7 decade thumbwheel resistor box (project #P1)
As already shown in the thumbwheel post, the initial resistor placement wasn’t perfect. I fiddled some more and I got a case and nice 4mm banana plugs for it. First, let’s see the final measurement results, done with a calibrated Keithley 2700 6 1/2 digit DMM in 4-Wire (Kelvin) mode.
Yep, the 1Ω data point still violates the 1%/10000ppm spec (1,9%). But that is probably the least used setting and I can live with that. Everything else is below 1,0% and even the geometric average is below 0.2%. Considering the parts used and their cost, that is pretty darn good and certainly sufficient for my applications. To go one order of magnitude down, one would even have to reconsider the test setup. As I have several full sets of characterization data, I can see the measurement repeatability of the entire system for the unchanged decades. And it doesn’t get better than 0.01%, which then would compete with the measured deviation. So I call it a day – electrical stuff done.
Ah, in case you wonder: This data is from the fully assembled unit. I didn’t make the mistake to characterize flying wires and then solder 4mm terminals onto it, forcing me to do the tedious 7 decade acquisition once again
Now to the mechanical side of things. Hardest part is choosing a proper enclosure. I went with a Hammond 1594 DBK unit, which is very thick ABS plastic (3mm) with threaded metal inserts. Size is 131mm x 66mm and 55mm height, which fits the resistor block very well and leaves some space for the banana terminals. I got it from voelkner for 7,46€.
The 4mm terminals are also from voelkner (type 630970), they are some kind of store brand, even though they are tagged as Hirschmann. They are specced at 20A/1000V, so their performance isn’t too shabby. Contact material is silver-plated brass, I didn’t want to gild the lily with fancy tellurium copper terminals (or the like) if I don’t need the current capability nor the last Milliohm of contact resistance. Price was 1,29€ a pop, which is about half of the high-end Hirschmann ones. Quality is good, feels sturdy and the assembly is pretty easy.
Well, cut out the center to fit in the thumbwheel block, smoothen with a file, put it in. If you work carefully, it will hold securely in place without glue (“German Precision”, even though I failed my goals, have a look). Next, set holes for the terminals, in my case these required a 12mm diameter bore. A stepped drill can come in handy. This is also true for the outer screw holes of the Hammond case, as I used my own Torx screws, which have a higher screw head than the original ones. So I needed to drill a little deeper, which is no problem considering the thickness of the case.
Last, whack in some wires to connect the terminals to the resistor block. Yeah, one could gild the lily again, I used the first two copper ones that I found. Depending on the terminal type, one could also use pluggable connectors, but I decided to just solder them to the 6mm flat terminals and have some extra wire length for convenient soldering and mounting. It’s a tight fit, but the terminals do not short to the thumbwheel blocks on both ends, so that’s fine. If you use different ones or a different case, keep that in mind.
* Case: 7,46€ – quality part
* Terminals: 2,58€ – quality part
* Thumbwheels: 5,33€ (7/10 used = 30% spares) – WHL parts
* 56 SMD Resistors: 3,87€ ((0.8*7)/128 used = 95% spares) – WHL parts
* 8 THT Resistors for the middle contacts: ?? a few cents if actually purchased new for that purpose… WHL parts if available
* Two 10cm wires and some solder – whatever you got handy
* Four M3x20 screws if you do not like the supplied ones…40 cents tops
…total? 15-20€, depending on your definition of parts usage and special requests. Pretty decent for a 1% 7 decade (1Ω to 10MΩ) resistor box, I’d say
For better spec cravings, have a look at EEVBlog #461. Be sure to fasten your seatbelts, because those pornographic single-Milliohm switches come with a price – as Dave always says: “If you have to ask for the price, you can’t afford it”. 4-5 digits for 0.01%, compared to 20€ for this puppy