Power supply via bus cable, calculation

Supposed all devices are supplied externally, the supply has to feed the PDP21 or PDP22 connected to the bus. The supply current depends on the voltage (typical values):

Supply voltage	19.2 V	24 V	31.2 V
FBP supply current typ.	46 mA	37 mA	31 mA

To calculate the needed current per S500-FBP slave equipped with a FBP, you should add the supply current of the remote station itself to the above FBP needed current.

Module	DC505-FBP		DI524, DC532, DX522, DX531, AX522
Supply voltage	19.2 V	24 V	19.2 V	24 V
FBP supply current typ.	18 mA	12 mA	5 mA	4 mA

Example of needed current

Station with only 1 x DC505-FBP + PDP22, total current needed with 19.2 V DC: 54 mA (46 + 18 mA)

Station with 1 x DC505-FBP + 7 x DC532 + PDP22 at 19.2 V DC supply: 99 mA (46 + 18 + 35 mA)

To simplify the calculation, the scheme below uses the highest values of the currents, but - on the other hand - does not regard the increased copper resistance and voltage drop for higher environment temperatures.

Note

All slaves, even the slave most distant from the supply unit, need to be supplied with min. 19.2 V DC including ripple. That means that the power supply unit at the beginning of the line has to provide a higher voltage to compensate the voltage drops due to the line resistance.

Use the Excel sheet “PROFIBUSâ DP Supply via Bus Cable Calculation.xls” from the FieldBusPlug tools to estimate the size, nominal voltage and power of the needed power supply. This Excel sheet can be found on the ABB website.

Practical example

DP Network with 1 master and 8 slaves located in 3 cabinets/installations with the following splitting:

• 1st cabinet with master device (AC500 + DP communication module) located in a cabinet also containing 2 remote I/Os DC532. Distance between the slaves about 2 m.
• The distance to the following cabinet is 200 m.
• 2nd cabinet with 3 remote I/Os (1 x DC505-FBP + 2 x DC532), distance between the slaves about 2 m.
• The distance to the following cabinet is 100 m.
• 3rd cabinet with 3 remote I/Os (1 x DC505-FBP + 2 x DC532), distance between the slaves about 2 m.

With a voltage at the end of the bus of min. 19.2 V DC, the current consumption per module is as follow according to the previous table:

FBP + DC505 + 2 x DC532 = 46 + 18 + 10 = 74 mA per remote station.

Use the calculation sheet of the FBP products, adapt the values to those defined before.

The calculation sheet delivers the following result:

The recommended power supply unit can be adjusted to 28 V DC:

Power Supply 24V / 5A adjustable, order code: 1SVR 423 416 R0100, type: CP-24/5.0