A look at two dimmer modules. One is a really cheap 1 $/£ eBay module and one is a better quality one as sold in the UK for home use. The main differences are in the component ratings and suppression of electrical noise.
I did some more testing of the unusual potentiometer and the resistor section bridging to the middle pin is the fixed 5K resistance that seems to be part of a filtering circuit. The large conductive area is a deliberate dead zone where the knob activates the switch as you turn the dimmer on, and then it gradually decreases in resistance to the point where the rotation is limited at about 29K. Definitely custom-made for the dimmer application.
The inductor will allow low frequency current to flow without much impedance, but will block very sharp current transitions at the switching point.
The most common component to fail in dimmers is the triac, which usually goes short circuit, jamming the load on all the time.
The triac in the cheap dimmer is a PH600E / BT134 rated at 4A if fitted with a heatsink.
The triac in the more expensive suppressed dimmer is a BTA06-600C rated at 6A if fitted with a heatsink.
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