Rockwell Automation 1770-HT8 SMART TRANSMITTER - Manual (Page 122)

Cable Length and Power Supply
Requirements
Appendix C
C-5
3. Connect the wire from the negative terminal in the power supply to
pin 3 in the 17 pin connector on the cable.
4. Insert the connector on the cable into the 17 pin connector header on
the Terminal Block.
5. Turn on the power to the power supply.
If loop power is to be supplied to HART field devices via a Terminal
Block, several factors have to be accounted for in the calculation of the
voltage required of the external 4-20 mA power supply. These include:
the supply voltage required by the HART field device (varies from
device to device)
the wire gauge and length of the 4-20 mA loop
the DC resistance of the Terminal Block in relation to the 4-20 mA
current loop
the voltage drop across one of the Terminal Block’s diodes
These factors have been incorporated in the following formula :
V
supply
= V
device
+ (2
i l r) + 514.25 i + 0.25
where:
l is the cable length in feet
i is the maximum loop current
r is the cable resistance per foot.
This formula was used to calculate the voltages required of power supplies
for the Rosemount 3051 and 3044 with different cable lengths and wire
gauge. The 3051 requires 10.5 volts and the 3044 requires 12.5.
Cabling and Power Supply
Requirements for Loop Power