8 decade resistor box

Building my own

I saw a video on YouTube about a resistor box and thought to myself: well, that’s a nice project… So I decided to go and build one. With my local electronics shop, Okaphone, I ordered the parts I needed:

  • Resistors of 1% precision:
    • 9 x 1 Ω
    • 9 x 10 Ω
    • 9 x 100 Ω
    • 9 x 1K Ω
    • 9 x 10K Ω
    • 9 x 100K Ω
    • 9 x 1M Ω
    • 9 x 10M Ω
  • 8 x 12 position rotating switches
  • 1 Enclosure

Now the building could start. With a little patience I soldered the 72 resistors to the switches. These switches by the way could be restricted in their movement, so I altered them to do only 10 steps, from 0 to 9 in every decade.

Photos

Soldering, as you might be able to see:
I soldered the first (rightmost) switch the wrong way around and corrected that.
And there it is, on the shelf above my workbench.

Lets measure.

As you can see, with all switches in position zero, my bench meter indicates 0,146 Ω

The bench meter fortunately has a ‘REL’ function, which switches to zero at the current resistance value, so all next meetings will be relative to this value.

Furthermore, I am using the 4 wire measurement method that eliminates the resistance of the measuring leads. This can be seen below, two wires (with banana plugs) from the decade box, and two red and two black wires to the meter (as seen above).

Measuring with the 4 wire measurement method
The meter set to ‘REL’, thus 0.000 indicating 0.000 Ω

Results

Now I started the measurement with all switches on the zero Ω position. For every individual decade I jotted down the measured values in an spreadsheet and generated a graph:

Graph of value deviations per ranger in %

As you can see, almost all value deviations over all ranges stay under or around 0,3%. Only the highest decade (0 – 90M Ω) runs up to just over 0,8% deviation. I ascribe that mainly to the lack of digits in the bench multimeter.

Conclusion

It was a fun project to do, with a rather nice result. It’s not necessarily a professional high precision instrument, but you would have to pay top dollar for those.