Re: 20mA current loop

On Jan 26, 9:00 am, kumailb...@xxxxxxxxx wrote:
I have a temperature measuring instrument giving 0 - 10 Volts (30 mA)
corresponding to 0 - 100 celsius. This voltage signal needs to be
transfered 500 feet away on an ammeter. Is this possible by just using
a series resistor? What are the drawbacks if neglecting environmental
temperatue and noise pickup disturbances.

You didn't say what sort of accuracy you need... Certainly it would
be easy to get decent accuracy if you keep the current in the wire
low, so that the voltage drop in the wire is low compared to the drop
in the fixed resistor you add (which of course should be chosen to be
stable). You need to account for changes in wire resistance and in
the resistance of the meter movement as temperature varies. A key
reason for using a 4-20mA current loop in industrial instrumentation
is that the sensors control the current, not the voltage, so voltage
drop in the connecting wire is irrelevant (up to a point). What you
propose is NOT a current loop, but should work fine anyway. Just pick
a meter with low enough full current range that the change in
resistance due to the wire and meter changing temperature doesn't
materially affect the current. The temperature coefficient of
resistance for copper is about 0.4%/C.

Cheers,
Tom
.

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