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I have been tweaking my MF PSU V3 & am considering adding a class X2 cap in .47uf from live to neutral. It has been suggested that a class Y2 cap can be added from live to earth & neutral to earth. Can anyone suggest a value to try in this class?
I live in the 240volt part of the world.
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The line-ground caps admit charging current that can upset ground fault interrupters, so they are only made in small sizes.I don't like the effects of straight capacitors in these applications. I've found that series R-C networks, where R is about 120 ohms, work better. The resistor should be flame-proof or mounted in a location where it will not start a fire if the capacitor should fail to a short.
I also don't like to couple line noise to the safety-earth wiring. You may want to evaluate the effects of line-ground filters before committing to them permanently.
So a 120ohm resistor in seris with a .47y2 cap across the live a nuetral gives better results then just the cap???
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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