Above is a dimmer circuit. In some of these circuits I see a toroid inductor together with a capacitor forming a low pass LC filter which is called choke filter.
My questions are:
Why is RL used instead of RC and is that something to do with the character of mains freq. and voltage? Inductors are expensive or big thats why we use caps around opamps to obtain low pass filters right?
What is being choked by the LC toroid cap couple?
Ideal capacitors and inductors don't dissipate any power, whereas resistors do. One advantage is therefore that L-C filters are more efficient than R-C or R-L filters. Often it's not the actual waste of power that is the driving factor, but the mechanics required to get rid of the heat. Lamp dimmers often need to fit into small spaces where keeping them cool would be difficult.
Another advantage is that L-C filters attenuate more in the stop band. These are double-pole filters. R-C and R-L filters are single pole. Well into the stop band, a single pole filter attenuates by the frequency ratio to the rolloff point. In log space, that's 20 dB per decade. A two pole filter is like two poles applied in series. They attenuate by the square of the frequency ratio to the rolloff point, which is 40 dB per decase in log space.
For low power (a few watts to 10s of watts) dimmers, the main reason to filter is to reduce the high frequencies so as not to interfere with radio communications. These are many multiples of the 50 or 60 Hz fundamental frequency. A L-C filter will attenuate the higher frequency harmonics relatively more than the lower frequency ones. This is useful since it's the higher frequency content that the regulators care about more.
LC noise filtering benefits were explained in other answers. This is an addition specific to triacs or thyristors.
One reason to use an inductor in series with a triac is that a triac has maximum allowed rate of change of current (\$\frac{dI_T}{dt}\$ rating) when it is triggered into conduction. This may or may not be an issue for a particular triac based circuit, but in general such problem does exist.
A quote from http://www.nxp.com/documents/application_note/AN_GOLDEN_RULES.pdf (page 5):
When a triac or thyristor is triggered into conduction by the correct method via its gate, conduction begins in the die area immediately adjacent to the gate, then quickly spreads to cover the whole active area. This time delay imposes a limit on the permissible rate of rise of load current. A \$dI_T/dt\$ which is too high can cause localised burnout. An MT1-MT2 short will be the result.
An inductor limits \$\frac{dI_T}{dt}\$, provided that the inductor does not saturate.
A dimmer typically works by using a triac to take only a portion of each 60Hz input power cycle. When the cycle gets chopped off abruptly by the triac this creates a pulsed power draw at the input.
The pulsed power draw will have harmonics of the 60Hz waveform in it. In order to comply with FCC regulations regarding conducted emissions a filter must be added isolate the mains power from those pulses.
So why use RC vs LC?
LC FILTER:
PRO: Dissipates less power than RC filter (so less heat)
PRO: Second order filter, which means better noise rejection
CON: May resonate if not properly damped
CON: More expensive than RC due to higher inductor cost.
RC FILTER:
CON: Dissipates more power than LC filter
CON: First order, worse noise rejection
PRO: Inherently free of resonance
PRO: Cheaper than LC due to low cost resistor
