Milliamp Transmitters; Powering 4-20 Milliamp Transmitters - ABB XSeries G4 6200 User Manual

6.3 4–20 milliamp transmitters
6.3.1 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 insure that they will be
able to drive 20 mA onto 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, but
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 only leaves 5 to 20
VDC (of the aforementioned 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 the 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. This
could be 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.
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 the
next figure – schematic A). Some meters use a small three-pin jumper to select an
onboard 250 OHM resistor (see the next figure – schematic B). Still, other meters
employ an electronic switch to select the 250 OHM resistor (see the next figure –
schematic C). In the figures , VCC represents the voltage source used to power
the transmitter. In many instances, VCC could be tied to the meter's VBAT.
With a solar paneled system, such as many 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.
Page 34
Assume 20ma
AI -
-
+
250Ω
5VDC
The following discussion does not pertain to 1-5 VDC
transmitters.
4-20ma
Transmitter
AI +
-
+
7VDC
remaining to
operate Transmitter
VCC
12VDC
source
2104349 rev. AD