URD watch lights light stay on in day

Would someone like to explain the theory behind street light or watch lights staying on all day with replaced photocells. Open neutral i suspect but explain the theory behind the electronic cell

Ummmmmmmm I kinda spect if you remove the photo cell and look closely at the bottom of it it will say " hencho en mexico". Most if not all of the electronic PCs also have a timer that delay the operation for a few seconds. to allow for say a lightening flash and keep the light from cycling during a storm. I bleve this timer is where they fail.

Sometimes the cells also switch contactors to control a lot of lights, and if the contactor contacts weld together they can stay in.

If it's definitely a new photocell you plug in and it's controlling the lamp directly then maybe there's an internal short between live and switched live, or maybe someone shorted them deliberately to get the light on because they didn't have a spare photocell.

One common cause for contact welding in contactors or cells controlling lights is the high inrush current to the power factor correction capacitors. They appear as a dead short initially when power is applied at anywhere other than the mains zero crossing point.

And just to confuse you more they make normal open and normal closed photocells. Depends on the application. Highway lighting around here used to always have normal closed photocells, when they failed the lights would stay on!!!!

around 2004-2005 when they quit using cadmium sulfide photocells and a contactor and started using photo diodes and Solid state switching.

The reason being is the latter uses far less energy in the control circuit.

Ummmmmmmm I bleve the only ones I ever seen wuz normally closed. thus the blink when ya energize em or the possibility of gettin in series with one.Light opens em n lack of it allows em to reman closed

You can see the difference because the Good ones have a CdS cell like this.

http://en.wikipedia.org/wiki/Photoresistor

The crappy ones have a photodiode like this.

http://en.wikipedia.org/wiki/Photodiode

The old Cadmium Sulphide photocell ones were super-simple. They had a cadmium sulphide cell in series with a heating element on a bi-metallic strip. In daylight the cell would pass enough current to heat the strip and make the contacts open. (They started off closed so the lights go on initially.)

Since the photocell and element need the neutral, if it's missing the heater won't heat up and open the switch.

Cadmium is one of the materials that they're trying to ban under the RoHS regulations. This is pointless since the cadmium sulphide cell is rugged and simple. It's right up there with banning lead from solder. Maybe they'll ban electricity next!

But I never really tore one apart and dissected its innards for analysis but I know they were way more reliable than what is on the market now.