Answer / carl_ellis

For a 2 wire loop powered transmitter, a DC power supply
supplies power to the transmitter to power its electronics.
The transmitter uses about 3.5mA of the loop current to run
its electronics.

The transmitter's electronics measures a physical property
(temperature/pressure) which creates a measurement value
associated with engineering units. The transmitter
regulates the loop current between 4.0 and 20.0 mA (by
varying its internal resistance to drop whatever voltage is
needed to create the correct current) so the current
represents a measurement proportional to the scale
configured for that physical property.

The controller/indicator is the primary 'load' with an
analog input that has a dropping resistor. The dropping
resistor creates an IR (voltage) drop the
controller/indicator uses for its A/D conversion.

A 2 wire loop powered transmitter has a lift-off voltage,
the minimum voltage needed to operate the transmitter with a
zero ohm load. As the loop load voltage increases, the
minimum voltage required to power the transmitter and the
loop load increases.

24Vdc is the typical, nominal power supply voltage used in
the process industries. Many 2 wire loop powered
transmitters will function with supply voltages up to about
40Vdc. Higher supply voltages are needed with long copper
wire runs where the wire resistance becomes a significant
load factor.

Carl Ellis
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