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Resistance Decade Box

This is surprisingly fragile and easily damaged by excess current, by accidentally setting it to ~0ohms. The maximum power dissipation is 1W per resistor; obviously the maximum voltage varies according to the selected resistance.

How could you do that by mistake? Examples include:

  • connecting it across a power rail, and accidentally selecting a low resistance
  • op amp circuits with a virtual earth configuration is equivalent to a PSU rail; large currents might flow through a low resistance
  • breadboards, besides being the work of the devil, are a fruitful source of misconnections and flapping wires with who knows what voltage on them
  • using it to limit current; just don't!

The principal uses of resistance decade boxes are:

  • in one arm of a Wheatstone Bridge
  • in series with another fixed resistor, to tweak that resistor's value

Do not, under any circumstances, look at the prices of replacement resistance decade boxes in the RS catalogue, unless there is a trained first-aider within earshot.

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