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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: Re: Well, not exactly posted by cporada@cabnr.unr.edu on November 4, 2006 at 06:03:48:
The basic idea is to load the AC line with resistance, but without burning up a lot of power.All cables, including the ones used to connect your house power outlets to your breaker panel, have a characteristic impedance. Their DC resistance may be very low, but RF noise sees an impedance determined by the inductance and capacitance of the cable. When RF noise encounters a change in impedance, it is somewhat reflected. This is why simple capacitor filters are not as effective as resistor-capacitor filters. The simple capacitor filters reflect some of the noise and it can buld up to higher levels somewhere else.
A straightforward approach would be to attach a resistor in series with a capacitor to an AC plug. The problem with this is that you need to use a proper capacitor and resistor to withstand AC voltage spikes without burning your house down. Surge protectors help, but do not completely eliminate AC spikes.
It turns out that transformers are good at reducing the magnitudes of AC spikes. Their cores have a limited ability to pass high frequency energy and this tames the spikes. Their cores are also good at dissipating high frequency energy instead of reflecting it back into the AC wiring.
A simple filament transformer can be used to make a parallel R-C filter. The larger and simpler the transformer, the higher its core losses and the more effective it will be. Thus, do not use toroidal transformers if cheaper square ones are available.
The math required to determine the R and C values is fairly simple. We want a resistance that looks like 120 ohms (I measured some Romex-type cable and got 120 ohms for the characteristic impedance) to the AC line. Transformers transform voltage by the turns ratio N and impedance by the square of the turns ratio. Thus, a transformer with 120-volt primary and 12-volt secondary has an N value of 10 and N-squared of 100. One needs a 1.2-ohm resistor on the secondary to look like 120 ohms at the AC input to the primary.
The capacitor is used to keep the 60 Hz power out of the resistor. Without a capacitor, the 1.2-ohm resistor in our example would dissipate 120 watts! This would require a very large transformer and heat up the room.
The size of the capacitor determines the actual power dissipated in the resistor. If we use, say, a half-watt size resistor, we want to keep the power down to a quarter of a watt or less to avoid overheating the resistor. The resistor power is the resistor value times the square of the current, so 0.25 watts in a 1.2-ohm resistor means the current has to be less than 0.45 amperes.
We can ignore the phase angle issues for this calculation, as the capacitor impedance will be much larger than the resistor at 60 Hz. The size of capacitor needed to deliver 0.45 amperes at 12 volts is determined by Ohm's Law: I = V/Xc, where Xc is the capacitor impedance at 60 Hz. Thus, we need a magnitude of Xc of 26.3 ohms.
Capacitor impedance is the inverse of [the angular frequency times the capacitance]. The angular frequency is 2 times Pi (3.14159...) times the frequency in Hz. For 60 Hz, the capacitor impedance is 1/(377*C), or 0.0027/C, where C is in farads. Thus, to get 26.3 ohms impedance at 60 Hz, we need a capacitance of 0.0027/26.3, or 100 microfarads.
Using a smaller capacitor will further reduce the power in the resistor but raise the frequency for which the resistor becomes effective. Please do not use an electrolytic capacitor for AC service: these are polarized and cannot withstand significant reverse voltage. They can explode if used incorrectly!
Oil-filled motor capacitors can be used. If the capacitor is too small, there will be little benefit. The transformer core will stop responding at a frequency lower than the effective frequency for the R-C filter.
If you build one of these, put some sort of fuse or circuit breaker on the primary side to prevent a fire if the capacitor or transformer windings should fail.
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Follow Ups
- OK, here is the parallel AC filter again. - Al Sekela 10:04:49 11/04/06 (1)
- "I measured some Romex-type cable and got 120 ohms for the characteristic impedance." - cdb 22:28:50 11/09/06 (0)
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