ABB XSeriesG4 User Manual page 113

Appendix A 4–20 mA transmitters
Appendix A 4–20 mA transmitters
A.1 Powering 4–20 milliamp transmitters
4-20 milliamp (mA) transmitters are essentially variable constant current sources. They need to have sufficient voltage applied to
them to ensure that they will be able to drive 20 mA into a 250 Ohm load. Different transmitter technologies will have different
powering requirements.
Many 4-20 mA transmitters claim to be operational between 10 and 30 Vdc. This can be somewhat misleading. If it is a two-wire
(line powered) transmitter and it is sourcing the maximum loop current of 20 mA, 5 Vdc is dropped across the 250 Ohm resistor.
Dropping 5 Vdc across the load resistor leaves only 5 to 25 Vdc (of the before mentioned 10 to 30 Vdc) to operate the
transmitter. There is a good chance that 5 Vdc will not be enough to properly operate the transmitter at its maximum output of 20
mA.
Example: Voltage dropped across the load resistor at maximum loop current:
E = I x R = 20 mA x 250 Ohm = 5 Vdc
The figure below shows a 4-20 mA loop being powered by a 12 Vdc source, possibly a solar panel with battery backup. If the
battery has been discharged to 12 Volts, then only 7 Volts would remain to operate the transmitter. This may not be enough to
fully operate the transmitter at higher loop currents.
4-20ma
Assume 20ma
VCC
Transmitter
AI - AI +
- -
+
+
250Ω
5VDC 7VDC 12VDC
remaining to
source
operate Transmitter
NOTE: The following discussion does not pertain to 1-5 Vdc transmitters.
Totalflow flow meters employ a 250 Ohm resistor to convert the 4-20 mA transmitter current loop to a 1-5 Vdc signal. In some
meters this conversion is made by attaching a 250 Ohm resistor across the 4-20 mA input pins (see next figure, schematic A).
Some meters use a small three-pin jumper to select an onboard 250 Ohm resistor (see next figure, schematic B). Still other
meters employ an electronic switch to select the 250 Ohm resistor (see next figure, schematic C). In the schematics, VCC
represents the voltage source used to power the transmitter. In many instances, VCC would be tied to the meter's VBATT.
With a solar-powered system, common for Totalflow installations, available voltage to power the 4-20 transmitter can become an
issue. Batteries in a battery-backed solar installation can drop to 12 Vdc or less. Some transmitters may not be able to drive 20
mA into a 250 Ohm load with only 12 Vdc applied to them. Fully charged batteries, connected to an operational battery charger,
may be sitting at about 13.5-14 Vdc.
XSeriesG4 Remote Controller User's manual 2103334 Rev. AB | 101