Ive heard that when you flick the light switch on and off repeatedly it damages the bulb. I know this is certainly not true for LED bulbs, but Im wondering if and how it damages other kinds of bulbs. I know that light bulbs run off of ac current so they are turning on and off 120 times a second so I find it hard to believe that this would damage them.

  • \$\begingroup\$ I've heard that the number of on/off cycles affects the life of compact fluorescents and that they last longer if they are left on for a period of time when they are new. I don't know if that is true or why it might be true. I don't recall hearing that about fluorescent tubes. \$\endgroup\$
    – user80875
    Commented Jan 30, 2016 at 22:41
  • \$\begingroup\$ Obligatory: youtube.com/watch?v=CU5mBpFD8GQ \$\endgroup\$ Commented Jan 31, 2016 at 3:41

2 Answers 2


Incandescent filament lamps resistance changes dramatically - up to ten times - as they warm up. The result is that there is a large inrush current initially but as the element heats the resistance increases and the current decreases to its nominal value. This explains why bulbs generally popped on switch-on rather than at a random moment when burning steadily. (The thermal inertia of the element is high enough that it doesn't cool down between AC mains half-cycles.)

During rapid switching the filament would not cool completely before the next switch-on so it would not be stressed so much. The longer the 'off' periods the harder it is for the lamp.

My domestic experience was that lamps on dimmer switches lasted 'forever' relative to the other lamps in the house. This was due to the rotary dimmer being turned up over, say, half a second and gradually increasing the current while the element warmed up. There was no sudden inrush current.

Lamptech have an article on the subject with some graphs and response times. They also contradict me regarding longer off times. (Ah well.)

  • \$\begingroup\$ the question's content part is about 120 hz flickering, in that case, lamp probably does not cool down enough to get shock \$\endgroup\$
    – qdinar
    Commented Feb 5, 2020 at 16:28
  • 1
    \$\begingroup\$ Thanks. I think I covered that in the last sentence of my first paragraph. \$\endgroup\$
    – Transistor
    Commented Feb 5, 2020 at 16:36

That statement originated (to my knowledge) in the damage that can be caused to incandescent lights (those that produce light as a result of [super] heating some element in the light).

  • The filaments in incandescent lights are rapidly heated to "white hot" during the first several ac cycles of power applied to them (through ohmic heating); remain at a relatively constant (hot) temperature, reardless of the 120hz "pulsing" of ac mains power through them (due to the damping of their thermal mass), until power to the light is "turned off;" then rapidly cool off again.
  • Repeatedly subjecting the incandescent element in one of these lights to this rapid heating/cooling causes thermal stress to the element, often causimg it to become physically brittle.

Fluorescent lights can also be damaged by repeated on/off switching of the "switch," still due to a startup-effect, but differently than incandescents.

  • Unlike incandescent lights, fluorescents don't have a hot element with significant thermal mass, so they do show a 60/120hz "flicker" in their light output as a result of ac mains cycling. They do, however, have starting circuitry that only runs while the light is "warming up" or "coming on."
  • Many fluorescent lights have ohmic heating "preheaters" that wear similarly to the incandescent element above (these also have "starter" switches that frequently wear out as well).
  • In some other fluorescent (and non-fluorescent "arc lighting" or "High Intensity Discharge" lights), there are special starting electronics necessary to provide a much higher voltage to "strike the arc" until a pseudostable plasma channel has developed through the arc-path gas(es). In the starting electronics only ever operate/wear when the light is switched on, and the electrodes imside the "bulb"/"tube" also wear more from the arcing of these first "strikes" than during steady-state (albeit 120hz) usage.
  • 2
    \$\begingroup\$ Just wanted to add: one example of where power cycling definitely does have a serious effect is incandescent stage lighting. As the bulbs are typically 500W-2kW, constantly cycling power has a negative effect. To fix this, a preheat is applied: the bulbs are never actually turned off, they are typically reduced to ~1% power or so, which keeps them effectively dark, but the filament temperature stays high. This is essential for extending the bulbs lifetime. \$\endgroup\$
    – uint128_t
    Commented Jan 30, 2016 at 21:59

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