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In Reply to: Ping Al Sekela posted by JB on October 20, 2006 at 05:48:36:

The X2 are more readily available in my local parts store, so that is the type I use. According to the link, the X1 designation is for caps tested to 4000 volts, while X2 is for 2500 volts. The X1 type would be for industrial use and cost more, but if it is the type you have, it will certainly not hurt to use it.
The size of the capacitor is to be calculated to limit the 60 Hz power dissipation in the resistor. For 120 volt application, a 1 microfarad capacitor allows 0.25 watts in a 120-ohm resistor. Use a half-watt or larger metal film or metal oxide resistor, and be sure not to cover the resistor body with insulating tape. Be sure the resistor is flame-proof, as the X- rated capacitors can fail to a short-circuit. If this happens, you want the resistor to fail without starting a fire.
I bypass larger capacitors with smaller ones to extend the useful frequency range. A 0.01 microfarad X2 capacitor I removed from a filtered IEC inlet device had a self-resonant frequency of 15 MHz. The larger sizes would have lower self-resonant frequencies, and be ineffective for noise frequencies above their self-resonances.
The whole point of the 120-ohm resistor is to load the AC power wiring, acting as transmission line for higher noise frequencies. 10 meters of wiring would have a fundamental resonance of about 5 MHz, with many harmonics. An R-C filter can reduce the resonant Q of the wiring, but only if the capacitors provide low impedance to the important noise frequencies, and the resistor has low inductance.
It may be difficult to find X-rated caps in sizes below 0.01 microfarads, but you can safely substitute Y-rated caps. These are usually found only in small sizes.
I don't know if it matters which side of the power line the capacitor goes to.

Link to tutorial on X- and Y-rated caps (Open in New Window)

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Topic - Ping Al Sekela - JB 05:48:36 10/20/06 (1)

Answers to your questions... - Al Sekela 21:51:02 10/20/06 (0)

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Answers to your questions... - Al Sekela 21:51:02 10/20/06 (0)

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