Specifications

Chapter 4 – Coupling Circuits
140 PL 3120/PL 3150/PL 3170 Power Line Smart Transceiver Data Boo
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Some products cannot rely on their enclosure as a safety isolation barrier and an alternate method of safety isolation must
then be provided. For example, a circuit board that used a non-isolated line-to-neutral coupling circuit in conjunction
with a PL Smart Transceiver whose I/O pins are user-accessible would present a potential electrical shock hazard.
Because 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-recognized
transformer having the appropriate communication characteristics in place of L101 (see Appendix C). 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 L102 can be incorporated into the isolation transformer by designing the leakage inductance of
the transformer to match the value of L102. A transformer-isolated coupling circuit is shown in Figure 4.20, where it can
be seen that the transformer isolates the PL Smart Transceiver from the line conductor and the neutral or earth conductor.
It is important to note that mains powered products that require circuit isolation must incorporate both an isolated
coupling circuit and an isolated power supply. An isolated coupling circuit can be used with a non-isolated power supply
(or visa-versa) but a mixed combination will not provide circuit isolation.
Figure 4.20 Functional Transformer-Isolated Coupling Circuit