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resistance-decade-box [2017/04/20 18:03] (current)
tggzzz created
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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.
 +
 +{{ :​test_equipment:​resistancedecadeBox.jpg?​direct&​200 |}}
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