Article: 12390 of comp.home.automation From: (Rane Curl) Subject: Signal-Bridge "Tuneup" Date: 14 Dec 1995 17:00:56 GMT Organization: University of Michigan Engineering, Ann Arbor Sender: I lived with an unreliable wall-switch light in my X-10 system, when I learned from this newsgroup about the problem of passing the control signal from one phase to the other phase of a three wire service. The X-10 FAQ suggests bridging the two phases with a 0.1 mfd capacitor at the service panel. However my house has an unused 240 V receptacle that could be used for a plug-in signal-bridge. In addition, it appeared to me that the 13.2 ohm impedance of a 0.1 mfd capacitor, at 120 KHz, might hinder obtaining equal signal strength on the two phases. A larger capacitor could be used, but that is more expensive, physically larger, and increases the reactive power load on the circuit from the 2.2 VA with 0.1 mfd. I decided to tune out the capacitor's reactive impedance at 120 KHz by means of a series inductor. The complex impedance of a series R-L-C cricuit is given by Z = [1/jCw][1 - CLw^2 +jCwR] where w = 2(pi)f and j is root(-1). If one chooses L = 1/(Cw^2), the reactive impedance of the capacitor is cancelled, and the circuit impedance is resistive with Z = R. One can also describe this as tuning the series circuit to resonance at the chosen frequency. The inductance required to bring a 0.1 mfd capacitor to resonance at 120 KHz is 17.6 mH (microhenries). A 18 mH inductance is an "off the shelf" value, and therefore that was chosen. The finished signal-bridge therefore consisted of a 0.1 mfd, 630 V, capacitor in series with a 18 mH inductor - plus a 1/4 amp fuse for "discretion" - all installed in a 240 V cord-plug housing, which can be plugged into the 250 V receptacle (it could also be installed in a box as part of an extension cord for the 240 V circuit). I tested this "tuned signal-bridge" and found a resonant frequency of 120.7 KHz. This is slightly higher than that expected from 0.1 mfd and 18 mH (118.6 KHz), because of the tolerances of the components. The non-reactive resistance of the bridge was measured to be R = 3.3 ohms, primarily from the resistance of the fuse. To attain the equivalent impedance with just a capacitor and the fuse would require C = 8.7 mfd. This circuit, however, does not increase the reactive power at 60 Hz above the 2.2 VA expected with 0.1 mfd. The circuit is also "wide band", with a stand-alone Q = 4, and a half-power bandwidth of ca. 30 KHz. The X-10 signal on the two power phases will be essentially in phase, so this does not address the problem of obtaining workable X-10 control across the two phases. Oh - incidentally - it works. Rane L. Curl N8REG
Scott Presnell
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