interference - LC Filter for Phase Control

Can you point me to a good online resource to learn how to understand and design LC filters?

The goal is digital forward phase control to dim a light bulb. I've read that the EMI produced by the voltage spikes when a TRIAC goes into the "on" state can be bad news for other electrical devices nearby on on the same supply circuit.

http://www.epanorama.net/documents/lights/lightdimmer.html gives an example circuit with a 50-to-100 microhenry inductor in series with the TRIAC, and a 100 nF capacitor in parallel with it, but I don't know how the author came up with those numbers.

Answer

Are you talking about this circuit?

     R1                       R2

     180                      1K
+---/\/\/\----------+   +----/\/\/-------------+------------+-----------> 230V
                   1|   |6                     |            |             Hot
                   +=====+ IC1                 | MT1        |
                   | MOC | TRIAC              +-+           |
                   | 3020| Driver           G | | TRIAC     |
                   +=====+                   /| | TIC226D   |
                   2|   |4                  / +-+           |
+-------------------+   |                   |  | MT2        |
                        +-------------------+  |            |

                                            |  |            |
                                            \  |            |
                                      R4    /  |            |  C1
                                      1K    \  |           --- 100 nF
                                            /  |           --- 400V
                                            |  |            |
                                            |   )           |
                                            |  (   L1       |
                                            |   ) 50..100   |
                                            |  (    uH      |

                                            |  |            |              Neutral
                                            +--+------------+----o    o--> 230V
                                                                  load

The values are standard "band-aid" values to improve ripple. They both store energy for the load to use during transitions. The author most likely picked these values from experience instead of calculations, just like one would choose decoupling values for digital IC Vcc pins ("Hmm, this one uses about 10mA max, so a single 100nF cap aught'a do'er!") . It's not like a SMPS, where an inductor's precise value is critical to the device's range of operation. One can make rough estimates by comparing the maximum energy used by the load during "dead time" (time between zero-crossing and SCR switch) and energy stored in the inductor: E_L = ¹/₂LI² (use RMS current). For this application, though, the inductor will likely saturate and empty very quickly, so the stored energy really just smoothes the transitioning voltages.