Echelon LONWORKS PLT-22, User Manual

Page: 46 / 128

hazard. Since the mains neutral lead is connected directly to the circuit board common,
the user could be exposed to a hazardous voltage at the I/O connector, especially if the
line and neutral connections are accidentally reversed. Additional circuitry is needed in
such a product to provide a safety isolation barrier between the user-accessible I/O
connector and the mains line and neutral conductors.
The most common solution is to provide isolation in the coupling circuit by modifying
the simple coupling circuit described earlier. This style of coupling circuit is referred
to as an isolated coupling circuit.
The preferred isolated coupling circuit uses transformer-isolation . Transformer-
isolation requires substituting a safety agency-approved transformer having the
appropriate communication characteristics in place of L1 (see Appendix A).
Transformer-isolation can be used for both line-to-neutral and line-to-earth coupling.
Transformer-isolated coupling has the advantage that the resonant inductor L2 can
be incorporated into the isolation transformer by designing the leakage inductance of
the transformer to match the value of L2. A transformer-isolated coupling circuit is
shown in figure 4.9, where it can be seen that the transformer isolates the PLT-22
transceiver from the line conductor and the neutral or earth conductor.
C3
+
Transmit Amp
Receive
Front
End
PLT-22
TXOUT
RXIN
RXCOMP
Line
Neutral or
Earth
V
Optional
circuit
D1
C2
T1
C1
L3
A
Figure 4.9 Transformer-Isolated Coupling Circuit
An alternate method of providing safety isolation is to add optical isolation between
the Neuron Chip and the I/O connector, or between the Neuron Chip and the PLT-22
transceiver. If optical isolation is used between the Neuron Chip and the PLT-22
transceiver, then a maximum of 150ns propagation delay may be added to the CP
4-12 Coupling Circuits