Q. Why is there a noticeable delay when I switch on a light bulb?
A. Chances are that a household now has a mixture of bulb types — incandescent, compact fluorescent (CFL) and light-emitting diode (LED) bulbs — and each has a different start-up process.
The speed they switch on depends on the type of bulb and the intervening mechanisms like transformers, ballasts and drivers that may be present.
With an old-fashioned incandescent bulb, the current goes at full strength directly to the tungsten filament, which very quickly heats up to the temperature that produces visible light.
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In a compact fluorescent bulb, actually a tube twisted into the shape of a bulb, a burst of current is needed make the initial electrical collection. Then the gas within the tube is heated to a temperature high enough to fluoresce and emit light. In old-fashioned fluorescent lights, a separate device called a ballast provides the surge, then damps down the current to a steady voltage that will not quickly burn out the light.
In modern CFL’s, this device is integrated into the bulb.
With LED bulbs, many have built-in mechanisms called drivers to gradually transform the current to the voltage needed to light up the bulb, as well as a resistor to keep it at a constant level thereafter. There is a significant delay while the driver does its initial work. The time lapse may increase if the bulb is connected to a dimmer switch, which only some LED bulbs are equipped to work with.