BEKA BA314E, Manual

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9.4 4/20mA output
The BA314G Tachometer can be supplied with a
factory fitted galvanically isolated 4/20mA output
which may be configured to represent any part of the
Tachometer speed display.
9.4.1 Intrinsic safety
The 4/20mA output has been certified as a separate
galvanically isolated intrinsically safe circuit complying
with the requirements for
simple apparatus. This
allows terminals C1 and C3 to be connected to any
intrinsically safe circuit protected by a certified Zener
barrier or galvanic isolator providing the output
parameters do not exceed:
Uo = 28V dc
Io = 200mA
Po = 0.84W
The maximum equivalent capacitance and inductance
of the pulse output is:
Ci = 2.2nF
Li = 4µH
To determine the maximum permissible cable
parameters, these figures should be subtracted from
the maximum cable capacitance and inductance
specified by the certificate for the Zener barrier or
galvanic isolator powering the 4/20mA output circuit.
9.4.2 System design
The Tachometer 4/20mA output is a passive current
sink i.e. not powered, but it is totally isolated from all
other Tachometer circuits. It is effectively a 2-wire
4/20mA transmitter requiring a minimum supply of 5V
with its current being controlled by the Tachometer
speed display. Subject to complying with intrinsic
safety interconnection requirements, the terminals C1
and C3 may be connected to another instrument,
which will accept a 4/20mA transmitter input. The
4/20mA current output may also be transferred to the
safe area via a galvanic isolator or Zener barriers.
Terminals C2 and C4 are internally linked and may be
used for joining a return 4/20mA wire.
Fig 17 shows how a 2-channel Zener barrier may be
used to transfer the 4/20mA current output into the
safe area, alternatively a galvanic isolator may be
used.
Fig 17 Application of 4/20mA output
9.4.3 Configuration and calibration
When a Tachometer is supplied with an optional
4/20mA output the configuration menu is extended
as shown in Fig 18. The 4/20mA output sub-menu
is accessed via the
4-20 oP
function.
The 4/20mA output is controlled by the Tachometer
speed display, the speeds corresponding to 4 and
20mA output are defined in the sub-menu.
Fig 18 4/20mA output configuration sub-menu
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