Specifications

Chapter 4 – Coupling Circuits
142 PL 3120/PL 3150/PL 3170 Power Line Smart Transceiver Data Boo
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Capacitor Charge Storage
The coupling capacitors depicted in the earlier figures can retain substantial charge even after a PL Smart Transceiver-
based device has been disconnected from the power mains. This can be of significant concern in applications where a
line cord could be touched by a user after being disconnected from the power mains. To minimize potential shock
hazard, coupling circuits should include a large value bleeder resistor to discharge the coupling capacitors following
disconnection from the mains. Even in applications where the connection to the mains is permanently wired, it is good
practice to include the resistor to protect service personnel. The example coupling circuits shown at the beginning of this
chapter include appropriate bleeder resistors. If an alternate path to discharge this capacitor exists (such as the primary
winding of a linear power supply transformer) then this bleeder resistor can be eliminated.
Fuse Selection
Safety considerations might require a fuse in series with the mains connection. For an end product to continue to function
(without user intervention) it is necessary that the selected fuse not open following a specified line surge. A minimum 6A
time-lag (“slow blow”) rating has been shown to be necessary to avoid unintentional fusing action for surge events in
branch circuit and power entry applications. For outdoor applications a 10A time-lag part is needed to avoid
unintentional fusing.
If a coupling circuit that incorporates varistor protection is selected, the recommendations of the varistor manufacturer
for maximum fuse current rating should be followed. Several varistor manufactures recommend a maximum fuse rating
of 6A to 6.3A for use with 1250A surge-rated varistors and a maximum rating of 18A for use with 4500A surge-rated
varistors.
If a current rating greater than 6.3A is required by the application then a varistor with a surge current rating of >2000A is
recommended.
For the purposes of communication signaling it is also important that the fuse add very little resistance (<0.1 ohms) to
the transmit signal path. Use of fuses in the 6A to 10A range satisfy this requirement.
3-Phase Coupling Circuits
When power line communication devices are located on different AC power phases, a significant portion of the overall
attenuation between these devices is caused by loss in crossing phases (typically 10-20dB). Most of this loss can be
avoided if one of two communicating devices connects to all power phases as illustrated in Figure 4.22. If all
communications are to (and from) a device located at a central distribution panel, then the use of a 3-phase coupling
circuit in that device is recommended to maximize communication distance. Due to the fact that this central device must
drive the parallel combined impedance of all three phases, a higher current transmit amplifier is recommended for use in
these locations.
The standard transmit amplifier recommended for use in most PL Smart Transceiver-based products includes circuitry to
limit output current to 1Ap-p (so that the amplifier will not be damaged when driving very low impedance lines). An
alternate transmit amplifier, which can provide 2Ap-p of output is recommended for use with 3-phase couplers. The
appropriate transmit amplifier can be implemented using an optional discrete interface circuit with the PL Smart
Transceiver. Reference design implementations for both the 1Ap-p and 2Ap-p interface circuitry are available, and are
summarized in Appendix A.
Note that the return path for a 3-phase coupling circuit can be either neutral or earth, whichever is appropriate for the
application. Note also that a 3-phase earth-return coupling circuit does not result in the same ground leakage current as a
single-phase line-to-earth coupling circuit. The ground leakage current of a 3-phase earth-return coupling circuit is
nominally zero. This is due to the canceling effect of the three leakage currents through C101A, C101B, and C101C,
which are 120 degrees out of phase with each other.