You could consider making your own - it's a switch, not rocket science.
Layout a radial contact pattern and get some PCB's made up at a cheap
PCB facility for $10 or so. Get them gold plated - possibly $20 or so?
Ideally the gold would be alloyed to make it harder and more durable and
2 or 3 um thick. Make the wiping contacts from gold plated copper strip
and fashion some sort of spring to apply an appropriate contact force.
Use several if necessary to reduce the contact resistance sufficiently.
Solder to the inner most part of the radial contacts to minimise thermal
EMFs (by keeping all solder joints as close as possible for best thermal
equilibrium).
With respect to thermal EMFs it should outperform those expensive
switches as:
a) It would be all copper - those Dallas suggested have silver-alloy and
brass contacts with copper terminals and the Elma datasheet doesn't even
specify the materials (ridiculous for such an expensive part).
b) The fixed and moving contact solder joints can be significantly
closer together, around the shaft rather than around the periphery, and
thus easier to minimize the temperature differences.
Indexing can be achieved in lots of ways including drilling holes in the
PCB and having one of the moving contacts, or a spring loaded ball
bearing of an appropriate size, to partially drop into the holes.
Alternatively, create notches around the circumference which a shaped
metal strip drops into under spring pressure.
The main concern for this construction might be leakage across the PCB
and durability depending on how well you can control the contact force.
With a bit more complexity you could even arrange for the indexing
mechanism to raise the moving contacts (to reduce, not eliminate wiping
action) in between switching positions.
If leakage is a concern you could even bolt pie shaped radial segments
cut from gold or silver plated copper sheet to a PTFE disc. That would
have the additional advantage that both the fixed and moving contacts
could be made from the same piece of copper, reducing thermal EMFs even
further. The contact area, most of each pie segment, can be quite large
compared to those at the circumference of commercial switches with the
potential for very low contact resistance.
Finally, use thin strips of the same copper for any interconnections
rather than copper wire of slightly different composition to eliminate
anther dissimilar metal to metal joint - at one end anyway.
Must be worth considering given the high prices being asked for the
commercial switches.
Tony H
On 26/03/2015 21:51, Stan Katz wrote:
Hi Dallas and Frank,
Dallas, the diameter of the switch you pointed me to is indeed too large.
It's only available at Digikey for a very affordable $92.00USD. The shaft
is also not compatible with the 720a construction.
Frank, I called up Fluke for a quote on the switch. Delivery in one week
for only $1060.00USD!!!! I'm still waiting for a quote from IET. The ELNA
04 doesn't look like a candidate, but I have requested a quote anyway.
Shame on Fluke for using an outside supplier for that faulty switch. This
costly, and impractical to repair 720a has taken away all of the thrill of
classical analog metrology for me.
I think that I should consider a used Fluke 5440b and give up on lash ups
needing the 720a.
Stan
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