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Simple Circuit Filters are used when the maximum broad band noise voltage amplitude at 127 kHz is less than 100 mV rms/ (30 kHz)^0.5 and there is absolute certainty that there is not and never will be any other PLC equipment that can interfere with the SPI PLC Lighting Control system.

Simple Circuit Filters have the advantage that their cost is about half the cost of Premium Circuit Filters.

A simple Circuit Filter increases the range and reliability of the PLC communications for lighting control and prevents PLC signals on one branch lighting circuit interfering with PLC signals on another branch lighting circuit.

A simple Circuit Filter functions by minimizing lighting control PLC signals that are impressed upon the circuit breaker panel electrical bus; by attenuating electrical noise that propagates from the circuit breaker panel electrical bus to a branch lighting circuit; and by preventing a low electrical bus impedance from attenuating the lighting control PLC signals on a branch lighting circuit.


Pictorial diagram for assembly of a 3 phase simple Circuit Filter.


In the simple Circuit Filter the size of the inductors is primarily limited by component availability and cost considerations. The inductor value choice for 120 volt lighting circuits is:
La = Lb = Lc = Ln = 100 uH.
The inductors must be rated for 15A or 20A depending on the circuit breaker size. The self resonant frequency of the inductors must be much larger than the 127 kHz operating frequency. An acceptable self resonant frequency is 1.8 MHz.

The capacitor values are chosen to provide 60 Hz power factor correction for the inductors with an average lighting load of ????? watts per 120 volt branch lighting circuit.
Ca = Cb = Cc = 0.47 uF.
The capacitors must be rated for continuous use with AC. A 600 to 1000 volt AC continuous rating is suggested, although a 300 volt AC continuous rating may be sufficient for a 120 volt lighting circuit. Dry film capacitors are used to avoid the use of oil filled capacitors. The issue is that over time the oil might deteriorate and become a potential fire hazard. The capacitors must be engineered such that their effective series resistance and distributed inductance are negligibly low below 500 kHz.


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The maximum 127 kHz noise coupling from the circuit breaker panel electrical bus to the branch lighting circuit occurs when the branch lighting circuit is short and contains only a few ballasts, so that its impedance is relatively high. Let Zb be the impedance of the branch lighting circuit. The transfer ratio T from the electrical bus to the branch lighting circuit is given by:
T = Zb / (Zb + 2jWL)

At 127 kHz the term 2WL has a value of:
2WL = 2 X 6.28 X 1.27 X 10^5 X 10^-4 = 159.51 ohms

Information provided by Systel indicates that the impedance of one two lamp ballast at 127 kHz is about 483 ohms inductive, so three ballasts in parallel will give an impedance of about 161 ohms inductive. This impedance makes T ~ 0.5. Hence,the Circuit Filter reduces the noise voltage coupled from the circuit breaker panel electrical bus to the branch lighting circuit by a factor of at least two (6 dB) provided that there are at least 3 ballasts connected to the branch lighting circuit. For design purposes it is convenient to impose the requirement that there are always at least three two lamp ballasts connected to a branch lighting circuit.

Note that this 6 dB reduction is helpful in reducing broad band noise but it is not sufficient for suppressing strong interfering narrow band PLC signals that are on the circuit breaker panel electrical bus.

In the worst case of a short branch lighting circuit with 3 connected two lamp ballasts, the externally produced noise on the electrical bus is 100 mV rms and the corresponding noise coupled onto the branch lighting circuit is 50 mV rms. As the branch lighting circuit is made longer its shunt impedance decreases, causing the coupled noise from external sources to decrease. In our analysis this effect is used to provide the operating safety margin, which is of the order of 12 dB. This safety margin allows for uncontrolled materials, imprecise electronic components, long term degradation, etc.


Assume the worst case which is a branch lighting circuit that is connected to the same phase of a lighting circuit breaker panel as is a high impedance load that is sensitive to the PLC signal. Assume the worst case that all the other connected electrical loads are off. Then the transfer function from the lighting circuit to the circuit breaker panel electrical bus is given by:
T = (Xc) / (2 Xl + Xc)
= -j / [WC(2jWL + (-j/WC))]
= WC / [WC(-2W^2LC + 1)]
= 1 /(-2W^2LC + 1)

For L = 100 uH, C = 0.47 uF, F = 127 KHz:
2W^2LC = 2 X (6.28 X 1.27 X 10^5)^2 X 10^-4 X .47 X 10^-6
= 2 X 63.61 X .47 = 59.79

T = 1 / (-59.79 + 1) = -1 / 58.79 =-.0170

For example, if the maximum PLC signal is 2.5 V RMS then the interference to the electrical bus is:
.0170 X 2.5 V = .0425 V rms = 42.5 mV rms

The above calculation implicitly assumes that both the inductors and the capacitors are ideal. In reality real inductors contain distributed capacitance and resistance and real capacitors contain distributed inductance and resistance. In order to ascertain the effect of these component non-idealities, on July 4, 2005 an experimental measurement of T was carried out using the same components that are contemplated for use in the 120 V Breaker Panel Filter.

The theoretical and experimental values of T as a function of frequency were tabulated as follows:
Frequency (kHz)---Theoretical T---Experimental T

In this frequency range T is nearly proportional to F^-2, as expected and the effect of component non-ideality on T is negligible. Hence the Circuit Filter will behave almost exactly as projected by its mathematical model.


If there are two branch lighting circuits connected to the same phase of a circuit breaker panel, and each branch lighting circuit is fitted with a Circuit Filter, then there are two .47 uF capacitors in parallel across the electrical bus. Assume the worst case that there are no other loads connected to this electrical bus. Then the transfer function from either of the branch lighting circuits to the electrical bus becomes: T = (Xc/2) / (2 Xl + (Xc/2))
= -j / [2WC(2jWL + (-j/2WC))]
= 2WC / [2WC(-4W^2LC + 1)]
= 1 /(-4W^2LC + 1)

For L = 100 uH, C = 0.47 uF, F = 127 KHz:
4W^2LC = 4 X (6.28 X 1.27 X 10^5)^2 X 10^-4 X .47 X 10^-6
= 4 X 63.61 X .47 = 119.6

Tr = 1 / (-119.6 + 1) = -1 / 118.6 =.00843

Thus if the maximum PLC signal is 2.5 V rms then the maximum signal impressed on the electrical bus is:
2.5 V X .00843 = .021 V rms.

The corresponding maximum noise impressed on the other branch lighting circuit is: 0.5 X .021 =.0105 V rms

Thus the Standard Circuit Filter design will reduce a 2.5 V RMS 127 kHz PLC signal down to below the background noise level commonly found on the electrical bus.

Consider the case of a 200 amp 3 phase lighting panel with 13 15A circuits per phase. If all of the circuits of one phase are fitted with SPI Lighting Control equipment then 12 circuits will be imposing noise on the 13th circuit of the same phase. The amplitude of this noise is:
(12)^0.5 X .0105 V rms = .0364 V rms.
This noise level is below the 50 mV rms noise level that could occur due to external broad band noise.

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This web page last updated September 20, 2005

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