Author Topic: Zero Ohm DIY 4-wire standard  (Read 37583 times)

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Offline 3roomlab

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Re: Zero Ohm DIY 4-wire standard
« Reply #25 on: November 25, 2016, 04:27:14 pm »
How do you balance the round tetrajunction (how do you decide where to file the piece)?

I ask because of all the shapes, the tetrahedral (2b) seems to me to be the easiest to work out the balance (in theory - mainly since a tetrahedral is a superposition of six Wheatstone bridges).

what you said gave me an interesting idea. the "balance" then turns out to be a physical puzzle to be solved, by which all 4 points must be connected with the same "resistance" no matter which point it intend to "go" to, and it can only be done by emphasizing/enforcing the "diagonal" length. then the perimeter length have to be "bent" but still carry the same length, see pic. do i make sense? now then, all 6 connections/paths have the same "resistance" and "distance". i hope this idea is not 1 of the patents anywhere?

ignore the scale in mm, then expand the diagonal/bends to achieve the 0.75 inch 4mm banana port?
BUT, now that this is all in equilateral "distance/resistance" WRT banana port wise, the insides of the DMM connection wires are not, or are they ? oh dear !
« Last Edit: November 25, 2016, 04:36:56 pm by 3roomlab »
 

Offline VintageNutTopic starter

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Re: Zero Ohm DIY 4-wire standard
« Reply #26 on: November 25, 2016, 04:56:48 pm »
I do not follow what you are explaining. Are you using the triangle shaped device as an example?

There are three unique configurations to force current and measure voltage.

Call them configurations 1,2 and 3. You characterize each configuration. Then you sort the resistances from low to high. The highest resistance is the worst one and is the limiting factor. You cannot make the highest resistance lower. You can only make the lower resistance configurations higher. You adjust the lower two configurations to be the same as the highest one.

This is my educated guess for the triangle shaped device. For the round device, I am not certain if filing one of the 120 degree arcs of the circle makes the resistance lower or higher for that arc.

The Ohm Labs device in my office/lab is specified at no more than 50 nano-ohms. The measured value is 4 nano-ohms. So, when you are adjusting, you are changing the value by a very tiny and very difficult to measure amount.


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Offline 3roomlab

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Re: Zero Ohm DIY 4-wire standard
« Reply #27 on: November 25, 2016, 05:17:05 pm »
im visualizing it as point to point (the 4 points are imaginery ABCD spots). i think it this way, is because i dont understand the triangular theory behind it. i assume in my own way the 4 points to be connected need to see each other as zero ohms (or shortest path?)  in which ever path they need to take, and each point has only 3 ways to reach the other 3 points. in my mind it is as if, all the 4 points need to exist in the same spot (= travel zero distance to reach each other), which is impossible.

this reminds me of ohms per square. odd corner to even corner resistance = diagonal tip to tip resistance?
« Last Edit: November 25, 2016, 05:22:13 pm by 3roomlab »
 

Offline VintageNutTopic starter

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Re: Zero Ohm DIY 4-wire standard
« Reply #28 on: November 25, 2016, 05:24:11 pm »
I think of the device this way.

The 4 points are labeled 1,2,3,4.

Let's start by forcing current through points 1-2 and measuring voltage across 3-4.

If you draw a straight line from 1 to 2 and another straight line from 3 to 4 you will see that the lines are perpendicular to each other and the lines do not intersect. This is how very little voltage is induced in line segment 3-4 when a current is forced through 1-2.

The other unique combinations are 1-3, 2-4 and 1-4, 2-3.

Perform the exercise of drawing the current path and you will see that the measured voltage is in an area where no electron should be flowing.
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Offline 3roomlab

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Re: Zero Ohm DIY 4-wire standard
« Reply #29 on: November 25, 2016, 06:07:00 pm »
hmmm yes i think i get it

in this pic, again i see the "ohm/square" inside, and each corner connected in a uniform way, and the other pair not "seeing" the induced voltage.

in my case, i think i have flatten the 4 point triangle into 6 wires, so the wire frame seem to have a "bypass" for 3-4 not being able to see induced 1-2 voltage (or rather sitting in the middle with "no" voltage). being a wireframe, does it subject itself to being sensitive to the perpendicular induced current? since 3-4 are equal path/distance and "perpendicular" in arrangement, a wireframe should be similar like a solid copper piece? (but with much higher resistance)
« Last Edit: November 25, 2016, 06:32:32 pm by 3roomlab »
 

Offline VintageNutTopic starter

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Re: Zero Ohm DIY 4-wire standard
« Reply #30 on: November 25, 2016, 07:00:56 pm »
The device is excited at the vertices. Since the two line segments share no copper, the only method to induce a voltage into line segment 3-4 by forcing a current into line segment 1-2 is via magnet flux lines around line segment 1-2 cutting line segment 3-4. The beauty of this geometric arrangement is that there is only one tiny flux line of 1-2 that cuts through 3-4 and the flux line is not in a direction that will induce a voltage between the points 1-2. Rather, the flux line will cause a voltage to appear across the thickness of line segment 3-4. Quite a clever device indeed.

I am not sure what your drawn ohm-square represents.

If are using segments 1-2 and 3-4, cut away all of the material except the two line segments. Then start adding material nearby to these two line segments. Mostly likely, the only electrons flowing are very near to these two line segments. So, the material very far away from these line segments probably is not used for this configuration.

I do not think that you can analyze this device all-together. You have to analyze it as three distinctly different  devices, each of which must be evaluated by itself.
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Offline 3roomlab

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Re: Zero Ohm DIY 4-wire standard
« Reply #31 on: November 25, 2016, 08:23:24 pm »
ohms/square? im confusing myself terribly about how to calculate the resistance  :-//

so unlike the tetrahedral, the flatten version forces 2 points to appear to be in the mid point of the other 2 if current passes.
although worlds apart, i think it may have some advantage over the normal short plug.

how does the keithley short plug achieve 0.5u-ohm? that trace is small, could it be 5000u-ohm? a 220mil x 1060mil x 1 oz @ 35C = 2500 u-ohm according to saturn PCB calculator.
 

Offline Assafl

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Re: Zero Ohm DIY 4-wire standard
« Reply #32 on: November 25, 2016, 08:32:51 pm »
In the tetrahedral scenario what you basically have is a Wheatstone bridge that has 4 resistors, and a resistor across the Up/Down (UD) and another resistor across left/right (LR). The shape matters little.

As usual, if all bridge resistors (ignore the top/down TD and left/right LR resistors) are equal - and you apply a voltage across the top/down there will not be a voltage across the LR resistor since the bridge is balanced.

Measure all options 12, 13, 23... Figure out which has the lowest -imbalance. The Top/Down resistor is the culprit for the imbalance in the other options. Fix it, redo the testing.

I don't believe that mOhms to nanoOhms can be done by filing the junction mOhms yes. uOhms - Ehhh. nOhms? No way...

Maybe they drill fine holes, tap them, and insert a screw into the tetrajunction. By adjusting the screw "depth" the resistivity will change ever so slightly. The smaller the screw, the finer the resolution... Then goop it as a fancy "do not touch" warning... 

Or parallel a few gigohm resistors in parallel to your thick tetrajunction arms.
(that would be amazing - a 0ohm in parallel with a glass embedded mega to multi GOhm resistor - talk about wasting money...).

Hmmmm - this process should also work for your tetrajunction.

So let's theorize a process:

1. Figure out which pair has the highest imbalance (say force 1-2 measure 3-4 results in the highest NV reading).

2. So your highest imbalance is either 1-3 2-3 1-4 or 2-4. (the imbalance is always in the bridge - not the resistors across TD or LR).

3. You can deduce the pair by the polarity. So if the 3 is positive (to 4) the offending "resistors" are either 1-3 or 2-4. Feed 1-3 (and measure 2-4) and Feed 2-4 (and measure 1-3). Whomever has the lowest imbalance - assuming feeding 1-3 results in a lower voltage across 2-4 - means resistor 1-3 needs to be lowered somewhat. Solder a resistor across 1-3. Repeat.

(Note: I am a bit on the fence about how to relate the imbalance nano volts to gigohm resistors, though.... Should be simple wheatstone bridge analysis, but having a superposition of 6 of these makes my head spin. The symmetry reeks of a simpler way to solve the equations...)

4. Perhaps one doesn't need a trick. Solder a resistor across 1-3. Measure 1-3 and 2-4 again. Decrease the value of the resistor until measurements 1-3 and 2-4 balance out. Repeat the entire exercise. Do this until all 4 balance out.

Note: The resistors need to be soldered where the wires connect to the tetrajunction. Not to the tetrajunction itself, though, as heating it will change its properties. A solder lug under the screw perhaps?

 

Offline Assafl

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Re: Zero Ohm DIY 4-wire standard
« Reply #33 on: November 25, 2016, 08:41:47 pm »
ohms/square? im confusing myself terribly about how to calculate the resistance  :-//

so unlike the tetrahedral, the flatten version forces 2 points to appear to be in the mid point of the other 2 if current passes.
although worlds apart, i think it may have some advantage over the normal short plug.

how does the keithley short plug achieve 0.5u-ohm? that trace is small, could it be 5000u-ohm? a 220mil x 1060mil x 1 oz @ 35C = 2500 u-ohm according to saturn PCB calculator.

By the nature of the symmetry all the shapes in the diagrams are the same "shorted bridge" as the tetrahedral. If they were not - the voltages would not zero out. However, the mechanics of achieving the symmetry change between the different geometrical shapes.

I like it. It can make a pleasant party quiz entry: What has a resistance for 2 wire measurements - but a real zero for 4 wire measurements? A superconductor? Nope - it is zero for both. Yup - a tetrajunction!
 

Offline VintageNutTopic starter

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Re: Zero Ohm DIY 4-wire standard
« Reply #34 on: November 25, 2016, 09:31:04 pm »
In the tetrahedral scenario what you basically have is a Wheatstone bridge that has 4 resistors, and a resistor across the Up/Down (UD) and another resistor across left/right (LR). The shape matters little.

As usual, if all bridge resistors (ignore the top/down TD and left/right LR resistors) are equal - and you apply a voltage across the top/down there will not be a voltage across the LR resistor since the bridge is balanced.

Measure all options 12, 13, 23... Figure out which has the lowest -imbalance. The Top/Down resistor is the culprit for the imbalance in the other options. Fix it, redo the testing.

I don't believe that mOhms to nanoOhms can be done by filing the junction mOhms yes. uOhms - Ehhh. nOhms? No way...

Maybe they drill fine holes, tap them, and insert a screw into the tetrajunction. By adjusting the screw "depth" the resistivity will change ever so slightly. The smaller the screw, the finer the resolution... Then goop it as a fancy "do not touch" warning... 

Or parallel a few gigohm resistors in parallel to your thick tetrajunction arms.
(that would be amazing - a 0ohm in parallel with a glass embedded mega to multi GOhm resistor - talk about wasting money...).

Hmmmm - this process should also work for your tetrajunction.

So let's theorize a process:

1. Figure out which pair has the highest imbalance (say force 1-2 measure 3-4 results in the highest NV reading).

2. So your highest imbalance is either 1-3 2-3 1-4 or 2-4. (the imbalance is always in the bridge - not the resistors across TD or LR).

3. You can deduce the pair by the polarity. So if the 3 is positive (to 4) the offending "resistors" are either 1-3 or 2-4. Feed 1-3 (and measure 2-4) and Feed 2-4 (and measure 1-3). Whomever has the lowest imbalance - assuming feeding 1-3 results in a lower voltage across 2-4 - means resistor 1-3 needs to be lowered somewhat. Solder a resistor across 1-3. Repeat.

(Note: I am a bit on the fence about how to relate the imbalance nano volts to gigohm resistors, though.... Should be simple wheatstone bridge analysis, but having a superposition of 6 of these makes my head spin. The symmetry reeks of a simpler way to solve the equations...)

4. Perhaps one doesn't need a trick. Solder a resistor across 1-3. Measure 1-3 and 2-4 again. Decrease the value of the resistor until measurements 1-3 and 2-4 balance out. Repeat the entire exercise. Do this until all 4 balance out.

Note: The resistors need to be soldered where the wires connect to the tetrajunction. Not to the tetrajunction itself, though, as heating it will change its properties. A solder lug under the screw perhaps?

In my opinion you cannot look the device as a resistance bridge. You have to look at as a collection junctions. Not semiconductior junctions but conductor junctions. The resistance of the junction is not zero but the voltage across the junction is nearly zero when current normal to the junction is forced.

These things are used to connect arrays of resistors together without adding significant resistance to the individual resistors.

I spoke with the owner of the company and yes a file is used to modify the contour of the device to change the behavior of one of the configurations. I discussed the device and how it is manufactured and tested when I was pacing the order. It is tested at the factory by forcing 5A and measuring the nanovots drop.My 4 nano-ohm device develops 20 nV when 5A is forced through it. They manually reverse the power supply wiring to achieve an offset-compensated ohms measurement.
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Offline Assafl

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Re: Zero Ohm DIY 4-wire standard
« Reply #35 on: November 25, 2016, 10:18:20 pm »
Is there a lack of equivalency between all the forms of tetrajunctions in the diagrams as far as DC network analysis is concerned? If so why would that be?

To me they are all a blob with 4 contacts that happen to be perfectly symmetrical (perhaps with the exception of 2a).

A tetrahedral clearly looks like a shorted bridge. The zero ohms stems not from any attribute of the copper, but from the perfectness of the symmetry.

I wonder how much filing they need to do to get to 20nV...

 

Offline VintageNutTopic starter

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Re: Zero Ohm DIY 4-wire standard
« Reply #36 on: November 25, 2016, 10:35:11 pm »
A network is a flat 2-D arrangement of elements that each have a well-known V-I behavior. When a current flows through an element, a corresponding voltage is developed across that element according to its V-I behavior.

In the tetrajunction, the current flows through element 1-2 while the voltage is measured across element 3-4.

Its more like a transadmittance amplifier than a resistor network. Actually three different transadmittance amplifiers in a 4-terminal package.

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Offline Assafl

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Re: Zero Ohm DIY 4-wire standard
« Reply #37 on: November 26, 2016, 07:14:00 am »
I think it it transimpedance if your ration is voltage output to current input.

The same can be said of a current driven Wheatstone bridge. Now you could argue that most Wheatstone bridges are voltage driven - true - but one could use Thevenin's theorem and substitute the current source with a voltage and resistor...

Be it lumped or not - all Thevenin requires is that it is a linear circuit. 
 

Offline VintageNutTopic starter

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Re: Zero Ohm DIY 4-wire standard
« Reply #38 on: November 26, 2016, 02:25:33 pm »
Sorry for incorrect term. You are correct. Transimpedance.

For voltage drive, the manufacturer of the commercial tetrajunction may be using a power supply and relying on the current limit for a constant current.

Since I own some SMUs, I prefer to use a SMU.

Here is the difficulty that I see trying to model this device using a bridge, etc. The device has 4-terminals while it has 6 distinctly unique current paths. If the terminals are labeled 1, 2, 3, 4, the unique current paths are

1-2
1-3
1-4
2-3
3-4
2-4

So, you could model as a bridge and add two resistors where the voltage source and galvanometer would normally go. That would probably be a good starting point.

As for finding the offending place to file material away by reasoning, that is giving me a headache. If I were going to try to modify my DIY device to balance it, I would construct more than one and experiment to find a reproducible procedure to balance the device.

This device is extremely difficult to measure. The error sources of induced AC and DC microvolts  are over 100X larger than the voltage being measured. The thermal drifts caused by touching the device and the wires to connect it require hours to settle out. Not seconds. Not minutes. Hours.

So, You decide to file some material off. You disconnect the circuit, take the device in hand and file some material. You reconnect the device and try to measure it. Thermal equilibrium is hours away and it will probably be the next work day before the device is stable enough to measure.

You cannot use any modern digital DMM style equipment to measure this voltage. Modern nanovoltmeters cannot. The only modern commercial instrument that can measure this voltage is the obsolete Keithley 1801 married to a 2001 or 2002 DMM.

The clever thing to do would be to construct a mounting apparatus that would allow filing the device without touching it. The mounting apparatus would have to be enclosed in an electrically and magnetically shielded enclosure that is able to be easily opened and closed for repeated adjustment of the device. Another low ohms project!

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Offline Conrad Hoffman

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Re: Zero Ohm DIY 4-wire standard
« Reply #39 on: November 26, 2016, 02:29:55 pm »
Long ago the late Bob Pease described some sort of resistive paper that could be cut out and used to analyze this sort of problem. Voltages could be applied, then the paper probed with a conventional meter. I suppose one could also stamp the shape out of resistive foil, manganin or something, and probe it. Can you tell math isn't my best subject?
 

Offline VintageNutTopic starter

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Re: Zero Ohm DIY 4-wire standard
« Reply #40 on: November 26, 2016, 03:27:21 pm »
Long ago the late Bob Pease described some sort of resistive paper that could be cut out and used to analyze this sort of problem. Voltages could be applied, then the paper probed with a conventional meter. I suppose one could also stamp the shape out of resistive foil, manganin or something, and probe it. Can you tell math isn't my best subject?

Interesting. It is worth considering.
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Offline guenthert

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Re: Zero Ohm DIY 4-wire standard
« Reply #41 on: November 26, 2016, 05:41:29 pm »
ohms/square? im confusing myself terribly about how to calculate the resistance  :-//

so unlike the tetrahedral, the flatten version forces 2 points to appear to be in the mid point of the other 2 if current passes.
although worlds apart, i think it may have some advantage over the normal short plug.

how does the keithley short plug achieve 0.5u-ohm? that trace is small, could it be 5000u-ohm? a 220mil x 1060mil x 1 oz @ 35C = 2500 u-ohm according to saturn PCB calculator.
By the nature of the symmetry all the shapes in the diagrams are the same "shorted bridge" as the tetrahedral. If they were not - the voltages would not zero out. However, the mechanics of achieving the symmetry change between the different geometrical shapes.

I like it. It can make a pleasant party quiz entry: What has a resistance for 2 wire measurements - but a real zero for 4 wire measurements? A superconductor? Nope - it is zero for both. Yup - a tetrajunction!
You guys are pulling my leg, aren't you?  *Again* (https://www.eevblog.com/forum/metrology/teardown-standard-resistors/msg999854/#msg999854:palm:

If you want a four terminal device with current going into one pair and no voltage across the other pair, there would be a much simpler solution, no?  What in the world is the use case of this (other then fooling the uninitiated?).  Hey, it's not even April 1st.  :-//

(edited due to erroneous quoting)
« Last Edit: November 26, 2016, 07:48:20 pm by guenthert »
 

Offline 3roomlab

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Re: Zero Ohm DIY 4-wire standard
« Reply #42 on: November 26, 2016, 06:33:26 pm »
https://www.eevblog.com/forum/metrology/teardown-standard-resistors/msg911075/#msg911075

oh gawd ! zlymex's tetrajunction !
and post #73 by quarks. "TRANS-RESISTANCE"
 

Offline VintageNutTopic starter

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Re: Zero Ohm DIY 4-wire standard
« Reply #43 on: November 26, 2016, 08:03:10 pm »
The ESI document states that the junction they use are less than 100 nano-ohms. Zlymex shared his DIY version that is something less than 50 nano ohms. The commercial version that I have from Ohm Labs is no greater than 4 nano ohms. My first attempt at the circular version with no adjustments yielded 20 nano-ohms or less in any orientation.

Having a geometry that is adjustable seems to make a big difference if you are trying to get as close to zero as possible.

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Offline Assafl

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Re: Zero Ohm DIY 4-wire standard
« Reply #44 on: November 26, 2016, 08:52:00 pm »
You guys are pulling my leg, aren't you?  *Again* (https://www.eevblog.com/forum/metrology/teardown-standard-resistors/msg999854/#msg999854:palm:

If you want a four terminal device with current going into one pair and no voltage across the other pair, there would be a much simpler solution, no?  What in the world is the use case of this (other then fooling the uninitiated?).  Hey, it's not even April 1st.  :-//

(edited due to erroneous quoting)

Hmm. Quite true. If you want zero volts there are other ways to get zero volts... And my guess is if the bias current into the D/A was indeed zero, those would indeed represent a zero ohm resistor.

My guess as to why this is a "better" approach is that since the bias current is not zero, how you take the entire current from the kelvin force connection, allow the bias currents as they would apply to the sense terminal, and yet maintain a true zero ohm resistor? (so that errors due to the bias current are indeed fully nulled). In this case it will be a perfect zero ohm resistor.

I say "better" in quotes as it will be large and heavy, and have large thermal EMFs if not hooked perfectly and allowed to stabilize for hours. It is also very tedious to calibrate. And it needs longer cables than a 4w short. So one has to be careful when using.

Just as to how to much of a difference does Fluke's 4w short approximate the same thing (putting a hairline short between the shorted force and shorted sense terminals) - I don't know - I always wondered if Fluke's positioning of the hairline short was random or intentional (about a 2/3 to 3/4 of the way down).

For me the beauty is of a mathematical nature and lies in that fact that a geometrical exercise can find a virtual zero ohm resistor that doesn't really exist. I really dig that - even if it happens to serve a none existing issue. 
 

Offline Assafl

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Re: Zero Ohm DIY 4-wire standard
« Reply #45 on: November 26, 2016, 09:20:41 pm »
As for finding the offending place to file material away by reasoning, that is giving me a headache. If I were going to try to modify my DIY device to balance it, I would construct more than one and experiment to find a reproducible procedure to balance the device.
Find the culprit "most to blame" leg of the bridge. I don't know if the process I postulated above will work but I see no reason it won't.

But first try to see if ultra high resistors can balance it (on the 4 wire zero side - not the power supply or nano voltmeter side).

Then try to balance it by using exceptionally high value resistors screwed to the binding posts of the short. Don't touch the short.

Once you are able to balance it that way - the question becomes how much drilling is a 100M vs 1G worth. Oh - and where to drill/ scratch/scuff....

Quote
This device is extremely difficult to measure. The error sources of induced AC and DC microvolts  are over 100X larger than the voltage being measured. The thermal drifts caused by touching the device and the wires to connect it require hours to settle out. Not seconds. Not minutes. Hours.
Maybe hook it to a bunch of reed relays so you could rotate the bridge without touching it...

Quote
The clever thing to do would be to construct a mounting apparatus that would allow filing the device without touching it. The mounting apparatus would have to be enclosed in an electrically and magnetically shielded enclosure that is able to be easily opened and closed for repeated adjustment of the device. Another low ohms project!

In a box with binding posts....

I applaud you trying to make this work. I neither have the equipment, nor the patience to do something like this.

Who needs Smith charts when one can have so much fun with DC?
 

Offline VintageNutTopic starter

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Re: Zero Ohm DIY 4-wire standard
« Reply #46 on: November 27, 2016, 03:04:25 am »
Are you suggesting placing a resistor across the current force pair or the voltage measure pair? I do not see how you could reduce the apparent resistance because the device is not a resistor. It is more like anti-resistor. Where the voltage is measured, no current is flowing. The current is flowing in a segment normal to where the voltage is measured.  If you try to place anything in parallel, you will probably induce microvolt errors.

i definitely like the reed relay idea. I have a KE7001 and a matrix card that could be used for this purpose. What I don't have is the cast iron pot and the design of a mechanism to hold the tetrajunction.

After constructing a DIY version, it makes me appreciate the commercially produced device. At $400 it was a bargain to have something mounted in a box with high quality binding posts that was 5X better than my DIY device. They do not stock these. They make them one at a time to order. I had to wait something like a month before they shipped it.
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Offline Conrad Hoffman

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Re: Zero Ohm DIY 4-wire standard
« Reply #47 on: November 27, 2016, 04:29:22 am »
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.
 
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Offline Assafl

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Re: Zero Ohm DIY 4-wire standard
« Reply #48 on: November 27, 2016, 06:14:43 am »
Such a beautiful FEA plot. Just like network DC analysis would have us predict. An infinite array of copper resistors.

No skin effect (to the horror of the audiophiles). No Leyden jars...

Can you imagine what this plot would look like at Ghz frequencies?
 

Offline 3roomlab

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Re: Zero Ohm DIY 4-wire standard
« Reply #49 on: November 27, 2016, 08:27:34 am »
You can download a free and surprisingly capable FEA program called FEMM.

wow this is a fun simulator
the slightly crooked X junction did show a skew in potential diff

*update
question for conrad : in the 2nd pic, is there a way to cross the horizontal segment over the vertical without then forming a new node? (so it mimics pic3).

it appears the program does not do multiple planes but its all fine,

but i guess you peeps can see what i am trying to mimic (a shorting plug with 6 legs of equal traces trying to be a tetra-thingy)
if i can get the FEMM to make the centre float off and not connect?
does my flatten tetra thingy make any sense?
« Last Edit: November 27, 2016, 01:15:21 pm by 3roomlab »
 


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