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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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In Reply to: Ping Al Sekela On R-C Filters For RFI Filtering posted by diyaudiophile on March 6, 2007 at 22:33:34:
I measured the inductance and capacitance of a piece of plastic-jacket house wire (similar to Romex) and estimated the characteristic impedance:Z = SQRT( L / C ), with L in Henries and C in Farads
as about 120 ohms. This could be off by quite a bit, as I do not own sensitive test equipment.
An exact match is not that important, so use my value, or 100 ohms, if you can't get an estimate for your in-wall cable impedance.
A 1 microfarad capacitor will cause 0.25 watts dissipation in a 120-ohm resistor with 120 volts, so the dissipation would be four times as much, or 1 watt, with 240 volts. Always use a resistor with a rating at least twice the actual load.
The capacitors used should be rated for continuous exposure to the line voltage. In the USA, these are called X-type for line to neutral, and Y-type for line to ground. They are designed to be self-healing after breakdown due to voltage spikes. The Y-types are supposed to fail to open-circuit only, to avoid a safety hazard, but the X-types may fail to a short-circuit under the assumption that the circuit breaker will trip if that happens. Y-types are only made in small values because excessive ground current would trip ground-fault detection circuits.
The resistor may or may not be flame-proof. If it is not, it will make a brief flame about as big as a kitchen match if the capacitor fails to a short-circuit. Please use an appropriate non-combustible enclosure if you are not sure the resistor is flame-proof.
I've found substantial sonic differences depending on the components in the capacitors and resistors. There are nonmagnetic capacitors and resistors, but this attribute is not usually listed. I find mine in surplus stores, with a magnet in hand. I don't know if the Mills resistors contain magetic parts. If they do, I would not use them. Noninductive resistors are preferred, to allow the filters to work at higher frequencies. I like the old-style Holco or PRP resistors sold by Michael Percy, but be careful of the power dissipation.
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Follow Ups
- Resistor value based on impedance of cable. - Al Sekela 12:24:55 03/07/07 (2)
- Re: Resistor value based on impedance of cable. - rick_m 08:48:17 03/08/07 (1)
- Try this Evox-RIFA tutorial on the subject. - Al Sekela 13:53:31 03/08/07 (0)
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