Triode Alternating Current switch
Electronic Semiconductor device which is an integral part of modern dimmers. When a current is applied to a triac, it starts conducting, and continues until the current passing through it falls to zero.

Electronic dimmers do clip the AC cycle. They use transistor-like devices called triacs to switch on the current to a lamp part way into each half-cycle. By shortening the time that power is delivered to the lamp, the dimmer reduces the total energy delivered to the lamp during each half-cycle and the lamp dims. But while a triac turns on easily, the only way to turn it off is to get rid of any voltage drop across it. The dimmer uses the alternating current itself to turn off the triac--the voltage of the power line naturally goes to zero at the end of each half-cycle and the triac turns off. The triac then waits until the dimmer restarts it, sometime into the next half-cycle.

Since the dimmer messes up the waveform of the electric current flowing through the lamp circuit, what you measure with a voltage meter depends on how that meter works. Since many AC voltmeters just measure peak voltage and assume that they are looking at a pure sinusoidal current, they don't give you an accurate sense for what is really happening to the voltage of this clipped waveform as a function of time. Unless an electronic dimmer is turned way down, the peak voltage it delivers will be close to the normal power line peak, a fact which tricks the voltage meter into reading a high value and which allows a properly designed fluorescent lamp to continue operating normally but at a dimmer level.

But if the lamp uses a normal bulb and obtains three light levels from it, then it uses the same technique as a dimmer switch. In this technique, an electronic switching device called a triac is used to limit the times during which electric current can flow through the bulb and deliver power to it. In the bright setting, the triac permits current to flow through the bulb at all times and the bulb appears as bright as possible. But in the dim or medium settings, the triac prevents current from flowing at certain times. The triac takes advantage of the fact that the power flowing through a household lamp is alternating current--current that reverses directions 120 times a second (in the United States) for a total of 60 full cycles of reversal, over and back, each second (60 Hz). At the beginning of each current reversal, the electronic devices that control the triac start a timer. This timer allows those devices to wait a certain amount of time before they trigger the triac and allow it to begin carrying current to the light bulb. Once triggered, the triac will allow current to flow through the bulb until the next reversal of current in the power line. Thus the amount of energy that reaches the bulb during each half-cycle of the power line depends on how long the electronic devices wait before triggering the triac. The longer they wait, the less energy will reach the bulb and the dimmer it will glow. In the bright setting, the triac is triggered immediately after each current reversal so that power always flows to the bulb and it glows brightly. But in the medium and dim settings, the triac is triggered well into the half-cycle that follows the reversal. A normal dimmer gives you complete control over this delay, but a three-way touch switch only provides three preset delays. The medium setting has a medium delay while the dim setting has a long delay.