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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 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 (part no. B9303)
Mains
Transformer
Relay
Board
Solenoid Valves
and Heater
Indicators
Internal
Interconnection
Cable Terminals
Mains Voltage Selector
110 V to120 V
220 V to 240 V
L
N
E
N/C
C
N/O
N/C
C
N/O
N/C
C
N/O
N/C
C
SERVICE
N/O
N/C
C
HEATER
N/O
+
-
+
-
Interconnection Cable from/to
Microprocessor Unit and instrument
components (wet section)
Fig. 2.3 Location of User Junction Box Components
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 number B7363).
Note. For reliable switching the minimum voltage must
be greater than 12 V and the minimum current greater than
100 mA.
MAINS
INPUT
RELAY
1
RELAY
User
2
Cable
CAL
Entry
Glands
OUT
OF
OUT
OF
SAMPLE
ON/OFF switches are situated on the
O/P 1
right hand side of the junction box.
O/P 2
External
User
Connection
Terminals
2 INSTALLATION...
Mains 'ON'
Indicator
MAINS
Mains
ON / OFF
Switch
OFF
MAINS
Mains Fuse
ON
2A
F1
OFF
Pump
PUMP
ON / OFF
Switch
ON
Cable Gland
✶
Note. The mains and pump
5
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