The next piece of the system is one of the simplest, yet one of the most important aspects of the system, that is the high voltage interface. For AC applications this interface is shown in Figure 4-2 and it is simply a transformer and high voltage capacitor.

This circuit serves one main purpose – to act as a bandpass filter to allow the communication signal to be coupled onto the main line while preventing the low frequency AC signal from feeding back into the protection circuit. This becomes clearer when the equivalent transformer circuit is used. This is shown in Figure 4-3.

The capacitor and transformer together create a bandpass filter. The goal of this filter is to filter out the AC mains frequency (~60Hz) while keeping the communication modulation frequency within the pass band. The parameters of the transformer can be found in the transformers datasheet where leakage inductance is L_leak_p + L_leak_s’, core resistance is Rc (if not shown assume RC to be open circuit), core inductance is Lm, the primary winding resistance is Rs_p, and the secondary winding is Rs_s. Please note that the above figure has “transformed” the equivalent circuit to remove the turns ratio – to convert secondary components to their “transformed” version, s, so that an accurate model is retained, divide the original impedance value by the turn’s ratio (Primary to Secondary) squared. For most transformer datasheets the only value you may have to transform is Rs_s which would be Rs_s’ = Rs_s/ (turn’s ratio) ^2.

After the bandwidth has been sorted out the capacitor must be rated for high voltage applications as it will take the brunt of the incoming signal. For a 110 VAC too 220 VAC application a 1.5 kV rated capacitor was used.

If the end power line is not AC and is high voltage DC – the transformer is unnecessary and only the high voltage capacitor is needed as the capacitor will block the high DC voltage.