Author Topic: Copper binding post shorting straps for low thermal EMF-- Alloy/tempering?  (Read 2100 times)

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Offline Vacuuminded

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Hi all!
Was searching online for some good low EMF shorting straps, and came up pretty empty handed.  The only easily found examples were nickel plated brass, which seem to be less than ideal for below e-6.  So, I have decided to have some solid copper ones milled and might have extras made if there is enough interest. 

I'm thinking .032" to .050" thick, but have two basic alloys to choose from.  C101, or C110 in various tempers.  C101 is "oxygen free", but C110 reportedly has the highest conductivity of all copper alloys so that on the surface would seem the better choice.  I don't think the level of tempering will effect conductivity or any thermal characteristics, but defer to you experts on this?? :-//  At face value, it would seem the hardest temper available would be ideal, mechanically speaking. 

Then there's the physical design...  I'm leaning more towards "B" or "C" in the below image, what would your preference be?

Regards,
Chris
 

Online texaspyro

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Re: Copper binding post shorting straps for low thermal EMF-- Alloy/tempering?
« Reply #1 on: November 08, 2017, 06:45:06 AM »
Shorting straps are almost always used only to connect voltage and sense lines on banana connector pairs.  The sense lines carry very little current, so conductivity is not an issue.  I'd go for a hard alloy.  Most quality binding posts use a tellurium-copper alloy.

I would also look into getting them gold plated.  When copper oxidizes the copper oxide has a VERY high thermal EMF voltage.  The thermal EMF hit you take for the gold-nickle-copper transitions is usually far less than the copper oxide issue.  Maintaining  clean bare copper surfaces is a never-ending royal pain in the ass.  You rarely see bare copper binding posts because of this.
 

Offline Vtile

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Re: Copper binding post shorting straps for low thermal EMF-- Alloy/tempering?
« Reply #2 on: November 08, 2017, 07:58:25 AM »
Copper and a good bastards file? I'm just curious and mixing the soup again.
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Offline engineer_in_shorts

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Re: Copper binding post shorting straps for low thermal EMF-- Alloy/tempering?
« Reply #3 on: November 09, 2017, 02:01:43 AM »
Flukes 8.5digit dmm the 8508A comes with a gold plated PCB to do this.

http://eu.flukecal.com/products/accessories/test-leads-probes-and-clips/8508a-lead

Not hard to make a clone to your required dimentions, I would assume 2oz copper or more
 

Online texaspyro

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Re: Copper binding post shorting straps for low thermal EMF-- Alloy/tempering?
« Reply #4 on: November 09, 2017, 03:35:48 AM »
Flukes 8.5digit dmm the 8508A comes with a gold plated PCB to do this.

Yes, I've been thinking about laying out a PCB to do this.  The size is small enough that OSHPARK prices should be fairly competitive to the Chinese suppliers.

The shorting clips on my MV-100 reference are a bitch to get off.  I don't think 2 oz copper is needed.  The current in the sense inputs is very small.
 

Online Vgkid

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Re: Copper binding post shorting straps for low thermal EMF-- Alloy/tempering?
« Reply #5 on: November 09, 2017, 04:01:59 AM »
I have never removed the shorting clips from any of my meters. I just push them out of the way.
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Online Cerebus

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Re: Copper binding post shorting straps for low thermal EMF-- Alloy/tempering?
« Reply #6 on: November 09, 2017, 04:14:13 AM »
... mixing the soup again.

I suspect that's a Finnish idiom that didn't quite translate*, but I also suspect we know what you mean. We'd normally say 'stirring' and it wouldn't be soup.

* Would that be a 'finished idiom'? It's OK folks, I'm here all week...
Anybody got a syringe I can use to squeeze the magic smoke back into this?
 
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Online Cerebus

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Re: Copper binding post shorting straps for low thermal EMF-- Alloy/tempering?
« Reply #7 on: November 09, 2017, 04:17:18 AM »
If you don't want fancy, a freshly cleaned length of solid core copper wire does the trick nicely. Some cores stripped out of 4mm2 electrical cable have done me nicely in the past.
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Online texaspyro

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Re: Copper binding post shorting straps for low thermal EMF-- Alloy/tempering?
« Reply #8 on: November 09, 2017, 07:32:30 AM »
I have never removed the shorting clips from any of my meters. I just push them out of the way.

The problem with the MV-100 is the clips don't want to push out of the way.  The cutout in the clip catches on the binding post.  It needs to have rounded corners.
 

Online texaspyro

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Re: Copper binding post shorting straps for low thermal EMF-- Alloy/tempering?
« Reply #9 on: November 09, 2017, 11:11:37 AM »
It looks like a 15x35mm PCB will work fine.   OSHPARK would charge you around $5 for three boards (1.6 mm FR4, 1 oz copper).  I checked a couple of China fabs.  I think I am going to have some built with 2 oz copper, 0.8 .. 1mm FR4, and ENIG gold finish... Looks like I can offer them for maybe $10 for 10 pieces shipped.  Anybody interested in some?

I'm going to try and lay out a board tonight.

I checked the clips on my MV106.  The reason they hang up on the binding posts is that they have stamped little divots on the corners of the gaps which reduces the throat of the gap to 0.255".  Binding posts are 0.250" wide, so there is basically no clearance. I ground down the tips of the divots and they work well now.
 

Offline Vacuuminded

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Re: Copper binding post shorting straps for low thermal EMF-- Alloy/tempering?
« Reply #10 on: November 10, 2017, 06:15:06 AM »
Thanks for the input, all.  I guess the consensus is that bare copper straps just aren't worth the hassle any longer, even for temporary shorts used strictly for cal/zeroing.  Agreed for sense lines gold plated whatever would certainly suffice.  If anybody would still want some please message me.  Otherwise, looks like this project is dead.  I guess advantages of a solid copper strap over a PC board or some other material must be completely mind over matter.

I thought about PC boards for this too.  Gold plated would be eaay, bare copper maybe not so much as HASL or gold plating seem to be the default options nearly everywhere now. 

Texaspro, so long as there is no soldermask which would make stacking them an issue, I would be interested in some at that price.  If for some reason it doesn't reach fruition, I will look into doing the same. 
Regards,
Chris
 

Offline nanofrog

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Re: Copper binding post shorting straps for low thermal EMF-- Alloy/tempering?
« Reply #11 on: November 10, 2017, 07:30:24 AM »
It looks like a 15x35mm PCB will work fine.   OSHPARK would charge you around $5 for three boards (1.6 mm FR4, 1 oz copper).  I checked a couple of China fabs.  I think I am going to have some built with 2 oz copper, 0.8 .. 1mm FR4, and ENIG gold finish... Looks like I can offer them for maybe $10 for 10 pieces shipped.  Anybody interested in some?
FWIW, I'd recommend hard gold for that particular application if you can find it at the right price (better wear resistance than ENIG).
 

Online texaspyro

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Re: Copper binding post shorting straps for low thermal EMF-- Alloy/tempering?
« Reply #12 on: November 10, 2017, 08:24:56 AM »
I once did a check on what hard gold would cost on a board... it was rather eye watering.  The lowest price I found was around $300 setup per run and $20+ for a hard gold edge tab.    You can replace a LOT of ENIG boards for that. ENIG gold costs around $15 setup per run, regardless of board quantity.

The boards I am thinking about doing would have no solder mask.   One thing is that most PCB makers want to silkscreen their part number on the board somewhere.  It can be a problem finding one that wont do that.
 

Online texaspyro

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Re: Copper binding post shorting straps for low thermal EMF-- Alloy/tempering?
« Reply #13 on: November 10, 2017, 09:00:52 AM »
I guess the consensus is that bare copper straps just aren't worth the hassle any longer, even for temporary shorts used strictly for cal/zeroing.

There's definitely a place for solid copper straps.  They are hard to beat for high-end volt-nutty goodness.  It would be good to have a source available at not unreasonable costs.

I think for 90%+ of applications that a PCB based design will work just fine... and it's hard to beat the cost.   At a low enough price, you can afford to sacrifice them to various evil experiments.

I started looking into doing the straps a couple of weeks ago when I was working on an MV100 voltage standard and the sense straps were very difficult to remove (a little grinding fixed that).  I went looking at some of my other equipment to steal a pair and found most either had captive straps that could not be fully removed, or were missing straps.  I figured there would be more people in the same boat.   
 

Offline nanofrog

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Re: Copper binding post shorting straps for low thermal EMF-- Alloy/tempering?
« Reply #14 on: November 10, 2017, 09:13:14 AM »
I once did a check on what hard gold would cost on a board... it was rather eye watering.  The lowest price I found was around $300 setup per run and $20+ for a hard gold edge tab.    You can replace a LOT of ENIG boards for that. ENIG gold costs around $15 setup per run, regardless of board quantity.

The boards I am thinking about doing would have no solder mask.   One thing is that most PCB makers want to silkscreen their part number on the board somewhere.  It can be a problem finding one that wont do that.
What I've seen for hard gold has been ugly as well, but I've not checked every source or for a board that small. So I certainly understand your logic.

For quick & dirty, I'd second using freshly stripped solid copper wire.

In regard to copper straps, I have copper sheet on hand, some quite thick (20oz IIRC). Thick is good for making buss bars, and thinner for trace repair.

And if you truly want/need hard gold plating, you can DIY that (i.e. nickel solution is $25 for 8oz, and hard gold starts at $99 for 2oz from Gold Plating Services).
 

Online texaspyro

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Re: Copper binding post shorting straps for low thermal EMF-- Alloy/tempering?
« Reply #15 on: November 10, 2017, 09:40:33 AM »
Also, you can get bare copper finish on PCBs.   Basically it's HASL without solder and hot air process.   A lot of PCBs used to be made using the SMOBC (solder mask over bare copper) process.  Now I think they usually use solder mask over nickel or tin over bare copper.  A problem with plain SMOBC was pinpoint defects in the solder mask and alignment issues around pads that left bare copper exposed that could corrode.
 

Online texaspyro

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Re: Copper binding post shorting straps for low thermal EMF-- Alloy/tempering?
« Reply #16 on: November 10, 2017, 12:04:51 PM »
Here's what they should look like.   Dots are via stitching between the top and bottom layers.   I'm sending them off to OSHPARK first as a test...  Their CAM processor was freaking out about having just top and bottom polygons... said I had no copper layers.
 

Offline Vtile

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Re: Copper binding post shorting straps for low thermal EMF-- Alloy/tempering?
« Reply #17 on: November 11, 2017, 02:17:10 AM »
... mixing the soup again.

I suspect that's a Finnish idiom that didn't quite translate*, but I also suspect we know what you mean. We'd normally say 'stirring' and it wouldn't be soup.

* Would that be a 'finished idiom'? It's OK folks, I'm here all week...
Hah. Indeed thank you for correction of the idiom. These similar, but not exact ones are the most hard to remember. "Mixing the pot" "Stirring the soup" ....   :D To be honest I'm not sure if there is even a typical version in Finnish as there is lots of variations in use.

There is somewhere an appnote which content were partly around PCB shorting bar usage in metrology, it did also contain longish discussion of various TC effects involved. Unfortunately I can not recall the name at all.
« Last Edit: November 11, 2017, 02:20:08 AM by Vtile »
Pick your point and call it as a ground.
 

Offline VintageNut

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Re: Copper binding post shorting straps for low thermal EMF-- Alloy/tempering?
« Reply #18 on: November 11, 2017, 03:59:21 AM »
... mixing the soup again.

I suspect that's a Finnish idiom that didn't quite translate*, but I also suspect we know what you mean. We'd normally say 'stirring' and it wouldn't be soup.

* Would that be a 'finished idiom'? It's OK folks, I'm here all week...
Hah. Indeed thank you for correction of the idiom. These similar, but not exact ones are the most hard to remember. "Mixing the pot" "Stirring the soup" ....   :D To be honest I'm not sure if there is even a typical version in Finnish as there is lots of variations in use.

There is somewhere an appnote which content were partly around PCB shorting bar usage in metrology, it did also contain longish discussion of various TC effects involved. Unfortunately I can not recall the name at all.

"Stirring the soup" in the USA is a certain kind of activity not safe for a family-friendly forum. 
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Online texaspyro

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Re: Copper binding post shorting straps for low thermal EMF-- Alloy/tempering?
« Reply #19 on: November 11, 2017, 04:35:43 AM »
I've ordered 6 prototypes from OSHPARK... they do offer 0.8mm / 2 oz copper, but it takes a week longer to get.  I'll uses these to do some performance, fit, and finish checks and look into adding more via stitching that would not provide a fracture line across the narrow parts of the board or interfere with the banana jack contact disks.
 
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Offline beanflying

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Re: Copper binding post shorting straps for low thermal EMF-- Alloy/tempering?
« Reply #20 on: November 12, 2017, 07:57:41 PM »
These might be worth looking at too. $1 USD each plus some filing or drilling. It means I guess you would then be relying on the upper and lower surfaces and not the hole for the best contact surface.

https://meniscusaudio.com/product/gold-shorting-barspr/
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Offline sarel.wagner

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Re: Copper binding post shorting straps for low thermal EMF-- Alloy/tempering?
« Reply #21 on: November 13, 2017, 01:40:06 AM »
Ready made, 3 boards and you can do 1 with 4 bananas and 2 ea with 2 bananas. https://oshpark.com/shared_projects/yOAPbUZF

Online texaspyro

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Re: Copper binding post shorting straps for low thermal EMF-- Alloy/tempering?
« Reply #22 on: November 13, 2017, 04:55:30 AM »
I've also done a layout with longer "legs" on the "E" shape.   It will let you short all 4 terminals.

I don't like shorts that use banana plugs... too many connections (some soldered, plus banana plug to jack connections are not the best),  too many potentially different metals involved, and lots of thermal mass.
 

Offline sarel.wagner

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Re: Copper binding post shorting straps for low thermal EMF-- Alloy/tempering?
« Reply #23 on: November 13, 2017, 05:29:08 AM »
On these, at least gold flash over copper board, and I used high quality Gold bananas.

Online texaspyro

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Re: Copper binding post shorting straps for low thermal EMF-- Alloy/tempering?
« Reply #24 on: November 13, 2017, 07:09:13 AM »
On these, at least gold flash over copper board, and I used high quality Gold bananas.

Mine also have heavy ENIG gold over the copper.   

Soldering the bananas to the PCB introduces an unnecessary  thermal EMF junction and the banana plugs add a lot of thermal mass to stabilize.   I think those are the reasons why Fluke sells a PCB based 4-way shorting strap.  Either way will work, but it's nice to minimize possible sources of error.
 


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