Echelon LONWORKS PLT-22, User Manual

Page: 44 / 128

Figure 4.8 shows additions to the coupling circuit which are required to make it fully
functional. The first is a 1.0mH inductor, L3, connected to the PLT-22 transceiver receive
filtering circuitry. The DC resistance of L3 can be up to 50 Ohms. The second is a diode,
D1, connected from the transmitter to the amplifier supply voltage (V
A
) to protect the
inputs of the PLT-22 transceiver from large (>15V) transients. This diode works in
conjunction with a diode internal to the PLT-22 transceiver that connects from the
transmitter output to ground. Bypass capacitor C3 also has been added to emphasize the
fact that it is an integral part of the coupling circuit. One of the functions of this
capacitor is to protect the V
A
supply line from excessive overshoot when positive going
line surges discharge through diode D1. The last addition is an optional circuit that
improves performance in environments where large (>50V) impulses may be present from
devices such as SCR-controlled light dimmers. This circuit consists of an LCR series
network that acts as a notch filter whose center frequency is at the characteristic
“ringing” frequency of the coupling circuit. This optional circuit should be included on
nodes containing an SCR or triac switching device, and in nodes operating from a nominal
V
A
supply of less than 12V.
Transmit Amp
Receive
Front
End
PLT-22
TXOUT
RXIN
RXCOMP
Line
Neutral or
Earth
V
Optional
circuit
D1
C2
L2
C1
L1
L3
C3
+
A
Figure 4.8 Functional Line-to-Neutral or Line-to-Earth Coupling Circuit
In instances where large ambient fields may be present (such as from switched mode
power supply open frame magnetic elements), it is possible that one or more of the
PLT-22 coupling circuit inductors may pick up these stray fields and conduct them
4-10 Coupling Circuits