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Old 1st Jun 2020, 5:36 pm   #1
Skywave
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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.

Al.
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Old 1st Jun 2020, 6:41 pm   #2
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Default Re: Designing a multi-range milli-ammeter

One way is a string of resistors across the meter, you tap up the string for lower current ranges. OK you get a little more voltage drop, but not much.
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Old 1st Jun 2020, 6:44 pm   #3
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Default Re: Designing a multi-range milli-ammeter

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Old 1st Jun 2020, 8:06 pm   #4
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Thumbs up Re: Designing a multi-range milli-ammeter

Thanks Merlin.
For some unknown reason I had considered that arrangement - which I know I've seen before (and have probably used it in the past) - but discounted it on the grounds that it didn't solve the problem. Obviously, I was mistaken . Perhaps that error was because I considered it during one night recently at 3 a.m. when I was subject to one of my insomnia sessions . OTOH, it might have been due to the 'advancing years'
Strange how my search of the 'Net failed to reveal it.
However, prompt answer now provided by you - thanks again.

Al
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Old 1st Jun 2020, 8:39 pm   #5
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Arrow Re: Designing a multi-range milli-ammeter

Edit to the above.

I've just reviewed my original idea. I had arranged the switching so that the meter was always in cct. with series-connected resistors in shunt with it and the junctions of those resistors to the selector of the switch. Therefore if that selector failed to make contact with the shunt chain, the meter takes the full current. That's why I discounted it.
But in your arrangement, the series-connected shunts are always across the meter with the switch selector connecting to the junctions of those shunt Rs - and that makes all the difference, of course!

The following extract from the book "Most Secret War' by R.V. Jones says it all:
Horace Darwin, the founder of the Cambridge Instrument Company, stated "Whenever you think that you have made a good design, try reversing the arrangement of some of the parts - it might work better".

'Nuff said!

Al.
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Old 1st Jun 2020, 11:51 pm   #6
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Arrow Re: Designing a multi-range milli-ammeter

My Q. has been answered; at present, I've nothing to add to this thread. However, others here might choose to.
So I'll leave it to the moderator(s) to decide if and when this thread is closed.

Al.
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Old 2nd Jun 2020, 10:49 pm   #7
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Default Re: Designing a multi-range milli-ammeter

In terms of meter protection, I recall seeing a simple circuit using an op amp which could not exceed certain output level to drive the meter. It may have been designed with the issue of Avo VCM 50uA meters in mind. I did note that you refer to "simple designs", but think this still qualifies?

B
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Old 3rd Jun 2020, 12:58 am   #8
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Default Re: Designing a multi-range milli-ammeter

There is another circuit that uses one transistor and shunt resistor to protect a meter movement.

As the current rises above a certain level, the transistor turns on and shunts the meter movement.

My google foo is failing me this morning and I can't locate an actual drawing as an example.

Of course, there is also the old standby of anti-parallel diodes across the meter terminals. While they may not stop the needle pinning, they will in most cases protect the coil from suffering damage.
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Old 3rd Jun 2020, 11:13 am   #9
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Default Re: Designing a multi-range milli-ammeter

If you have the meter always in circuit and use the switch to connect various shunt resistors across it, then dirty switch contacts will effectively increase the resistance of the shunts, and seriously degrade accuracy. The circuit in post #3 avoids that problem while still using only a single pole switch. That’s why pretty much every analogue multimeter from the AVO 8 down uses it.

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