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#35928 03/24/04 06:30 PM
Joined: Dec 2001
Posts: 444
Sandro Offline OP
Hey all, I've been doing alot of maintenance work lately with outdoor lighting and photocells. Anybody have any info websites on photocells and how exactly they work? Why do they require a neutral? What makes them fail? Are there any moving parts inside one? Guess I'll have to take apart the next bad one I replace.

I've been wiring these things for years yet I couldn't explain how one actually works.

#35929 03/24/04 06:57 PM
Joined: Jan 2003
Posts: 4,391
Heres the best I could come up with.

Bob Badger
Construction & Maintenance Electrician
#35930 03/24/04 07:58 PM
Joined: Sep 2001
Posts: 806
Why do they require a neutral?

Because the electronic circuitry inside the unit needs power to operate, which is drawn from the hot and neutral wires, like any other load.

What makes them fail?

Besides improper installation, transient voltage spikes would probably be the leading killer, along with corrosion/short circuits from water leakage into the case.

Are there any moving parts inside one?

Only the armature of a relay in most units. Some units use a TRIAC or other solid state switch, eliminating the mechanical relay.

Guess I'll have to take apart the next bad one I replace.

"Reverse Engineering" is always instructive... [Linked Image]

I've been wiring these things for years yet I couldn't explain how one actually works.

Basically, just a comparator circuit. A comparator looks at 2 different voltages (signal and reference), and turns on an output if the signal input equals or exceeds the "reference" voltage. In a photocell switch, the input signal comes from a cadmium sulfide photocell, which produces a voltage proportional to the ambient light level. The reference voltage is set by the "sensitivity" adjustment pot. When the sensor voltage drops below the setpoint, a relay is turned on, energizing the load.

#35931 03/24/04 08:57 PM
Joined: Jan 2004
Posts: 615
who needs "how stuff works" when we have NJwirenut?

#35932 03/24/04 11:37 PM
Joined: Mar 2001
Posts: 85
Kinda off topic but....we're using something called an Aube Timer now instead of photocells. Basically its a timer that works off Latitude and Longitude and automatically readjusts itself everyday. It also gives you on and off options if need be. On the commercial stuff it save running that 3-wire to the outside of the building and can also double as a hand/off/auto switch.

#35933 03/25/04 06:00 PM
Joined: Dec 2001
Posts: 444
Sandro Offline OP
what is the price difference between the two though?

#35934 03/26/04 03:29 AM
Joined: Oct 2000
Posts: 2,722
Broom Pusher and

Just saw your post regarding Photocells, and would like to add a little more info.

Simple models - such as the common directional types with the ½" threaded base, 3 wires (normally Black, Red and White), and rated 1.0 - 1.5 KW @ 120 VAC - work simply as a 1 pole contactor, with a light level sensor which turns on or off the contactor.

The Photo Electric Cell portion may be a Photo Resistor, or a Photo Diode. When Light falls on it (a significate level of Light), it - the Photo element - will either block the flow of current, allow a high flow of current, or create a flow of current from its self.

If the element blocks the flow of current, then the contactor will have N.O. contacts between Line input and Load output.
In this case, the contactor will remain open until ambient light levels fall enough to allow current to flow through the Photo element and energize the contactor - making the contacts close.

If the element allows a large current to flow under daylight conditions, then the Contactor will have N.C. contacts between Line and Load.
During times when daylight is abundant, the contactor is energized with the contacts held open - due to the N.C. arrangement.
Once ambient light levels drop low enough, the contactor is de-energized (due to no current flowing through the Photo element and through the coil circuit of the contactor), so the contacts close - resulting in a complete circuit between Line input and Load output.

With the case of an element "creating its own power", the element drives an amplifying device - either a small relay or a Transistor, then this amplifying circuit controls the large load carrying contactor.

For the common type PCs, here are the functions per wires:


[*] Black Wire: Line input for both the lighting controlled by the photocell and to the contactor within the device,

[*] Red Wire: Load output to whatever loads are controlled by the device,

[*] White Wire: End of the device's internal contactor's coil - carries very little current.

The Photocells used on 208/240VAC work the same way - only the "White Wire" is not normally connected to the system's grounded conductor - but instead is connected to one of the opposing circuit (the "Other" Ungrounded Conductor of the 208/240VAC circuit).

I thought we had schematics of Photo Cells posted here, but it looks like none have made it in!
Might be something to add.

Sure hope this message is helpful.

I'll double check it tomorrow after getting some sleep, and edit where needed!
[Linked Image]


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
#35935 03/26/04 02:06 PM
Joined: Oct 2003
Posts: 172
hey canadian sparky
have you got a web site so we can view this timer that you speak of.

#35936 03/26/04 07:27 PM
Joined: Mar 2001
Posts: 85
Sorry guys....been busy.
These are american prices Sandro couldnt get the canadian price yet, our order desk guy is in Mexico.

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