FF Device current

[sri1234]

Is there any one can specify what is the maximum bus current that a FF device can consume if it is bus powered? At the same time how much current it should source when it is tranmitting the data?

[Mike ONeill]

The individual product manufacturers datasheet(s) should specify the current draw for the device(s) on the segment, and the total segment current requirement is that total plus 10mA for data transmission.

[aagostin]

In the to my knowledge only study ever performed, the highest current consumption was 26mA. See this thread here: http://forums.fieldbus.org/showthread.php?t=2205
Note that most wiring blocks today limit the nominal instrument current to a maximum of 40mA.

[Zonderman]

The power consumption of each field device is specified in de CFF file of each device as powerequired. I did do a small investigation based on the capability files posted at the Foundation website and datasheets.

The majority of the devices require less than 20 mA, it peaks to 42 mA, and even one device specifies 200 mA. I do have the impression that these figures are not always properly maintained. As I know some of these devices are externally powered, not drawing any current from the segment while still a current is specified.

Not sure why you were asking the maximum current of a field device. Most device couplers have a short circuit protection at 45 to 58 mA.

Please also be aware that device couplers with intelligence like short circuit protection or automatic termination also consume power from the segment. This varies from just less than 10 mA to over 50 mA.

Kind Regards,

Jan Rieks Zonderman

[jberge]

The standard does not set any max upper limit. The manufacturer can theoretically make devices that draw a lot of power. For instance, with FF would be possible to make two-wire magnetic flow meters for large line size, low conductivity, and noisy stuff like paper pulp and other suspended solids. Two-wire coriolis flow meters also for large line size and gas would be possible. Analyzers also require more power and can benefit. The drawback of higher current is shorter wire due to larger voltage drop. The fact that FF is not limited to 4 mA opens up many posibilities for new innovative devices. I personally believe that as the number of FF installations are now growing, we will see more and more innovative devices becomming available that make use of the additional power and the "always on" communication. This is a good infrastructure to build plants on so you can take advantage of these products as and when they become available.