VEGA Z728 Zener barriers - Manual (Page 20)

20
Zener barriers - operating instructions
Application examples
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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Solenoid valves
The simplest and most economical solution is a single channel
Zener barrier, with the power supply grounded on its safe side.
If the valve requires 30 mA at a minimum 12 V, then at a supply voltage of 27 V, 4 V would remain for the voltage drop
through the field wiring. The system is approved for
[EEx ia] IIC.
If the switch is in parallel circuit with the nominal mains voltage,
it is usual to use a barrier combination of 28 V, 300 Ohm and a
28 V diode return. In this solution, special attention has to be paid to the voltage drop in the barrier, since the diode return
causes an additional loss of voltage. The system is approved
for [EEx ia] IIC.
Switch status
In the traditional method of switch status detection, the switch
is provided with noble metal contacts suitable for low voltages
and currents. A ground fault in any field wire leaves the relay in
the de-energised state, despite the switch being closed. This
problem is solved by the use of quasi ground-free wiring. At a nominal voltage of up to 27 V, a typical coil with 12 V and
approx. 350 Ohm can be used to match the power. The Zener
barrier is approved for [EEx ia] IIC. Negative polarities can be
accommodated with the Z887.
Hazardous area Safe area
Z728
x3
CL
1
2
8
7
Hazardous area Safe area
Z787
x3
x3
1
4
3
2
8
5
6
7
Hazardous area Safe area
Z787
Z887
x3
x3
1
4
3
2
8
5
6
7