Answer / carl_ellis

Yes, V=IR, but that happens because the transmitter
functions as a transistor, varying its resistance in order
to regulate the loop current.

It is expected that the power supply voltage remains constant.

The transmitter acts as a transistor and regulates the loop
current as long as there is sufficient power supply voltage
to drive the current through the loop resistance (ohms law).
If the power supply voltage is not adequate, the loop
current will be limited to something less than a full 20mA
and in some cases the transmitter power-cycles itself and
starts through repeated initializations, trying to drive
current, but failing to do so.

Changing the power supply voltage will not change the loop
current as long as the power supply voltage change is above
the minimum needed, because that is the design
intent/purpose of 2 wire loop transmitters: to regulate the
loop current to match the process variable the transmitter
is measuring. Any excessive power supply voltage is
'dropped' in the transmitter in order to regulate the loop
current.

Every 2 wire loop powered transmitter has either an equation
for calculating the voltage required for a given loop load
resistance or a graph called a 'load line' that shows
voltage versus loop resistance. The minimum required
voltage to light up the transmitter is called "lift-off"
voltage, which is the minimum required when a power supply
is directly connected to the transmitter, like on the shop
bench, with no load; no analog input, no appreciable wire
resistance.

A typical 24Vdc power supply will drive 500 ohms of loop
resistance with most 2 wire transmitters.

Carl Ellis
Measure First

Answer / manohar m chotrani

In an electronic transmitter power supply varies with the
load attached.Normal power supply is 24V DC for a
transmitter. of this 12 volts are required to run the
transmitter without any load.This will give 4-20 mA as
output as we go on increasing the load we have to increase
the supply voltage to get a 4-20 mA current. Every load of
50 ohms require 1 volt increase in the supply voltage but
upto a certain limit like ABB made transmitter can take load
upto 2020 ohms with the 55 volts as supply.Beyond 55 we can
not increase the supply because operating voltage is 12-55
volts. If we increase the load beyond 2020 ohms with 55
volts current output will be affected.There is no voltage
regulator in the transmitter

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