Zero Ohm DIY 4-wire standard

[VintageNut]

This thread is in response to a PM asking for me to provide pictures of the inside of my Ohm Labs balanced tetrajunction.

Sadly, the Ohm Labs device case is epoxy sealed at the screws. To open the case I would have to remove the epoxy and thus void the warranty.

What I did for a DIY project is to try my hand at making a tetrajunction. Attached is an excerpt of US patent 5867018 showing various configurations of physical manifestations of a balanced tetrajunction.

The final device is mostly figure 2(d).

Details to follow in separate posts in this thread.

 

[VintageNut]

The main portion of the device is a solid copper disk. Diameter is about 2" or 50.8mm. The thickness is about 3/8" or 5 mm.

Four holes are drilled and tapped. The three perimeter holes are 120 degrees apart. The fourth hole is in the center of the disk.

The screws are solid copper tattoo machine screws. 

[quarks]

thanks for sharing, to bad the Ohmlabs Standard cannot be opened

BTW this would fit into to the zlymex posts
https://www.eevblog.com/forum/metrology/teardown-standard-resistors/msg997453/#msg997453

[VintageNut]

The device was measured using a Keithley 2450 to force 1A while a Keithley 147 was used to measure voltage.

The measurements obtained are attached. It must be noted that to connect this device and measure it is an endeavor that requires hours. The instruments must warm up and stabilize. The microvolts offsets created by connecting the device and cables requires quite a long time to settle. The microvolts are everywhere in the current path and are temperature dependent. Touching anything creates a thermal imbalance that must come to ambient equilibbrium.

[doktor pyta]

I see You have a lock-in amplifier. Have You tried to provide AC excitation current (eg. current source chopped by two MOSFFET) and measuring nanovolts using lock-in ? This should provide quick (few minutes) information about the geometrical symmetry. Do You think it is a bad idea?

[quarks]

As I do not have a Nanovoltmeter, I have just made a quick meassurement with my DIY Zero Ohm Standard
and tried the following setup:

Calibrator  10,00000 A (both AC and DC)
DMM 200mV range (zeroed before meassure)
around 0,0005 mV DC
around 0,0095 mV AC/100Hz (not really a stable reading)

I meassured several times to confirm it
If I did get it right my calculation gives 50 nOhm with DC  and about 950 nOhm with AC current

[VintageNut]

I see You have a lock-in amplifier. Have You tried to provide AC excitation current (eg. current source chopped by two MOSFFET) and measuring nanovolts using lock-in ? This should provide quick (few minutes) information about the geometrical symmetry. Do You think it is a bad idea?

I have never thought to try something like this. It will require some research and thought to construct the setup. If you have a proposed schematic and instrument connections that would help.

The lockin amplifier was used to perform some demodulation of phase modulated distortion. Eventually I found that a software defined radio Windows app was more robust than the lockin for my requirement at the time. 

[VintageNut]

As I do not have a Nanovoltmeter, I have just made a quick meassurement with my DIY Zero Ohm Standard
and tried the following setup:

Calibrator  10,00000 A (both AC and DC)
DMM 200mV range (zeroed before meassure)
around 0,0005 mV DC
around 0,0095 mV AC/100Hz (not really a stable reading)

I meassured several times to confirm it
If I did get it right my calculation gives 50 nOhm with DC  and about 950 nOhm with AC current

The problem with using a DMM is that you are using the very bottom of the range. I suspect that the absolute uncertainty of the 200mV measure range of your DMM is larger than your measurement. You cannot rely on a 0.0005 mV reading of a 200.0000 mV range. You want your measurement to be a significant percentage of the measure range. This is why a very old school nanovoltmeter is ideal for nano-ohms measurement.

The KE147 uncertainty is 2% of full scale. For the most sensitive 30nV range, that uncertainty is 0.6nV. The noise is specified at less than 0.6nV RMS.

There is a KE147 on eBay right now for USD$40. That is very cheap. Most likely it will require some repair. I own multiples for future spare parts if the chopper wears out.
http://www.ebay.com/itm/Keithley-Nanovolt-Null-Detector-model-147-/262615923728?hash=item3d2520fc10:g:ZnYAAOSwvzRX0bCY

[quarks]

I only did this as a quick and dirty test with what I have and was not ment to measure absolut values, only to get an idea how close to zero it might be

[VintageNut]

I only did this as a quick and dirty test with what I have and was not ment to measure absolut values, only to get an idea how close to zero it might be

What exactly is the range uncertainty of your DMM for the 200mV range?

The KE2002 has a 200mV range uncertainty of 9ppm. That is 1.8 uV. At 10A of drive current that makes your uncertainty 180 nano-ohms. The uncertainty of the measurement is 3X what you are measuring at 50 nano-ohms.

If your DMM has a greater than 9ppm range uncertainty then your measurement uncertainty is greater than 180 nano-ohms.

[HighVoltage]

Very interesting thread and too bad you can not open one up.

Where die you buy this Ohm-Labs standard? I might buy one for a reasonable price and open it up.
Are there other companies that make a similar zero ohm standard?

[quarks]

My DMM is much better than K2002, but as I only checked the relative change after zeroing it, it does not matter.

[VintageNut]

Very interesting thread and too bad you can not open one up.

Where die you buy this Ohm-Labs standard? I might buy one for a reasonable price and open it up.
Are there other companies that make a similar zero ohm standard?

I bought it directly from Ohm Labs. They build them one-at-a-time to order. There is a waiting time which was not a problem for me.

As far as I know, only Ohm Labs makes these. They are used for 4-terminal junctions of resistor transfer standard arrays. They decided to place one into a box and sell it as a zero ohm standard.

[3roomlab]

this reminds me of this thread on DIY plug. but compare to fluke plug, how many micro-ohm is fluke's?
https://www.eevblog.com/forum/testgear/diy-34401a-calibration/

[VintageNut]

That is a good question. I do not think that you can measure one of these with a DMM because these are used to calibrate zero of a DMM.

On my DMM7510, the Keithley Cal Short measures about 1/2 mico-ohm which is going to be way less than the uncertainty of the measurement. 1/2 micro ohm is 1/2 ppm of the 1 ohm range.

I will put this on my list to measure with the KE147 nanovoltmeter and a SMU. It will require an enclosure to shield electric and magnetic interference as well as thermal interference from the very un-regulated lab temperature. Its on my list to acquire a cast iron pot for measuring low ohms devices.

[3roomlab]

That is a good question. I do not think that you can measure one of these with a DMM because these are used to calibrate zero of a DMM.

On my DMM7510, the Keithley Cal Short measures about 1/2 mico-ohm which is going to be way less than the uncertainty of the measurement. 1/2 micro ohm is 1/2 ppm of the 1 ohm range.

I will put this on my list to measure with the KE147 nanovoltmeter and a SMU. It will require an enclosure to shield electric and magnetic interference as well as thermal interference from the very un-regulated lab temperature. Its on my list to acquire a cast iron pot for measuring low ohms devices.

EMF definitely is a itch in my brain somewhere.
i had been curiously browsing taobao about mu-metal sheets. i am guessing, the higher the flux handling the better the shield? but iron POT = WIN !
https://item.taobao.com/item.htm?spm=a1z0d.6639537.1997196601.67.vHJ5X8&id=539901820269

[lukier]

I wonder how well it would work. Machining mu-metal (bending these sheets, hammering, riveting etc) would disturb the crystalline structure thus you would have to perform the annealing again, see here:

https://en.wikipedia.org/wiki/Mu-metal

Heat treatment in a hydrogen atmosphere is not something I would DIY at home

[acbern]

It does not really make much sense to shield the zero-ohms-standard in mu-metal if the cabling and meter does no have the same shielding.... and they dont, while having the majority of magnetic flux exposure simply because of mechanical dimensions.
sounds a little like audio-voodoo

[VintageNut]

It does not really make much sense to shield the zero-ohms-standard in mu-metal if the cabling and meter does no have the same shielding.... and they dont, while having the majority of magnetic flux exposure simply because of mechanical dimensions.
sounds a little like audio-voodoo

The front-end copper pieces of the KE147 are inside an aluminum enclosure that is also Mu metal shielded.

The first time I used the KE147, the meter needle oscillated. The instrument was sitting on top of the SMU forcing current. Not a good idea. Creating distance between the SMU and the KE147 was the solution.

Magnetic fields hitting the the inside of the instrument will cause beat frequencies.

The KE147 cable is shielded. Doubtful is there is any Mu metal in the cable shielding.

[3roomlab]

It does not really make much sense to shield the zero-ohms-standard in mu-metal if the cabling and meter does no have the same shielding.... and they dont, while having the majority of magnetic flux exposure simply because of mechanical dimensions.
sounds a little like audio-voodoo

i definitely have no way of measuring its induced effects properly, and from what i can see here (the materials presented in the video), even the materials are so varied. i assume only in very specific conditions will it be a very bad problem.

[Vgkid]

I haven't watched the video yet. I recall that steel is one of the most cost effective shielding materials for low frequency shielding. Not as good as copper, but thickness does play into it.

[ap]

MU metal is a solution to shield low frequency magnetic fields where other solutions are not possible (e.g. for audio transformers). The preferred way to do this when cables are involved (and also within the zero ohms standard) is to use twisted cables. The twisting causes noise voltages to be induced in the cable pair with opposite polarity, cancelling each other out. In an amplifier PCB, for obvious reasons, MU-metal is the choice if the complete circuitry should be shielded, as the layout can hardly be equivalent to a fully out-cancelling design (but many nV-meters still dont do that, e.g. the 34420 iirc). Mu-metal shields for cables do exist, but are pretty exotic. It can enhance the effect of cable twisting, especially when high current conductors cause strong magnetic fields to be attenuated.
The zero ohms resistor, btw, as mentioned in the patent, solves the problem to generate zero ohms in applications where the 4 connected signals are physically located in certain positions (e.g. in the SR1010). If one just wants to make a zero ohms resistor with 4 (low emf, if need be) banana posts in a case, an equivalent solution would be to connect a wire to each of the posts, twist them together as pairs, and connect them at one point in an appropriate manner. That way, no voltage can be developped within the sense wires by current in the force wires. No need for a triangular piece of metal or so as in the patent.

[Kleinstein]

For strong magnetic fields steel is good, as it has a saturation, but it is not good at low fields as it stays magnetized. So than at least use electric steel like that used for transformers.

For low field Mu-metal  is the right material. If subject to mechanical stress / strain than some amorphous or nanocrystaline materials can be better, as Mu Metal is sensitive to deformation.

For the zero Ohms shielding is tricky as the magnetic material can also increase the coupling for certain combinations. So it makes some sense if the current and voltage contacts are fixed - but can cause more trouble than good if you want the contacts interchangeable.
The non interchangeable zeros are much easier and can get lower resistance values, as they don't rely only on symmetry but can use aspect ratios too.

To distinguish resistance, induction voltage / inductive coupling and thermal effects one might have to look at the time dependence. So a lockin amplifier might be Ok, but with care to the phase.

[Assafl]

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). 

[VintageNut]

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).

Very good question. I do not have an answer. I did not attempt to balance my DIY version. I only wanted to characterize it as built. The intended purpose is to have something better than the PCB plug-in banana "short" that is used to calibrate DMMs to zero ohms. The actual resistance of that "short" is something on the order of 2 micro-ohms (from memory).

If someone wants to explores this filing of the geometric shape, I will assist as I can.