Mains Filter – Effective Power Line Communication Filtering

impedancemainspower line communicationptc

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.

schematic

^{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.

Best Answer

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.
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.

Related Solutions

Electronic – How to terminate 50 ohm transmission line without drawing DC

After much experimentation in LTSpice, I devised a passive network that should work. R9, R10, R11 and C5 are internal to L3. R15, R16, R17 and C8 are internal to L4.

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Here is the wideband AC analysis:

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DC – How to Couple a Signal onto a DC Line

Yes, the standard bias-T looks something like this:

DC power source >>---L1--+                 +--L2->> "unregulated" DC power to regulator
                         |                 |
bidirectional RF ----C1--+- mixed RF + DC -+--C2---- bidirectional RF

GND--------------------------------------------------------------------GND

where L1 and L2 are equal-size power inductors and C1 and C2 are equal-size data-coupling capacitors.

I would avoid connecting anything else to the "mixed RF + DC" line other than those two inductors and two capacitors. (Or perhaps four capacitors, if I had a separate "transmit capacitor" and a "receive capacitor" at both ends).

Since you likely have some sort of connectors between the two devices that typically have 0.1 Ohm of resistance each, coupling capacitors that give an impedance of less than 0.1 Ohm (across the entire bandwidth) will be more than adequate (and perhaps overkill).

So a capacitor with capacitance at least 1/(2*pi*90 kHz * 0.1 Ohm) =~= 18 uF is more than adequate (and perhaps overkill). You'll want a cap with low parasitic resistance (ESR), so mica, film, or ceramic -- rather than tantalum or electrolytic. You'll want a cap with low parasitic inductance, so surface-mount -- rather than through-hole.

Standard off-the-shelf capacitors and inductors are more than adequate up to 10 MHz or so. People that work with higher frequencies use striplines and resonating stubs that may appear to be black magic. Although there are a few people who claim it isn't. a b c d e

EDIT:

capacitor sizing

Inevitably, not all the energy sent out by the transmitter will make it to the receiver. If I cut the cable between the transmitter and the receiver and add "a few more connectors" in the path between them, a little energy is lost each time the signal crosses a connector.

Practically all digital communication systems can tolerate a lot more loss than that caused by "a few more connectors". So keeping the distortion to something less than the loss of "a few more connectors" is overkill. (I prefer to get my first prototype working with oversized components, rather than pick stuff that is right on the verge of not working).

inductor sizing

Alas, I don't have a rule of thumb for figuring out how much inductance to specify. Perhaps whatever you have that is generating or receiving your G3-PLC data might have some sort of datasheet with some recommendations?

Ed Mullins and Anass Mrabet in "Analog Front-End Design for a Narrowband Power-Line Communications Modem Using the AFE031" have many tips you might find useful. In particular, their figure 27 seems to indicate that, with PRIME or G3-PLC, a standard off-the-shelf voltage regulator will work, one where the only inductance between the power line and the large bulk power storage capacitor that powers all the electronics is a standard EMI filter.

TI power line communication schematic showing coupling transformer

(via http://www.ti.com/solution/power_line_communication_modem , via http://www.ti.com/plc ).

The datasheet for one particular such voltage regulator has a detailed list of materials; its EMI filter consists of a

1 L1 Inductor, AC line, common choke, 27 mH, 54P512-276 Vitec Electronics Corp.
1 L2 Inductor, high current choke, 3.3 μH, HCP0703-3R3-R Coiltronics/Cooper

(nearby datasheets also mention a "54PR515-146" and a "AF5169-146" AC-Line Common Mode Choke).

It is getting easier to search popular electronic suppliers for "common mode chokes". Alas, while I see many with "at least" 27 mH, and many "at least" 3 A, finding one that meets both specs is difficult. Perhaps the Bourns Inc. 7122-RC (4 A, 25 mH) might be adequate?