Typical dimming system for incandescent lamps

AN2590

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Typical dimming system for incandescent lamps

A typical dimming system consists of the lamp(s) and dimmer (Figure 3). The dimmer

consists of the triac and a few components, see Figure 1. Dimmers of this type are

appropriate for incandescent lamps and are very popular in the market.

Figure 3.

Typical dimming system consisting of incandescent lamp and triac

dimmer

The triac conducts once it has been triggered and holds latching current. The triac shuts

down when the current is less than the holding current. The dimmer works fine with a

resistive load. The triac can be triggered at any timing of the sinuous voltage (AC line input),

and can be kept in conduction state until reaching zero line voltage. This allows the lamp to

be dimmed nearly to 100%.

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Formal CFL ballast and its disadvantages

Since the formal CFL is not a resistive load, the ballast has no power factor correction circuit

(PFC). If the incandescent lamp (Tungsten filament light bulb) is replaced by a formal CFL

(Compact Florescent Lamp) directly, the CFL cannot fully illuminate and may have

intermittant blinking or no light at all. Figure 4 shows the block diagram of a formal CFL.

Figure 4.

Block diagram of formal CFL ballast

The formal CFL ballast consists of a rectifier, storage capacitor, ballast driver circuit, half

bridge and resonant circuit. The source energy stored only to the storage capacitor from the

AC input at the timing nears its peak voltage per half cycle. When the formal CFL ballast is

connected to the triac dimmer (see Figure 1), the triac conducts only after having been

triggered, that is, when the rectified voltage is higher than the voltage across the storage

capacitor. At this extreme moment the capacitor can be charged with the same peak from