Black 0-9 Digits Single Unit Pushwheel Thumbwheel Switch (WHL #11)
Folks, this is a big one. I will split into two articles to make the item review available now and show the finished project later (once I got a box around it). Maybe even three articles to post DMM comparison data. Today, I will show the unit and the first measuring results.
Back in January ( ) I bought a 10-pack of these dials for 5.33€ including shipping at AliExpress. I saw these in the EEVblog forums and decided to have a go. And now you know the purpose of the assorted SMD resistors in WHL #10
First things first: Shipping took ages, and quality is very Wan Hung Lo’ish. While the case is of good quality and does stack well, the internal springs that make the transitions between the digits possible are totally out of spec. I don’t know if they are of the wrong material, or thickness, or length, but they do not work properly for almost half of the units. So I complained and got a partial refund. Good thing that I won’t need all of them for the resistor box project.
The construction is straight forward, and schematics are provided in the EEVblog forums as linked above. Note that there are other versions of these pushwheel switches that have different codings (and therefore less contacts) and are not usable for such a project — but maybe for others.
These here have one contact for every digit on the wheel, the main contact is placed in the middle. Once a digit is set, the wiper will short circuit the main contact with the corresponding contact (nicely labelled, btw). There is a (+) and a (-) button, so adjusting is possible in both ways. This is especially important if you have a setting that will switch back to all-zero when going up, and maybe you do not want a short-circuit while doing that…so you might wanna go 900Ω – 1900Ω – 1000Ω instead of 900Ω – 0Ω – 1000Ω
So building a resistor box is quite easy. Put a larger (THT) resistor across the center tap, tin the other contacts and throw your SMD resistors onto it. I started soldering these units as first practice session with my new Ersa i-con Pico soldering iron, and the result could be worse Note that the THT resistors should of course match your SMD ones in quality – I didn’t have precision types for every resistance value, so I selected with my multimeter. Again, could be worse, my picks turned out very nicely even if some are 5% types.
Now do that for all decades — it is a good idea to label the finished units so that you don’t put them into the final stack in the wrong order. Also, test each individual unit before final assembly, as changes are much easier in this stage. I did soldering in the final stack, but that is very touchy with SMD parts in that limited space. I also had the great idea to use the hot air gun for smooth reflow…mmh nope, don’t do that. Now my 1M decade is a tad roasted
Finished stack from the front:
And the back:
Adding wires, connecting to proper terminals and putting everything into a box: Coming soon…
However, I did measure the stack with a Keithley 2700 multimeter at work during lunch time (several, actually, as it takes a while to switch through all decades), and did a comparison with my cheap Peaktech 2010 DMM and Uni-T 203A at home. Even though the Keithley is a pain in the ass to work with when programming that thing, I very much trust the results, especially when doing 4-wire measurements. And they are not too far away from those of my own multimeters, so I’m very happy they do work accurately and don’t drift much. I will post the results in a future WHL article.
Speaking of drift: I know that the resistors are specced at 1%, but temperature coefficient is unknown. So in addition to the 10000 ppm general spec, there might be 200 ppm/K on top of that. As I have no temperature-controlled office, that can add another 1000 ppm or 10% to the initial resistor spec per 5°C. And that’s easily achieved by just holding the resistor box in your hands, or putting it near a warm power supply, your soldering iron, or below some old-fashioned lights.
First round of test results:
Not too shabby in general, but also not entirely inside specs…
This however does include one important lie: The first resistor, that is, the one that would switch from 0Ω to 1Ω, is just a dead short. This is due to the contact resistances of the individual switches. I actually measured them once I found everything off by one Ohm, and what do you know, the total contact resistance of all units is around 0.9Ω or 0.13Ω per unit. Exact numbers are 97 mΩ min, 180mΩ max, avg (132±26) mΩ — throw away at least one decimal place for measurement confidence, please. But thanks for the resolution, my dearest Half Ohm
If you keep that in mind, this also explains the high readings of the 10Ω range. The (correct) 10Ω resistor is offset by 0.9Ω, so that’s 9% off in theory. The larger the value gets, the more the effect of this offset diminishes. You can still see that on the 100Ω range, but this time, we’re already in spec (10000 ppm = 1%). All readings above that are basically governed by the individual resistor tolerances and not so much by this offset, which is why they have mixed high and low readings. Especially down in the MΩ range, there are some impressive sub-1000ppm readings – YAY!
Losing the 1Ω setting isn’t that tragic – these are 0805 SMD resistors with 1/8 or 1/10 watt rating, so they aren’t intended for power applications at all. Also, I wouldn’t throw much power onto spring contacts that are 1/8 Ω…actually, I made another 1Ω -9Ω range unit with 1/4 W THT resistors (5% generic ones), and they turned out to be horrible. One even had close to 2Ω, hilarious
At this stage, one would have to keep in mind the 1Ω offset for the 10Ω and 100Ω ranges — no big deal as usually no more than E12 resistors are in use, so transferring the experimental box value into a single static resistor would fail due to the large gaps in E12 series. But there’s always room for improvement. Results are in, but I will wait until this stack has a proper housing and specs are final.
To sum that all up: Yes, those switches make great dials for resistor boxes, if you
a) order more than needed (to equalize Wan Hung Lo quality goals)
b) can live with the contact resistance (in terms of absolute precision and power rating of the entire assembly)
Considering they are 50 to 60 cents a pop…go, get some and build your own resistor box!