McQuay MicroTech Unit Controller Installation And Maintenance Data page 38

SOLID-STATE RELAY SWITCHES
Recommended Testing Procedure
Solid-state relay switches may be depicted as shown in the
McQuay schematic wiring diagram; de-energized, with con-
tacts open. The contacts are not 'open', however, in the sense
that conventional electrical-mechanical relays open their
contacts.
When energized, the relay 'contacts' lower their resistance
and pass current freely. De-energized, they create a barrier
to current flow; but permit voltage to exist at downstream ter-
minals under certain conditions.
If for example, a voltmeter is used to test Logic output relay
number 14, with power applied to L1 and L2 terminals, and
the 3 amp circuit breaker 'closed', the voltmeter will display
115 volts AC when connected to terminal 30 on the relay and
L2. Similarly, if the relay is de-energized and the SW1 switch
or oil pump auxillary contacts open, a voltmeter connected
between terminal 29 on the relay and L2 will also display 115
volts.
On the other hand, if a test light, or a relay coil, such as
the oil pump contact coil (OP), chilled water (CHWR) or con-
denser water (CWR) pump relay coils are permanently con-
nected to the downstream relay terminal and to ground, a
voltmeter connected between the solid-state relay's
downstream terminal and ground, will not display any voltage
The absence of a voltage reading in this case is due to the
fact that the voltage is being bled off to ground by the con-
nected load.
A thorough understanding of this departure from the
characteristics of conventional relays is required to permit
prompt and accurate service analysis.
Each solid-state output relay used in the MicroTech control
panel is equipped with its individual 5 amp fuse. Fuses resem-
ble tiny resistors, and are located directly above the relay they
serve on the relay base. Fuses are pressed into place, and
can be removed by lifting them out with needle nose pliers.
Note the location of the fuse in the wiring circuit. Consider
its location and the characteristics of the solid-state relay when
electrically testing the circuit for a fault.
The LED present in the solid-state relay base responds to
the relay's optically isolated "coil" circuit. If the LED doesn't
glow, the relay probably is not being energized. Relays re-
quire the existance of 5 VDC to initiate closure. When the relay
is energized, the LED should glow.
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Solid-state Relav Board.
FUSE
(TYPICAL)
INPUT
CONTROL
POWER
5VDC
SOLID-STATE
RELAYS