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Old 1st Jun 2020, 5:36 pm   #1
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Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 6,850
Question Designing a multi-range milli-ammeter

Here is question (and its corresponding answer) which I am surprised I've never encountered before, but it is a basic Q. about analogue instrumentation.

For a simple multi-range milli-ammeter, it is usual practice to incorporate some type of switching so that various resistances can be selected in shunt with a basic movement to obtain the necessary FSDs. However, a moment's consideration shows that such a switching arrangement - typically a single-pole, multi-way rotary switch - will need to be of the 'make-before-break' type, so that at all times there is some resistance in shunt with the indicating meter. However, should a fault develop with that switch - even if intermittent or a resistive dirty contact - whereby for even a very brief period of time during a range-changing operation, a resistance fails to be in shunt with the meter, that meter is highly likely to suffer serious and permanent damage. E.g.: a 1 mA movement, with shunt to produce an FSD of 5 Amps. If, during measurement on that range, the shunt effectively goes O/C . . . exit the meter!

Thus my Q.: what alternative methods of shunt selection (and / or protection) can be used to assuredly prevent such a disaster? The answer may, or course, be simple, but I cannot think of one; can you?

All replies will be appreciated.

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