On Sat, 17 Sep 2016 20:00:16 -0500, Don Foreman
wrote:

On Sat, 17 Sep 2016 07:34:24 -0700, Larry Jaques
wrote:



Those only last a couple years, yeah? I've replaced quite a few
photosensors over the years in PIR lamp fixtures.

Didn't know that! We'll see, I guess.

The failure rate is ghastly, and invariably they short. Some last a
year, some 20, but in most cases, I've just turned the light on at
night & off in the morning. Often, I've moved to a PIR, and once the
sensor shorts, I just let it shine (5 min timer) the few times I pass
it in the day.

The marginally-designed Chinese 117-volt switch-on-when-dark modules
may fail, but CdS photocells are very reliable. The CdS photocell is
just a photoresistor that varies from a low resistance (a few K) in
presence of light to a high resistance (megohms) in total darkness.

The Chinese dark switches in lights probably use a triac to switch AC
line voltage. Triacs can get zapped by transients, and they'd fail
short when that happens. The remedy is to use higher-voltage triacs,
but that'd cost another few pennies. Designers and mfrs of traffic
controls never use less than 600-volt triacs, and they are protected
from transients with tranzorbs and MOVs.

Yeah, but the advice from Legal is to cheap out and make more money by
selling all those replacement units whose failure was caused by
knowingly cheaping out. Business Anti-Ethics 101, I guess.

--
The goal to strive for is a poor government but a rich people.

--Andrew Johnson