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CFL Lamps with Day Night Switch  

I received an email from a man with a classic problem.  The guy indicated that he could not seem to get compact fluorescent lamps to last more than a few weeks in an outside porch light.  He wondered if I had a solution.  I asked a simple question.  Did the porch light have a day/night switch?  The answer was yes, it did.  Bingo! I told him that standard day/night switches will quickly kill standard CFL lamps. 
The killer is the nature of the conventional day/night switches.  These devices have very little hysteresis.  When night time approaches, the switch senses the darkness and turns on the light.  The added light from the lamp causes the switch to think it is no longer dark and turns off the lamp.  The dark condition again causes the switch to turn on the lamp.  This cycle causes the lamp to rapidly flutter on and off in the morning and again at night.  The rapid fluttering puts a great strain on the CFL lamp’s control circuit, which results in a dead lamp within days or weeks.  There are two solutions.  The cheapest solution is to use only “dimmable” lamps in porch light fixtures.  Dimmable lamps use a different circuit and can tolerate the fluttering mode. 
Porch Light  Compact Fluorescent Lamp
A second solution is to replace the two wire day/night switch with a three wire device, which has built-in hysteresis.   The hysteresis feature is not often advertised so some experimentation may be needed.  I have yet to find a suitable day/night switch which does not flutter but I have been told they do exist.
 Dimmable CFL 3 Wire Day/night Switch Twilight Switch

May 2011     Issue 17

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