have experimented with a crimped wire solution as described
if I remember correctly the "triangle" showed more than an order of magnitude lower values

[Conrad Hoffman]

Everything depends on what you intend to do with this device. I'm not sure it's been explicitly stated, but a "zero" ohm standard built this way may or may not have low actual resistance. Built with something other than copper, it could be quite high and still work. It just fools a 4-terminal ohm meter into thinking it has zero resistance by putting the sense wires on equi-potential points. It simulates a far lower resistance than you can achieve with real physical materials. If you want actual low resistance yes, plant some stout terminals in a small lump of copper, shape not relevant. The junction is a way to insure that two circuits don't influence each other, but have a common connection, as most meters require. The tetrajunction is elegant in that any terminals can be chosen, but there are various ways to accomplish the same thing. The thicker and lower the resistance of the material used, the less sensitive construction will be. The PCB version is certainly overkill for my 5.5 digit meter that resolves 0.1 milliohm. OTOH, if you've got a more serious low ohms meter, it might not be, nor will it be sufficient for the connections in a low value transfer standard. Building one where the connections lie on perfectly equi-potential points is just a matter of mechanical precision. Press-fitted terminals seem like the best way to go, but other ways will certainly work.

[VintageNut]

In my first post of this thread, the sphere is figure 2(c). I discarded the sphere only because of the practical limitations of my machine shop capabilities. The flat massive disc is easy to acquire and not too difficult to drill. I only broke three drill bits before I resorted to Google to find out how to drill copper. Tiny bits at a time and slow. Nobody sells a massive copper triangle.

comparison of excess in material around the 4 points. more material seem to suggest a spread of the density lines around the sense pin making for more/less allowance in construction error (or maybe im interpreting it in reverse? less interference lines? or more?). so bulk/solid copper = more advantage than thin PCB, but it would also suggest mechanical positioning need to be better than PCB fab accuracy in some way.
i do suppose with solder mask, it is possible to control the uniformity of the solder interference with the copper, but then the thickness of solder = uncontrolled (which could become a form of fine adjustment?)
maybe instead of screw bolts with threads, solid pins put in place can be smooth pins set in by friction fit? like a tapered peg in a fitting hole?
if we are looking at this for what it is, then wouldnt the perfect shape for this "device" be a solid sphere?

[VintageNut]

Hello Conrad

DMMs have a limit for the potential between FORCE HI and SENSE HI. This precludes anything with appreciable resistance difference between the two HI terminals. The same goes for the LO terminals. You cannot have an appreciable resistance between these terminals.

Everything depends on what you intend to do with this device. I'm not sure it's been explicitly stated, but a "zero" ohm standard built this way may or may not have low actual resistance. Built with something other than copper, it could be quite high and still work. It just fools a 4-terminal ohm meter into thinking it has zero resistance by putting the sense wires on equi-potential points. The junction is a way to insure that two circuits don't influence each other, but have a common connection, as most meters require. The tetrajunction is elegant in that any terminals can be chosen, but there are various ways to accomplish the same thing. The thicker and lower the resistance of the material used, the less sensitive construction will be. The PCB version is certainly overkill for my 5.5 digit meter that resolves 0.1 milliohm. OTOH, if you've got a more serious low ohms meter, it might not be, nor will it be sufficient for the connections in a low value transfer standard. Building one where the connections lie on perfectly equi-potential points is just a matter of mechanical precision. Press-fitted terminals seem like the best way to go, but other ways will certainly work.

[VintageNut]

Please post pictures and measurements. Everybody can learn from your efforts. It would be good to have some documentation in this thread for different devices that are low ohms.

I will post pictures and measurements of my crossed wires devices. This week I am traveling for work and it may have to wait for next week.

4 short wires drilled together and soldered/crimped well in a single spot will be superior to any triangle...

before I build my "Zero Ohm Standard" I have experimented with a crimped wire solution as described
if I remember correctly the "triangle" showed more than an order of magnitude lower values

[quarks]

Please post pictures and measurements. Everybody can learn from your efforts. It would be good to have some documentation in this thread for different devices that are low ohms.

it is already there, if you follow the link in my first reply to this topic
https://www.eevblog.com/forum/metrology/zero-ohm-diy-4-wire-standard/msg1064308/#msg1064308

what I did not document was the 4 wires experiment, as it is not well made and I do not like it, but att. is a pic.
I used speaker cable 4 mm² all 4 with equal length and crimped with a 16 mm² ferrule and soldered on ring lugs
then screwed Pomona 3770 bindig Posts

meassured/calculated value of this was around 10µOhm
I compared this with identical setup
to my final "Zero Ohm Standard" it was clearly well below 1µOhm

[Conrad Hoffman]

DMMs have a limit for the potential between FORCE HI and SENSE HI. This precludes anything with appreciable resistance difference between the two HI terminals. The same goes for the LO terminals. You cannot have an appreciable resistance between these terminals.

Certainly true, and the question is how much? This is something I've never seen in a meter data sheet unless I don't know what I'm looking for. My WAG is that it might be quite different for different meters, unless everybody happened to settle on the same basic circuit.

[tszaboo]

DMMs have a limit for the potential between FORCE HI and SENSE HI. This precludes anything with appreciable resistance difference between the two HI terminals. The same goes for the LO terminals. You cannot have an appreciable resistance between these terminals.

Certainly true, and the question is how much? This is something I've never seen in a meter data sheet unless I don't know what I'm looking for. My WAG is that it might be quite different for different meters, unless everybody happened to settle on the same basic circuit.

Example:
http://literature.cdn.keysight.com/litweb/pdf/03458-90014.pdf
Appendix A, page 286.

I spare you the looking around. It is +/- 200V.
Seriously, what is the deal with this "tetrajunction"? Is it really hard to make 0 ohm on a multimeter? You need 2 shorting bars, three if you must be proper, and a Pomona shorting bar is 5 EUR on Farnell. Can someone tell me what is the deal with it? Is this being used in turboencabulators?