Echelon LonWorks USB Network Interface - Manual (Page 21)

L
ON
W
ORKS
USB Network Interface User’s Guide 15
The supplied wall-plug coupling circuit/power supply includes a step-down
transformer and a 12uH-leakage coupling transformer. Two diodes are used to
rectify the AC on the step-down side of the power transformer. The schematic for
the wall-plug power supply is shown in Figure A.2.
AC
INPUT
LINE
NEUTRAL
COUPLING
TRANSFORMER
STEP-DOWN
TRANSFORMER
COMBINED
VA SUPPLY/COUPLING
Figure A.2 Wall-Plug Supply Schematic
You will note the absence of a capacitor on the output of the supply. This is
intentional. Placing a capacitor from the combined VA Supply/Coupling line in
this schematic to the supply return would place a low impedance across the
coupling circuit. This low impedance path would attenuate the power line
communications signal, greatly reducing the communications performance of the
U20 interface. Instead, the required capacitor is included inside the U20
interface with a series inductor to raise the impedance at the communications
frequency. The schematic for the coupling circuit and power supply input inside
the U20 interface is shown in Figure A.3.