I have two questions about Mains filters and their impedance effect on PLC.

1) With respect to signal propagation relative to the Line impedance:

As the signal is sent, via the power line, it will take the path of least resistance. Therefore the line to the PSU needs to have a high impedance (greater than 50 Ohm) to ensure the signal is not sent to the PSU. What is the Max impedance I can add to my filter before the PSU stops working?

There are two PSU's I am using. The 5 Watt PSU and the 10 Watt PSU.


simulate this circuit – Schematic created using CircuitLab

2) With respect to the prescribed circuitry in the 5 Watt and 10 Watt data sheets.

There are a fuse and an NTC used in series with one another. The fuse is required to prevent fires from starting. I would like to know if a PTC could replace the function of the fuse. If so would having a PTC in series with an NTC be contradicting in their functionality, or would they just need to be placed far enough apart so that their temp's do not interfere with one another?

For question one the PTC and NTC resulting impedance will be included in the overall impedance of the filter.

  1. The more optimal solution is to make an impedance from a parallel LC circuit so that at the transmission frequency the impedance is high. The parallel LC network would go in series with the line feeding the power supply. Choose the L so that at regular AC frequencies the impedance it presents is just a few ohms then it won't impede AC power voltages reaching the power supply.
  2. You need to have the fuse if you are going to obey regulations for usage of this device - pretty much all SMPS devices have a fuse - it's the last line of defence against your house burning down. Install a fuse.
  • \$\begingroup\$ 1) Okay, I see where you are coming from, should I put the LC circuit after the Filter or before it. I do not want to attenuate my signal in any way. 2) One of the reasons why I tried to avoid a fuse is that the system will not easily accessible and it will be annoying if I had to change the fuse. Could I have an SMD Fuse with a 3A limit and then a PTC with a lower limit? would this allow me to conform to regulation? \$\endgroup\$ – Gareth T. Aug 27 '18 at 9:39
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    \$\begingroup\$ You need to select the fuse as defined by the SMPS supplier. If this is not clear or there is doubt, choose a different SMPS supplier (there are several). The parallel LC goes after the fuse and before any power supply filtering. \$\endgroup\$ – Andy aka Aug 27 '18 at 10:29

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