User guide

Hardware Installation
890 USE 108 00 August 2001 131
Distributing Field Power to A120 I/O
Field Power
Distributed to
A120 Modules
The following figure is an example of the recommended external 24 Vdc power
supply wiring and fusing scheme for discrete, relay, and analog input modules. The
24 Vdc power supply ground is connected to the common functional ground
(building ground). A separate circuit is recommended for external loads (outputs),
and that circuit should have its ground connected to the common functional ground.
Fuse values are determined by the modules and field devices used. DX, RX, and
CX are not required but are recommended in the event field devices are causing
noise spikes on the power line which may affect operation. For a 24 Vdc supply, DX
may be rated at 30 V minimum. Power rating is load-dependent. CX may be a 0.1
microfarad disk but results should be verified with an oscilloscope and the value
adjusted accordingly.
RX = 1 Ω / 3 Ω
DX is load- and power supply-dependent
CX is load-dependent
+24 VDC
Analog Output Power Supply
Relay Coils Power Supply
Power to Field Devices
DX
RX
CX
24 V Common
DEO 216 DEP 216 DAP 204 DAP 212
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