Laurel Electronics LT SERIES TRANSMITTERS PULSE INPUTS - Manual (Page 24)

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13. DUAL RELAY OPERATION
Dual solid state relays are standard and can operate in a basic alarm mode, in a hysteresis band
mode, or in a deviation band mode, as explained below. Setpoint operation is referenced to the
digital reading in engineering units that is internal to the transmitter. For example, rate alarm or
control would be referenced to a setpoint in RPM.
Time
1000
D
ig
it
a
l
R
e
a
d
in
g
Setpoint
OFF OFF
ON
Time
1000
Setpoint
OFF
ON ON
D
ig
it
a
l
R
e
a
d
in
g
Active High Basic Alarm Active Low Basic Alarm
A basic alarm changes state automatically when the reading rises above a specified limit, and
changes back automatically when the reading falls below that limit. A red LED indicates the
relay is in an alarm condition, which can be active high or active low, as programmed.
Time
R
e
a
d
in
g
1050
950
Pass
Fail
Fail
Time
1050
Setpoint = 1000
Hysteresis limit = 50
Hysteresis band = 100
Heater
OFF
R
e
a
d
in
g
ON ON ON
OFF OFF
950
Setpoint = 1000
Deviation limit = 50
Deviation band = 100
Hysteresis band Deviation band
A hysteresis band controls relay action symmetrically around a setpoint. The relay closes (or
opens) when the reading goes above the setpoint plus one hysteresis limit, and opens (or
closes) when the reading falls below the setpoint less one hysteresis limit. A narrow hysteresis