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in transmiters(rose mount) how to check output 4-20ma ? and
input 24v? i need exact explanation?
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Answer Posted / carl_ellis
1) Answer #1 is wrong about testing the power supply voltage
With a 24Vdc power supply, the voltage across the
transmitter terminals will never be 24Vdc because a 2 wire,
loop powered transmitter is a current regulator.
A current regulator operates by changing the voltage in the
circuit to regulate the current. The transmitter consumes
some current to operate (3.5mA) and regulate the loop current.
To repeat, to regulate the current, the voltage MUST change.
A minimum loop current of 4mA MUST drop voltage across the
transmitter terminals, it's basic physics.
If one reads 24Vdc across the transmitter terminals, then
there is no loop current flowing because the loop has an
open circuit!
Typically, the 24Vdc power supply positive connects to the
(+) terminal on the transmitter.
If the transmitter is operating and is wired to a typical
analog input with a 250 ohm input resistance, then the
voltage across the (+) and (-) transmitter terminals will be
23Vdc at 4mA loop current (1 volt dropped across the input
at 4mA)
19V at 20mA loop current (5 volts dropped across the input
at 20mA).
The sum of the voltage drops through the loop must equal the
power supply voltage. 23V + 1V = 24V; 19V + 5V = 24V
If the loop is opened by removing the wire from negative
terminal so that no current flows in the loop, then a
voltage test measuring from positive terminal to the loose
hanging wire (that eventually connects to the negative side
of the power supply) will show 24Vdc because the high
impedance of the voltmeter limits any current to the
pico/micro amp level, practically an open circuit.
So, making a voltage measurement across the transmitters (+)
and (-) terminals will NOT show 24Vdc (the power supply
voltage).
2) To check the milliamp output, it is correct that one can
remove the wire.
But nearly all 1151 and 3051 Rosemount pressure transmitters
have a 'test' terminals whose purpose is to allow a loop
current check without opening the loop.
When a milliameter (+) probe is connected to the (+) test
terminal and the (-) probe is connected to the (-) common
terminal, the milliammeter will indicate the loop current.
Using the test terminals does not break or open the loop
circuit.
A HART communicator will show the loop current, also.
Carl Ellis
Measure First
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