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.
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.
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