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In Reply to: You can only find small ones. posted by Al Sekela on January 30, 2007 at 10:42:41:
So a 120ohm resistor in seris with a .47y2 cap across the live a nuetral gives better results then just the cap???
Follow Ups:
The 0.47 microfarad cap will likely have a fairly low self-resonance frequency, say about 10 MHz, so it will not couple the higher frequency noise on the line to the resistor very well.What I've found to work best is to use multiple R-C pairs in parallel. Each capacitor is about a factor of 10 smaller, so, for example, use 0.47, 0.047 and 0.0047 microfarad X- or Y-rated caps, each with a 120-ohm or so resistor.
The idea of using a resistor in series with the capacitor is an old one, called Dynamic Loading. It provides a way of dissipating noise energy instead of reflecting it. A straight cap is a short for higher frequencies (up to the self-resonance) and reflects noise. It may reduce the noise in one part of the system but increase it in other parts.
If you are a dedicated scrounger, look for capacitors and resistors that do not contain steel parts. These exist in some surplus parts stores.
Would you like to suggest a resistor wattage? I take it something small would be sufficient?
A 120-volt line will dissipate 0.25 watts in a 120-ohm resistor connected to a 1 microfarad capacitor. The rating of the resistor should be twice the actual dissipation. With smaller capacitors, the power is even less.If the capacitor fails to a short (I know that Y-rated caps are only supposed to fail to an open-circuit, but X-rated caps may fail to a short-circuit), the resistor will burn out. Some resistor types will create a flame about as big as a kitchen match. If you use this type, be sure the filter is in a place where such a flame will not set your house on fire.
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