Why does a photoeye stop working?

[Kdog76]

Had a customer ask me that yesterday. I told her that they are subject to failure but really what it is that causes it to stop working?

 

[mcclary's electrical]

Had a customer ask me that yesterday. I told her that they are subject to failure but really what it is that causes it to stop working?

Were the relays solid state?

 

[realolman]

i just replaced one the other day. Didn't amount to much.... had a bi-metallic strip switch that turned the lamp on and off. The strip was wrapped by a heating element that was in series with the photocell... a resistor that changed resistance with the light or dark.

I don't know why they usually fail, but in this one the solder on the leads of the photocell just kinda rotted away from the photocell, so the heating element didn't heat, the bi-metallic strip didn't bi-metal, and the switch didn't switch.

 

[charlietuna]

Photo eyes fail mainly due to overload ! Next photocell you buy figure double the load and you'll never have aproblem with it.....

 

[e57]

A short ciruit through it could do some damage - failure of another part of the circuit could make it seem as if it is not working - and so could dropping a large rock on it too....

That said - I don't know of too many that are 'prone to failure' - so to speak - in fact they are usually pretty resiliant - for what they are....

 

[jeremysterling]

Someone told me the sun burns out the photocell. The person also stated that's why the manufacturer suggests facing them north.

But I'm pretty sure the manufacturer suggests north because of better responsiveness to ambient light.

I think sometimes that the contacts of the photocell switch weld shut and the light stays on all day.

Other times, something in the circuitry opens up, perhaps from overheated solder joints?

 

[ceb58]

The majority of the bad ones I have had to replace for people have been after bad lighting storms. Seems they dont like it very much

 

[cschmid]

it has to do with the amount of light being recieved on the photo electric sensor pad. most people install and open the sensor wide open. when it is not necessary. it is called over exposure. so more is not better on photo cells and if everything was designed to last forever we would not have jobs..

 

[cadpoint]

It's faced north to protect the sensitive nature of a copper ribbon layed on a plastic sheet or mylar (for the techno buff's). The ribbon of copper is worn down due to the movement on this sheet.

The circuit is balanced when created; the wear (of Sol) on the sensor element causes the imbalance thus tripping the light on, which it picked up by the electronic's.

Un-related; ok sudo related, look up "aged glass", Everything moves... One engineers to compensate.

: Not Googled!

 

[K8MHZ]

It's faced north to protect the sensitive nature of a copper ribbon layed on a plastic sheet or mylar (for the techno buff's). The ribbon of copper is worn down due to the movement on this sheet.

The circuit is balanced when created; the wear (of Sol) on the sensor element causes the imbalance thus tripping the light on, which it picked up by the electronic's.

Un-related; ok sudo related, look up "aged glass", Everything moves... One engineers to compensate.

: Not Googled!

And all this time I thought they were cadmium sulfide.

Googled:

Photoresistors come in many different types. Inexpensive cadmium sulfide cells can be found in many consumer items such as camera light meters, street lights, clock radios, alarms, and outdoor clocks.

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

 

[cschmid]

the variable is the heat generated on the photo eye housing even if it is faced north and I would bet that only 60% are actually north. this causes the copper to become softer and wear faster. I did not use all the technical terms but the theory is correct. I suppose I could go back to a book and use the correct terms but is it really necessary? I do not do engineering everyday so my technical terminology is old and rusty kind of like me. :grin:

 

[Article 90.1]

Many of the ones that I have replaced have had moisture as at least a key ingredient of the failure. When a photocell is mounted on the side of a box and you take it out and the water runs out of it that is your first sign. Other common failures are due to weed whacker carelessness! Electromechanically, I'd agree with what the engineers have stated thus far.

 

[danickstr]

I would think that typically the triac (solid state relay) which does the heavy lifting of turing on 120v with a load each time it is told to, would be the place where the thing wears out. Sort of like points in a car burning out over time due to arcing.

 

[fondini]

I always size mine same as I do low-voltage lighting transformers, load=1/2 rating of photoeye,never had a problem

 

[CFL]

I would think that typically the triac (solid state relay) which does the heavy lifting of turing on 120v with a load each time it is told to, would be the place where the thing wears out. Sort of like points in a car burning out over time due to arcing.

Solid state relays don't have contacts, they don't weld shut like someone else mentioned, and I don't think any that I've used have bimetallic strips. Please correct me if I'm wrong.

 

[K8MHZ]

Solid state relays don't have contacts, they don't weld shut like someone else mentioned, and I don't think any that I've used have bimetallic strips. Please correct me if I'm wrong.

You are correct.

They use triacs, FETs or thyristors.

 

[cschmid]

I wonder does anyone here have a schematic of a photocell. FET Field effect transistor, triac's being a dual flow device, thyristors are multiple layers devices..would not the size of the device be rather large to handle the current and how does the heat and moisture cause these devices to break down..

I just can not visualize thyristors in this application..

 

[ELA]

Solid state relays don't have contacts, they don't weld shut like someone else mentioned, and I don't think any that I've used have bimetallic strips. Please correct me if I'm wrong.

You are correct.

They use triacs, FETs or thyristors.

Correction as requested:

Here is a picture of a bi Metallic switched Photoswitch:
I don't like them but they are very common.

The cell is high resistance ( > 50K ohms) when dark and in series with a small heater ( 7.7k ohms). Contact is normally closed to the red wire so light is on.
As the ambient light increases cell resistance decreases to ~ 3K ohms and the heater dissipates ~ 1.5 watts. Bi Metallic strip expands and switches the lamp off.

 

[realolman]

Solid state relays don't have contacts, they don't weld shut like someone else mentioned, and I don't think any that I've used have bimetallic strips. Please correct me if I'm wrong.

Pop the plastic off of a photo electric switch that twists like a twistlock plug into the top of a mercury vapor "pole light".

I got the replacement off the shelf at a local True Value hardware store, so I think they're pretty common. Electronics have replaced mechanical stuff all over the place, so it could be that the bimetallic strips are out now... maybe the replacement was electronic... but the bi-metallic switch stuff is what was in this one, and that's what was in them 35 years ago

As I said in another thread, something can always be made cheaply, but a triac based switch should do it's current switching as the load voltage waveform crosses zero, in order to prevent noise .... which also prevents the triac from switching under load.

Thanks ELA, for the picture.... I didn't see your post until I had written mine, but I can certainly see all that mechanical stuff being replaced with a 10 cent electronic component.

 

[e57]

The triac/SCR based ones are instant switching - which can be problematic - and yes the bi-metal are more common as it privides time delay - but I have to say - these do die - but not every day... Most installed will last many years - sometime decades with no problems... Unless the sealed enclosure has been hit with a bat or something, or switching a load over recommended...