902B Abs Piezo
Operation manual
14
Setpoint relays
The 902B can be ordered with either 0, 1 or 3 mechanical relays that can be used for controlling external
process equipment. Each relay has closing and breaking contacts and the contacts are rated 30 VDC, 1A
resistive load. If the transducer is supplied without setpoint relays, the setpoint commands can still be
accessed. Refer to part number definition page 3 to verify if setpoint relays are included.
Inductive relay load
Special precautions should be taken when driving inductive loads with the relay contact. When an inductive
load like a solenoid is energized, the in-rush current is significantly higher than the regular load current. In-
rush currents exceeding the relay contact rating can cause reduction of relay contact life time or contact
reliability.
When a solenoid is de-energized, the collapsing magnetic field can cause significant voltage spikes. These
spikes can couple capacitively from cable to cable and interfere with measuring electronics or transducer
signals.
Driving inductive loads via the setpoint relay contacts requires de-energizing spike
protection. Inadequate protection can cause permanent damage to the transducer
or interfere with the analog output signal.
Always ensure that inductive in-rush currents do not exceed relay contact rating.
An arc suppression network as shown schematically to the right is
recommended. The values of the capacitance C and the resistance R
can be calculated by the following equations:
C = I
2
/(1 x 10
7
)
R = E/I
a
where:
C is in Farad. R is in Ohm
I is DC or AC
peak
load current in Ampere. E is DC or AC
peak
source voltage in Volt
a = 1 + (50/E)
Note that R
min
= 0.5
Ω
and C
min
= 1×10
-9
F. D is a fast transient suppression diode.
PDR900 controller relays
The PDR900 controller has power relays that can drive higher current loads and voltages than the
transducer relays. If the transducer is used with the PDR900 controller refer to PDR900 manual for setup of
relay output.
Do not connect any external sources to the transducer relay pins when using it
together with the PDR900 controller. Always use the PDR900 relay outputs.
Setpoint functionality
The set point relays can be activated either above or below the set point values. The graphs below show the
different relays stages in either below or above configuration. The NC contact will always be closed in case
of power failure.
