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SCR Assembly
SCR solid state relays are used to switch a single heater
stage on and off. An intelligent (Master) relay is used
for all single phase applications. For three phase
applications, a Master-Slave configuration is used to
switch two legs of three phase power to the heater
stage. The Master Relay controls one leg
independently, and provides a control signal to the
Slave Relay, which controls the second leg.
Figure 48. Typical heat sink fin orientation
Every Master or Master/Slave relay combination is
mounted on a heat sink to prevent the relays from
VAV-SVX08R-EN
overheating during normal operation. The SCR
assembly must always be mounted with the heat sink
fins oriented vertically (see above) with a minimum
clearance of 0.5 inches on all sides of the heat sink for
cooling. Where additional cooling slots are provided in
the heater, the VAV unit must be mounted with a
minimum of 1 inch clearance in front of the slots.
When the 0-10 VDC control voltage is present at the
Master relay in both single and three phase
applications, an ON indicating LED light on the Master
relay will blink continually (approximately 0.5 seconds
on, 1.0 seconds off). If the load side voltage is also
present at the relay(s), the Master relay LED and the
Slave relay LED blink rate will increase as the control
signal increases. When the control signal reaches a
maximum of 10 VDC, all LEDs will be lit continuously,
indicating that the relays are full-open and
continuously conducting. If the control signal is present
in three phase applications while the load side voltage
is not, only the Master relay LED will light and the blink
rate will be constant.
If the SCR assembly does not appear to be functioning
properly, verify that all required voltages are present
and all that all wiring is properly connected. If all these
conditions are verified, and the SCR LED does not light
and/or the SCR relay does not conduct, the entire SCR
heat sink assembly should be changed out. Individual
Master or Slave relays should not be replaced.
M M a a i i n n t t e e n n a a n n c c e e
61
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