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Installation
MXD70 Auxiliary mA Input
Setup and Operating Guide
- 7 -
Ins
tallation
Installation
The MXD70 series Auxiliary mA Input Card allows the user to read the current output of a variety of
loop powered and self powered transmitters.
Self Powered Transmitters
For self powered transmitters the current input of the input card is isolated from the instrument‘s
power supply thus allowing the input to be connected in series with other devices on the loop if the
loop is fed from a single ended transmitter.
Loop Powered Transmitters
For loop powered transmitters the following information may need to be considered:
Loop Voltage Drops
One of a current input instruments most important specification is the total resistance or burden it
presents to the connected transmitter’s output driver. Most transmitters’ data sheets specify the
maximum loop resistance the transmitter can drive while still providing a full scale 20mA output (the
worst case level with regards to burden).
Therefore knowing the input impedance of the MXD70 Auxiliary mA Input Card and assuming the
maximum current developed in the loop will be 20mA. By using ohms law the maximum voltage drop
of the current input is as follows:
Current Input Maximum Voltage Drop = 100Ω x 0.020A = 2 Volts
Transmitter Ratings
The maximum power dissipation of the transmitter can be calculated by combining all the voltage
drops in the loop with the minimum operating voltage of the transmitter, take this number away from
the current loop operating voltage and them multiply it by the maximum loop current. If the power
dissipation is too high then the user will need to externally power the current loop with a lower
voltage.
Wiring Resistance
In addition to any voltage drop caused by the transmitter and the current input circuit the user will
also have to take into account any voltage losses caused by the wiring resistance. This voltage loss can
be calculated by multiplying the combined resistance to and from the transmitter by the maximum
current flowing through the wire. This figure along with the voltage dropped by the transmitter and
current input circuit will define the minimum operating voltage of the loop.
V
min
= V
Tmin
+ V
CIPmax
+ (0.02 x R
Wiring
)
Where: V
min
= Loop minimum supply voltage
V
Tmin
= Transmitter minimum operating voltage
V
CIPmax
= Current Input Maximum Voltage Drop
R
Wiring
= Wiring resistance = 2 x Distance x Cable Resistance (typically 0.035Ω/m)
3 Wire Transmitters
For low powered 3 wire transmitters the input card can supply a 24V 30mA output via the 24V
connection, thus allowing for the removal of an additional external power supply to the transmitter.