I've seen the answer to questions asking why some bulbs can't be used with dimmers [1][2], and that makes sense, but why can't some light bulbs be used with timers? Aren't most timers equivalent to an on/off switch, or do they somehow leak current like a dimmer? I've thrown caution to the wind and tried such bulbs in a lamp attached to a timer and it works fine, and lasts as long as the same bulb in a non-timer controlled lamp.

  • I've never heard of a bulb that can't be used on a timer. Do you have an example?
    – DA01
    Commented Apr 21, 2015 at 16:48
  • @DA01 - Not at the moment. I'll see if I can find a box that has this printed on it.
    – j08691
    Commented Apr 21, 2015 at 16:53
  • 2
    Some timers switches do not require a neutral, just like some motion detectors. See diy.stackexchange.com/questions/30359/…
    – longneck
    Commented Apr 21, 2015 at 17:47
  • 3
    Your linked questions refer to non-incandescent bulbs, CFL and LED. Timers that work without a neutral connection must get power by trickling current through the bulb when off and dropping some voltage (bulb is slightly dimmed) when on. This works well with incandescent and halogen bulbs but not necessarily with CFL or LED bulbs.
    – DoxyLover
    Commented Apr 21, 2015 at 20:20
  • 1
    @longneck is that talking about timers or dimmers?
    – DA01
    Commented Apr 21, 2015 at 22:46

1 Answer 1


Timers come in two varieties:

  1. Timers with a neutral wire. These are connected between the hot and neutral wires of the circuit, parallel to the load. They're powered like any other device, and work by switching the hot (and possibly neutral) wires of the controlled circuit. These timers can control any device, but require a location with access to both the hot and neutral wires.

  2. Timers without a neutral wire. These are connected in series with the load, and work by restricting the current through the hot wire of the controlled circuit. Because they use the load as part of their own power circuit, they can only control resistive loads (essentially, light bulbs and some electric heaters), but can be installed anywhere (in particular, as a drop-in replacement for a light switch where only the hot wire passes through the switch box).

LED and fluorescent bulbs (and motors, and...) don't work properly with the second type of timer, either because they require a sufficient current to let any electricity flow, or because too low a current will damage them.

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