Answer / carl_ellis

The measured voltage across a regulated power supply will
remain at 24Vdc. If the voltage across the power supply
drops below 24Vdc, then either the power supply is too small
to carry the combined load or it is faulty.

The measured voltage across the transmitter will vary
according to the loop current.

Carl Ellis
Measure First

Answer / carl_ellis

You describe a situation with a 24Vdc power supply, a 250
ohm dropping resistor and a 2 wire loop powered transmitter
at 4.0mA

To get the voltage to vary across the transmitter, you have
to change whatever the transmitter is measuring to get its
output to change, or force an output with a HART
communicator. The loop current does not vary on its own,
it varies in response to whatever the transmitter is
measuring.

Carl's Loop Rules

The voltage across the power supply remains constant: 24Vdc

The voltage drops across the transmitter and analog input
vary with the current in the loop.

Assuming a bench setup with very short wire runs and a very
low wire resistance, then if the analog input dropping
resistor is 250 ohms, then

at 4.0mA the transmitter drops 23.0V, the AI resistor drops
1.0V

at 12.0mA the transmitter drops 21.0V, the AI resistor
drops 3.0V

at 20.0mA the transmitter drops 19.0V, the AI resistor
drops 5.0V

The sum of the voltage drops must equal the power supply
voltage.

You can bank on it.

Carl Ellis
Measure First

Answer / abhi

voltage vary according to the current vary across the
transmitter. if u connect multimeter to the transmitter in
line it shows only 23v dc nearer.

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