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Ask Mr. Tight-Watt: CFLs and Photo Eye Fixtures  Smart Choices Archive

Ask Mr. Tight-Watt: CFLs and Photo Eye Fixtures

CFLs and photoelectric eye fixtures don't work well together.

Ask Mr. Tight-Watt

Question: Why will a new compact fluorescent lightbulb (CFL) not work on a photoelectric eye light?

Richard's question is answered by Rob Kirkpatrick, Service Concepts, our CFL expert:

Answer: Richard, this is an excellent question.  There's an overly simple answer, and a detailed answer.

The overly simple answer is: There is usually a trickle of current through a photoelectric eye fixture in the off position that will cause a standard compact fluorescent bulb to wear out sooner than it should, so manufacturers don’t want you to use their CFLs in that way.

As to the details, your CFL may work in a photoelectric eye fixture, but it may not. The ballast in a CFL is a sensitive electronic instrument, designed to work as an expected voltage level and do an expected job.

A photoelectric eye uses a small current to operate. Many photo-eye fixtures permit this small current to trickle through the bulb as a single circuit. With an old fashioned incandescent bulb, you'd never know it. The filament in the bulb just acts like another wire in the circuit for this trickle, which is nowhere near enough to cause the bulb to light up.

With a CFL, however, running that small current through it—through it's sensitive electronic ballast—would be like trying to run your stereo on a very low voltage circuit. It wouldn’t work properly, but with that small current available it would try to work. In the process, components will be stressed, or heat up, and eventually fail prematurely. 

Alternatively, it may be possible that a photo-eye circuit saps enough power from the circuit in the “on” position that it only delivers a reduced amount of power to the light bulb. This may or may not be noticeable with an incandescent bulb, but it might not be enough to light the CFL. Even if it doesn’t light, it will keep on trying and eventually fail.

So, based on the details of the CFL and the photo-eye fixture, your light may look like it’s coming on and off normally. Or it may not come on at all. Either way, it is likely to fail completely well before its design lifespan.

Only CFLs that are labeled as “dimmable” (and they’re quite expensive) can be used effectively (in most cases) in fixtures with either dimmers or photo-eye controls.

Ask Mr. Tight-Watt a question or view other questions he has answered


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