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Use power line for communication despite noisy neighbours

Posted: 04 May 2012     Print Version  Bookmark and Share

Keywords:Power-line modems  CENELEC  transformer  luminaire 

In cases where the operating frequency of the switching power supply cannot be adequately predicted, perhaps due to the use of an OEM module or the desire to leave open future design changes in the switcher, an LC circuit rejecting the modem's signal bandwidth can prevent switching noise from infiltrating the modem bandwidth (figure 1). In many cases, this network can provide a second function, in that capacitance (C) dampens switcher transients while inductance (L) supplies power to the switcher's input.

In some designs, the LC filter may be built-in as the switcher's EMI filter, thereby saving cost over extra components. The filter need not resonate at the modem transmission frequency, but should isolate the communication band from any noise created by the power supply.

Figure 1: LC circuit rejecting the modem's signal bandwidth can prevent switching noise from infiltrating the modem bandwidth.

2) Another kind of noise is caused by zero-crossing interferers, that is, circuits that switch near the point where the mains waveform crosses zero volts. This kind of AC load is primarily represented by modern light dimmers and electric motor drives. Variable speed motor drives, once the province of industrial systems, are frequently found in new appliances and equipment intended for residential use, such as condensing units and furnaces.

Filtering or tuned circuits mentioned previously can be used, but suitable inductors and capacitors will be more costly at HVAC power levels. It is therefore advantageous to select a modem that does not depend on mains-waveform timing to send or receive data, that is, a modem that will communicate on demand without regard to AC phase (figure 2). Modems that do not require a zero-crossing are also suited for use on DC power lines.

Figure 2: It is advantageous to select a modem that does not depend on mains-waveform timing to send or receive data.

3) When the modem must be on the secondary side of a large power distribution transformer, the carrier frequencies may be chosen to be lower so that they may pass more easily through the particular transformer.

Each transformer type is different and requires evaluation. With a transformer larger than 3-5 kVA, especially those with widely spaced or "open" windings, signal transmission is not always satisfactory because of excessive attenuation and dissipation of the modem signal in the very-large transformer.

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