BD Sensors AX18-DPT200 Betriebsanleitung Seite 46

EN
6.6 Screening criteria for the selection of the Zener barrier
To avoid having the supply voltage fall below U
supply voltage is available when the device is being driven with full modulation. Full modu-
lation, that is, a maximum or nominal output signal (20 mA), can be achieved by applying
the maximum physical input signal (pressure).
An answer for the selection of the Zener barrier can usually be found in the technical data
for the barrier. It is, however, also possible to determine the value by calculation. If we as-
sume a maximum signal current of 0.02 A, then a certain voltage drop arises across the
series resistor of the Zener barrier in accordance with Ohm's Law. Subtracting this voltage
from the voltage produced by the power supply then gives the clamping voltage that is
present at the device when it is driven with full modulation. If this voltage is less than the
minimum supply voltage, then either a different barrier or a higher supply voltage should
be chosen.
When selecting the barrier or supply isolator, it should be noted that many ballasts
are not suitable for HART
vices that are specially developed for this type of application.
When selecting the ballasts, the maximum operating conditions as set out in the
type examination certificate must be taken into account. Refer to the current data
sheets when assessing the ballasts, so that the entire system of interconnected in-
trinsically safe components remains intrinsically safe.
, it is important to check what minimum
Bmin

communication. Most manufacturers offer a family of de-