Watlow MLS-PM, User Manual

Page: 46 / 144

Installation
MLS User’s Guide 33
PID Control and Alarm Output Connections
Typical digital control outputs use external optically-isolated solid-state
relays (SSRs). The SSRs use a 3 to 32 Vdc input for control, and you
can size them to switch up to 100 amps at 480 Vac. For larger currents,
use these optically-isolated relays to drive contactors.
NOTE
Control outputs are sink outputs. They are Low
when the output is On. Connect them to the nega-
tive side of Solid State Relays.
The next figure shows sample heat/cool and alarm output connections.
System Safe (Watchdog Timer) constantly monitors the MLS CPU. It is
a SINK output located on RTB terminal #6. (Do not exceed the 10
mAdc rating for the System Safe output.) Its output is Low (on) when
the CPU is operating; when it stops operating, the output goes High
(off), de-energizing the SSR.
Here's the recommended circuit for the System Safe output:
SSR
+
_
+5 Vdc
(RTB pin 2)
System safe
(RTB pin 6)