At 09:20 AM 3/6/01 -0800, Dan wrote: >The resistors are little 1/4W jobbers with values 100 and 220 ohms. >I just scraped the insulation off one, and it is metal film. These typically have a voltage rating of 350-500 V. >In general, the mains voltage doesn't actually appear across the >snubber R's, but largely across the reactance of the capacitors, >which is quite large at 60hz, so the mains current flow thru the >R's is tiny: > >Xc = 1/[2*pi*60hz*0.1uF] = 26K The capacitor is there to block 60 Hz current from flowing through the resistor and to move the resonant frequency of the inductance >> 60 Hz. >If I understand it correctly, the resistors are just in there to >limit impulse current levels through the snubber caps when the >relays switch. In this case, I would imagine that the wattage values >are not as important as how much voltage you can safely put across >the resistors - related to insulation, construction material, and >how far apart the leads are. The resistor is there to damp the back-emf resonance between the inductor and the capacitor when the triac turns off. The energy flows back and forth between the capacitor and inductor and the resistor guarantee a rate of decay. The wattage rating of the resistor is based on its conduction of the charged capacitor energy (at peak voltage). This energy is simply 0.5 C V^2 in 1/120 sec, or 60 C V^2. The larger the capacitance, the larger the resistor wattage required. >Regarding R values, they have to be small enough that enough >current will flow in the cap to effectively squelch the switching >transients, but large enough to limit instantaneous currents thru >the relay contacts to non-destructive levels. The 100 ohm R was used >in the switching ckt to a larger load than the 220 ohm R's. The optimum damping R is ~ 0.5 sqrt (L/C). ================================================================ Robert A. LaBudde, PhD, PAS, Dpl. ACAFS e-mail: ral@lcfltd.com Least Cost Formulations, Ltd. URL: http://lcfltd.com/ 824 Timberlake Drive Tel: 757-467-0954 Virginia Beach, VA 23464-3239 Fax: 757-467-2947 "Vere scire est per causas scire" ================================================================ -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body