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2.7 Relay Contact Protection and Interference
Suppression – Fig. 2.5
If the relays are used to switch loads on or off the relay
contacts can become eroded due to arcing. Arcing also
produces radio frequency interference (r.f.i.) which can cause
instrument malfunctions and incorrect readings. To minimize
the effects of r.f.i., arc suppression components are required;
these are resistor/capacitor networks for a.c. applications, or
diodes for d.c. applications. These components can be
connected either across the load or directly across the relay
contacts.
For a.c. applications the value of the resistor/capacitor
network depends on the load current and inductance that is
switched. Initially fit a 100 R/0.022 μF RC suppressor unit
115V Position
110V min. to 120V max. AC
50/60Hz
Mains
Transformer
Relay
Board
Solenoid Valves
and Heater
Indicators
Internal
Interconnection
Cable Terminals
Mains Voltage Selector
110V to120V
220V to 240V
F3
F2
0.5A (T)
5A (F)
L
MAINS
N
INPUT
N/C
RELAY
C
1
N/O
N/C
RELAY
C
2
N/O
N/C
C
CAL
N/O
N/C
OUT
OF
C
SERVICE
N/O
N/C
OUT
C
OF
SAMPLE
HEATER
N/O
+
O/P 1
-
+
O/P 2
-
Interconnection Cable from/to
Microprocessor Unit and Instrument
Components (wet section)
Fig. 2.3 Location of User Junction Box Components
(part no. B9303) as shown in Fig. 2.5A. If the instrument
malfunctions the value of the RC network is too low for
suppression and an alternative value must be used. If the
correct RC suppressor unit cannot be obtained, contact the
manufacturer of the switched device for details of the RC unit
required.
For d.c. applications fit a diode as shown in Fig. 2.5B. For
general applications use an alternative IN5406 type (600 V
peak inverse voltage at 3 A – part no. B7363).
Note. For reliable switching the minimum voltage must be
greater than 12 V and the minimum current greater than
100 mA.
230V Position
220V min. to 240V max. AC
50/60Hz
MAINS
OFF
MAINS
ON
F1 250V
OFF
PUMP
ON
User
Cable
Entry
Glands
Note. The mains and pump ON/
OFF switches are situated on the
right hand side of the junction box.
External
User
Connection
Terminals
2 INSTALLATION...
Mains 'ON'
Indicator
Mains
ON / OFF
Switch
Mains Fuse –
1A, 250V AC (T)
1A
Pump
ON / OFF
Switch
Cable Gland
5
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