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Aug 06, 2015, 01:36 AM
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uCurrent mods

Reviewing the schematic for the uCurrent, the key to its ability to change settings without interrupting the circuit is switches with 2 contact points. The resistor network is always in the circuit path. Only the SHORT option gives the current a shorter path, so avoiding accidentally switching to nA requires being in SHORT mode.

The sacrifice in using dual contact switches is 2 resistors always have to be in parallel. Working around that requires the ranges to be powers of 1000. The resistances have to be in ascending order & all 3 resistors have to be populated. The output of each resistor is always amplified by 100x using the same amplifier chain.

It really is a bit of slight of hand that lets the uCurrent work the way it does. It was one of those unique inventions born from a once in a lifetime observation developed over a lifetime of experience.

The easiest way to get the coveted 2mA-10mA range is replacing the 10R with 0.1R & replacing the 10k with the 10R. This would create a new 10mA range with 9.9mV per mA. The mA range would then give 0.9mV per mA & the uA range would still give 1mV per uA. It might be good enough, but hard to read in the 10mA range.

Unfortunately, the 0.01R resistor can't be trimmed to get 1mV per mA again. It's a special resistor with sense terminals on the substrate.
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