Fred,
By definition, a current is measured by sticking a voltmeter across a series
resistor...albeit a very very small-valued one, so as not to impede the
current flow in the circuit. It is small enough to be thought of as a wire.
Similar to what Graham was saying, I too, have used the voltage drop across
a wire to measure current. However, a ready-made shunt is preferable. It is
designed to be more stable (linearized) than a wire (over heating effects,
etc). A store-bought current meter usually has a calibrated shunt already
installed between the two terminals in the back of the meter. Large wires
need to run to this type. Alternatively, the 'Resistor' could be located
remotely i.e.. by the battery. This way large wires do not need to be run to
the instrument on the panel. It can be complicated for sure. Might be easier
to just run the heavy wires to the pre-made Current meter and call it good.
My explanations act like they've had a six-pack sometimes. Hope this makes
sense.
Greg
-----Original Message-----
From: owner-europa-list-server@matronics.com
[mailto:owner-europa-list-server@matronics.com] On Behalf Of Fred Klein
Sent: Sunday, January 19, 2014 8:20 AM
Subject: Re: Europa-List: Ammeters & Voltmeters
Frans...thank you for your thoughtful reply to my concerns, especially by
spelling things out in crayon to a guy who is
electronically-challenged...just what I needed, although, when you write:
> To measure amps, you need to have a resistor somewhere in the wire. I
> prefer to have this directly at the battery because this resistor is
> doing the least harm at that place, and if it fails, has the least
> impact on your flight. And it gives you the most useful information at
> that location.
...I must admit that I have no idea WHY a resistor is essential, nor can I
imagine what criteria are appropriate for sizing such a resistor, nor do I
understand why failure would cause minimal harm if it's located directly at
the battery...
All the best,
Fred
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