EEVblog #211 IET Wide Range Resistance Box

[Conrad Hoffman]

FWIW, if you look at an old Julie Research 1 ppm Kelvin Varley divider, they used the same trick to get within tolerance- add a small piece of resistance wire to the lead of a resistor. My guess is that's what IET did on that one odd connection, though I have to agree, the entire construction isn't up to a really professional standard.

[hacklordsniper]

Im somewhat suprised at cost & construction detail on such expensive decade, it looks like mine altrough much cheaper is better.

[ciccio]

It is possible to build such a decade box with only 4 resistors per decade, using commercially available thumbweel switches,   like those made by Crameda (I fear that they are expensive, being Swiss made...)
The catalog is here http://www.crameda.com/pdf_files/eng/multiswitch/MS-U-E-060915.pdf
You need the version 632 (page 4) but beware that there is an error in the schematic : the 4 resistors' values should be  3R, 4R 1R and 1R, and not 3R, 4R, 2R and 1R as indicated.
EDIT: the values shown on the data sheet do not result in the correct values when the switches are set according to the truth table,  or at least this is what I get after filling two A4 pages with schematics.. If another member wants to show me where I'm in error, he's welcome.
You need  the values 3R, 4R, 2R and 1R if you want to have a total resistance value from 0 to 10, as shown at the end of Dave's episode #212, but a decade starts form 0 and ends at 9, value 10 is the second first one of next decade.. (and this requires a different switch arrangement)

We used a similar construction for the thumbweel set-point selector of temperature controllers, but it was many years ago, when electromechanical components were cheaper..
Weeks ago I've seen one of them at a customer's plant: the controller is still working after more than 20 years, the only problem is the dim HP LED display...

[Bored@Work]

(I fear that they are expensive, being Swiss made...)

They have an online shop. Depending on configuration they quote in the range of 23 to 35 Euro per Series U, 632-type switch, excl. VAT (they might bill in CHF, I didn't check, and since they aren't in the EU, you might have to pay some import tax).

What I find great about their offerings are the dummy switches - if you need some non-functional switches for "aesthetical reasons". Almost 12 Euro per U-series dummy.