ProtEX-MAX PD8-7000 Explosion-Proof Temperature Meter Instruction Manual
19
Relay Connections
Relay connections are made to two six-terminal connectors labeled RELAY1 – RELAY4 on Figure 2.
Each relay’s C terminal is common only to the normally open (NO) and normally closed (NC) contacts of
the corresponding relay. The relays’ C terminals should not be confused with the COM (common) terminal
of the INPUT SIGNAL connector.
Figure 13. Relay Connections
Switching Inductive Loads
The use of suppressors (snubbers) is strongly recommended when switching inductive loads to prevent
disrupting the microprocessor’s operation. The suppressors also prolong the life of the relay contacts.
Suppression can be obtained with resistor-capacitor (RC) networks assembled by the user or purchased
as complete assemblies. Refer to the following circuits for RC network assembly and installation:
Figure 14. AC and DC Loads Protection
Choose R and C as follows:
R: 0.5 to 1 Ω for each volt across the contacts
C: 0.5 to 1 µF for each amp through closed contacts
Notes:
1. Use capacitors rated for 250 VAC.
2. RC networks may affect load release time of solenoid loads. Check to confirm proper operation.
3. Install the RC network at the meter's relay screw terminals. An RC network may also be installed
across the load. Experiment for best results.
Figure 15. Low Voltage DC Loads Protection
RC Networks Available from Precision Digital
RC networks are available from Precision Digital and should be applied to each relay contact switching an
inductive load. Part number: PDX6901.
Note: Relays are de-rated to 1/14th HP (50 watts) with an inductive load.
C
NO
NO
NC
NC
C
RELAY4
RELAY3
4
3
6
5
2
1
C
NO
NO
NC
NC
C
RELAY2
RELAY1
4
3
6
5
2
1
C
R
C
R
Use a diode with a reverse
breakdown voltage two to three
times the circuit voltage and for-
ward current at least as large as
the load current.
