VEGA Z728 Zener barriers - Manual (Page 2)

2
Zener barriers - operating instructions
Operating principle
Subject to reasonable modifications due to technical advances. Copyright Pepperl+Fuchs, Printed in Germany
Pepperl+Fuchs Group • Tel.: Germany +49 621 776-0 • USA +1 330 4253555 • Singapore +65 67799091 • Internet http://www.pepperl-fuchs.com
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The operating range of a Zener barrier must therefore be such
that it is below the Zener voltage, so that the leakage current is
restricted to a minimum. Zener barriers are normally tested to
ensure that at the prescribed voltage the leakage current is
smaller than 10 µA.
Figure 1.2 This figure shows a selection of leakage currents through
the Zener barriers under normal circumstances. The
Zener barriers conduct a maximum of 10 (1) µA leakage
current so long as the supply voltage is less than 25.5 V.
This is normal and has very little effect on the load. If the
voltage exceeds 25.5 V, the Zener diodes start to
conduct more current. This can have an effect on the
operating current and the accuracy. It is therefore
recommended that a controlled voltage source be used,
which maintains the voltage under the value at which the
diodes will start to conduct.
(A 24 V, 300 Ohm barrier is represented here as an
example)
These voltages are stated in the data sheet for a given barrier,
together with the leakage current. If the leakage current for a
given voltage differs from 10 µA, this is specifically stated.
Figure 1.3 This figure shows that if the maximum permissible input
(supply) voltage is exceeded, the total current drains
through the Zener diodes, without reaching the explosive
surroundings.
Pepperl+Fuchs Zener barriers have a low series resistance,
given by the sum of the resistance R1 and the resistance value
of the fuse F1. Due to the low series resistance, an inadvertent
short-circuiting of terminals 1 and 2 can cause the fuse to blow.
In order to avoid this, some barriers are available with
electronic current limitation (CL-version).
If the Zener barriers are provided with a resistance, this limits
the short-circuit current to a safe value in the event of a short-
circuit of the connecting wiring in the hazardous area or a
connection to earth of the wiring attached to terminal 1, as the
fuse blows.
Many barriers are available with a resistance connected
between the output terminals. These are used in
4 mA … 20 mA transmitter circuits. The resistance converts
the current in the intrinsically safe circuit into a voltage that can
be measured in the safe area.
Pepperl+Fuchs Zener barriers can be used in many
applications. In the simplest case, a single channel barrier with
a ground connection is used. But in many applications it is not
desirable that the intrinsically safe circuit is connected directly
to ground. If the circuit in the safe area is grounded, under
some circumstances grounding of the intrinsically safe circuit
can lead to faults within the system. In this case, quasi ground-
free intrinsically safe circuits can be constructed with two or
more barriers. This floating circuitry can be simply achieved
with 2- or 3-channel barriers.
Double grounding of intrinsically safe circuits is not permitted.
The insulation voltage of the wiring and field devices,
measured with respect to ground, must be greater than
500 V AC. The permissible ambient temperature of the Zener
barriers is between -20 °C … 60 °C.
Power
supply
Hazardous area Safe area
Load
≤25.5 V
50 mA
24 V
(+)
(-)
10 µA
Power
supply
Hazardous area Safe area
Load
>25.5 V
50 mA
24 V
(+)
(-)