This circuit, also informally known as the poor man’s variac, is used in many applications in both consumer and industrial electronics, where a perfectly sinusoidal waveform is not required and a real variac (variable autotransformer) would be too heavy and bulky. This is the case of light dimmers, vacuum cleaners, motor speed control, heater elements and similar. It was also one of the first of my creations in the electronics field and has surely helped me along the way.
However, the real difference between a circuit like this and a classic light dimmer from your local [insert chain store name here]; is the possibility to regulate also inductive loads. Basically, cheap commercial SCR- (thyristor) or triac-based dimmers are used for resistive loads only, i.e. incandescent/halogen light bulbs, heater elements etc.
With the addition of a protective RC circuit in parallel with the triac to filter out inductive kickback, new possibilities unfold: you are able to regulate fans, mains transformers, vacuum cleaners, drilling machines, angle grinders and the list goes on and on.
The principle is quite simple. Depending on triac’s conduction angle, which is set with the potentiometer, the circuit chops the input sinusoide in segments, this is best viewed on an oscilloscope. At full conduction the output has a sinusoidal waveform; yet the lowest angle yields only its very first portions to the output.
The regulation range itself depends on the load. Pure resistive loads start to work almost directly from the minimum setting onwards; however, transformers and whatnot are working only after e.g. quarter-way through.
Choose a triac with a 400-600 volt rating, at least. With a decent heatsink, up to 3 kilowatt appliances can be controlled.
Despite the simplicity of this circuit, it also has some minor disadvantages. Aside from the load-dependent smoothness of regulation, the regulator makes quite an interference in shortwave radio.
In case of improper RC-circuit matching with your particular triac, or if an inductive load with wiggly physical mounts is connected to the output, for example mains transformers with their cores insufficiently tightened, they might vibrate and make a growling sound, resemblant to that of a combustion engine. This is because the appliance is fed with deformed waveform.
Do not try to regulate capacitive loads or anything which has a switched mode supply. In the former case you might short your triac out, in the latter, the supply will behave erratically at throttled output. Examples of capacitive loads are e.g. appliances with power factor correction or starting capacitors (big motors, fluorescent lighting, etc.); switched-mode supplies are found in battery chargers, computer power supplies, CFLs, televisions and basically everything, that lacks a “classic” transformer and works at lower/higher voltages than mains.