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A 6-Decade Resistance Substitution Box

Get a million resistance values with this . . .

By Jim Rowe

Yes, we know about those little “resistor substitution wheel” gadgets, which you can pick up for around $25. Generally they offer a selection of 36 different resistor values, covering a very wide range, usually between 5Ω and 1MΩ. They’re OK but you will usually find that the value you need is not present in that limited range of only 36 values.

Then you dive into your resistor stock and hope that you can find a value that will work. We’ve all been there and know how frustrating it is to find that Murphy’s Law is applicable – there are none left in the drawer concerned. In any case, you tend to end up with a motley collection of resistors on the bench, all of which have to be put back in their drawers afterwards. That’s so boring.

Resistor substitution wheels have another drawback which is that their internal resistors are usually only 5% tolerance. So even if one of the 36 nominal values turns out to be suitable for the circuit you’re working on, you still need to check the actual value with your DMM before making your final selection of the value to be used.

So what we really need is more like an old-fashioned “decade resistance box”, with a much larger selection of closer-tolerance resistance values. But those old decade boxes were big, clunky and expensive. Even the latest models are quite expensive.

So why not build your own? We have produced a compact 6-decade resistance box using readily available rotary switches and 1% metal film resistors, all mounted on a PCB to make assembly a cinch.

A million resistance values

This unit allows you to dial up a million resistance values between 10Ω and 10MΩ, selectable in 10Ω increments. It uses only 54 resistors, so if you use standard 1% metal film resistors they’ll cost you less than $3.50.

Add in the cost of a UB1 jiffy box, six standard rotary switches and knobs, a pair of binding post terminals and a PCB and it is still not a large amount – a small fraction of the cost of a commercial decade box, in fact.

For even higher accuracy, you can use 0.1% metal film resistors instead of the 1% types. These will bump up the total cost to over $100 but it will still be much less than the price of a comparable commercial unit.

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