DIY Cheap resistor decade substitution box

[Uncle Vernon]

Here's my take on them...

Nice work George. A good beginner tutorial.

[george graves]

Thanks - that's kinda what I was going for!

[Johenix]

First off, the lower resistance decades should be higher wattage resistors. (One hundred volts across one ohm would be one hundred amps or ten kilowatts, in a really ridiculous case.)

That said, let us get to the real heart of the matter.

So called "One percent resistors" are never accurate to 1%, they are just called that. They are made on 96 values in each step between 1, 10, 100, 1000, 10 000, 100 000, 1 000 000, and 10 000 000 ohms. They are spaced by the 96th root of 10, a sort of logarythmic progression, in a system that allows voltage divider ratios to be scaled up or down for more or less current.

Oh, by the way, "20% resistors" are seperated by the 6th root of 10, "10% resistors" are seperated by the 12th root of 10, "5% resistors" are seperated by the 24th root of 10, and "2% resisrors" are seperated by the 48th root of 10. 

[mzzj]

First off, the lower resistance decades should be higher wattage resistors. (One hundred volts across one ohm would be one hundred amps or ten kilowatts, in a really ridiculous case.)

That said, let us get to the real heart of the matter.

So called "One percent resistors" are never accurate to 1%, they are just called that. They are made on 96 values in each step between 1, 10, 100, 1000, 10 000, 100 000, 1 000 000, and 10 000 000 ohms. They are spaced by the 96th root of 10, a sort of logarythmic progression, in a system that allows voltage divider ratios to be scaled up or down for more or less current.

Oh, by the way, "20% resistors" are seperated by the 6th root of 10, "10% resistors" are seperated by the 12th root of 10, "5% resistors" are seperated by the 24th root of 10, and "2% resisrors" are seperated by the 48th root of 10.

What is this nonsense?

[amspire]

Johenix confused the explanation of the standard resistance ranges and the tolerance. They are two different things.

Just to be clear, a 1% resistor is definitely better then 1% accurate. Typically most of the resistors will be within 0.5% accuracy.

Accuracy doesn't always take account of long term drift, so you can have a 1% resistor that is within 1% at the time of manufacture, but after 2000 hours of use, it may be no longer 1% accurate. So it is important to look at the long term stability spec as well as the accuracy spec.

It is true that there is not much point making the full E96 range of 20% resistors but the selection of the resistance ranges is a choice for the manufacturer, and is not dictated by the tolerance.

For example, if you look at the very expensive, very accurate resistors (0.005% for example), they are often made with a relatively few standard values. The reason is that the customers often need very precise values (like 3,459.3 K) and so the customers often order custom the exact value they require rather then values from a standard range.

Johenix is correct in that the choice of standard values of the E12, E24, E96, etc ranges are approximately logarithmically spaced, but if you do the calculations, you see that the values are often a bit out - probably because someone thought that the calculated value wasn't as useful as a nearby value.

Theoretically, the E12 range should be 1, 1.2, 1.5, 1.8, 2.2, 2.6, 3.2, 3.8, 4.6, 5.6, 6.8, and 8.3.

However the actual values are 1, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, and 8.2

Richard

[babysitter]

About 1 year ago, I decided to make my own R substitution box which was basically a VTL clone, but using SMD resistors in the middle range soldered to the switches. It hasn't disappointed me yet. After my company is having a trainee who wants to go into electronics, I decided that a couple of such boxes are one of his first projects.

I should show off mine asap, but it is waiting at my desk at work to be the role model for this guys project

[kripton2035]

so should look like this one ?

[babysitter]

@kripton: very close to that, but the 1R, 10R, (100R IIRC?) and 1M as thruhole also.
The small values because of wattage, the 1M because available.

BR
Hendrik

[gerrysweeney]

If anyone is interested, I came up with an alternative low cost solution. Not perfect for everyone by any means but works for me.

http://gerrysweeney.com/seven-decade-programmable-resistor-a-low-cost-solution/

Gerry

[george graves]

First off, the lower resistance decades should be higher wattage resistors. (One hundred volts across one ohm would be one hundred amps or ten kilowatts, in a really ridiculous case.)

What kind of mad man would pump 100 volts into a substitution box - at any setting, let alone 1 ohm!?!?!?  That's crazy talk.

Anyways.  If you're interested in a pre-built unit - I have a few for sale - but the labor is a bit too much, so I can't keep them in stock for what little I'm charging:  https://www.tindie.com/products/redrocketlabs/resistance-decade-box-resistor-substitution-box/

[curiouscane]

Hello Everyone Out there,

'I too would like to build a cheap Decade Resistor Box ranging from 0.1 Ohm to 100kOhm . So i intended to purchase a "KSA-2 Decimal 10 Position Single Unit Thumbwheel Switch" but what was delivered to me was a "KM-1, 0-9 Pushwheel Thumbwheel Switch " from Ebay last week. Now i don't know how to exactly connect using a C,1,2,4,8 (pls refer the pic attached) !!! Is there any guidance you can provide on the connecting the resistors ? 

[Bored@Work]

There is no simple way to use BCD switches to build a decade resistor box. Use the switches to build a decade capacitor box http://www.electricstuff.co.uk/capbox.html or a simple constant current source / sink http://www.edn.com/design/power-management/4325377/Programmable-current-source-requires-no-power-supply

[nachbelichtet]

I've created an enclosure for 3D printing. You can grab it at Thingiverse:

http://www.thingiverse.com/thing:1079283

Thank you for the great ideal. It's really helpful!

Greetings
Markus

[timofonic]

Sorry for the necroposting

I have a ten or more of those switches from an going-to-be-repurposed CDROM SCSI box. I wanted to use them to make a resistor decade substitution box.

The big problem is that the buttons are very small, so I'm not sure if it's feasible to enlarge them or better to buy others,

Any ideas?

[kripton2035]

those buttons are so cheap from ebay that I doubt it will be interessant to try to modify yours...

[Old Grey]

The 1W is as tight as a fish's ahole.
I think I got Chinese-ed. The 1% resistors don't seam to measure 1%, but that seams to be only in the lower values.

Ah close enough, and anyhow I'll measure it and compensate with the actual value.

[Blastcap]

Here is another similar thread, for a SMD version.
https://www.eevblog.com/forum/projects/yet-another-resistance-decade-box/

[Bzzz]

Hey guys, I just published an article with a lot of BLAH about these dials in my blog, including resistance measurements of my 7-decade box. That plot is also attached for general convenience, and to spoil the results:
a) Each switch has about 1/8 ohms contact resistance, which adds roughly 1 ohm to my box -> deadly for the precision of the 1-9 ohm dial if not corrected
b) Once corrected in the first three decades, I'm able to achieve 10000 ppm over the entire range, which is the 0805 resistor spec of 1%. For values greater than 10 ohm, most of them are closer to 1000 ppm = 0.1%. And I have some lucky shots that are below 100 ppm (remember the temp co...!).
Stuff was measured with a Keithley 2700 DMM in 4-wire mode, 100 readings within 0.01% window and then rolling avg until the reading gets stable (don't breathe or move)

[advancednewbie]

I made these resistor adapter PCBs available on Tindie:  https://www.tindie.com/products/13690/

Makes it way easier to solder up all the resistors, and you can use higher wattage resistors as well.