BEKA BA678C, Руководство пользователя, Страница: 5 / 27

4. ELECTRICAL SYSTEM DESIGN
The BA678C is a smart HART

enabled loop
powered 4/20mA Indicating Temperature Transmitter.
For installation in poorly illuminated areas the
BA678C is available with an optional factory fitted
display backlight.
No additional wiring of power
supply is required, but the minimum operating
voltage of the transmitter is increased as shown
below.
without backlight 9.0 to 28V
with backlight 15.5 to 28V
Fig 2 Typical BA678C loop
When designing a transmitter loop it is necessary to
establish that the sum of the voltage drops caused by
the transmitter, load and the cable resistance is less
than the minimum supply voltage.
For the transmitter loop shown in Fig.2
Minimum operating voltage of BA678C 9.0V
without optional backlight.
Maximum voltage drop caused by 5.0V
250 Ω load.
(250 Ω x 20mA)
Maximum voltage drop caused by 0.2V
cable resistance.
(10
Ω x 20mA)
______
Total maximum voltage drop
14.2V
The power supply voltage must therefore be above
14.2V at all output currents.
4.1 Use as a temperature indicator
The BA678C may also be used as just a temperature
indicator by ignoring the instrument’s 4/20mA output
current. The load shown in Fig 2 may be omitted and
terminals 5 & 6 connected directly to the power
supply.
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