Post by thread:[EE] Long-term reliability of cds photo-resistive

I have a few projects that I'm using cds-type photocell resistors in them. Specifically they are all digikey #PDV-P9001-ND in a simple voltage divider circuit. I'm running them to spec, 1ma max-current etc. They've all gotta last at least 10 years. My definition of failure would be their specs drifting by more than about 30% In the applications they are at worst completely uncovered and *may* be subject to UV light, but would generally be used in a standard office environment. I mentioned this to someone who said that photocells are unreliable and won't last more than a year or two. He said they all fail due to light exposure within a few years. Seems pretty dubious to me, photocells are used in streetlamps after all, and the datasheets don't mention anything worrying, but... What do you guys think? -- http://www.petertodd.ca

On Sun, Apr 01, 2007 at 10:26:20AM +1200, Jinx wrote: > > I mentioned this to someone who said that photocells are unreliable > > and won't last more than a year or two. He said they all fail due to > > light exposure within a few years > > Sound pretty pessimistic. If you're really worried about UV then just > about any plastic will block it. Cut up a sunglass lens, everyone's got > a busted pair of sunnies in the drawer Thanks, that's good to hear. As for the guy I heard it from... Well, suffice to say I've had my suspicions that he can't stand being anything but the smartest guy in the room, even when the subject is electrial engineering and he's a Java programmer... But, wanted to see what you lot thought first. -- http://www.petertodd.ca

> Thanks, that's good to hear Hey, don't mistake me for an expert !! ;-) "8 Year Standard Warranty" http://www.imexcom.com/p2-electrical-photo.html Silicon vs CdS http://www.precisionmulticontrols.com/PDFdownload/Sulfidevssilicon.pdf http://www.selcoproducts.com/CFM/photocell_toc.cfm www.selcoproducts.com/CFM/photocells/photocell_PDF/Selco_PhotoCells_C onstruct.pdf OPERATING LIFE Figure 13 illustrates the change in illuminated resistance (at 10 lux) of CdS cells with operating time, showing slightly increasing curves. If used within the maximum ratings specified in this catalog, the CdS cell recovers from this time change and maintains stable values for quite a long period. Exceeding the maximum ratings can cause deterioration or damage. If this is kept in mind, the life of CdS cells can be expected to be quite long www.selcoproducts.com/CFM/photocells/photocell_PDF/Selco_PhotoCells_p recautions.pdf Use at high temperature and humidity shortens the cell life and should be avoided Avoid usage that exposes the CdS cell to strong ultraviolet light

Silicon type photocells (tiny little solar cells, essentially) have some of the same problems that power solar cells have - degraded output current over time. CDS (Cadmium Sulfide) should be much more stable over long periods of time. As far as longevity, consult your manufacturer - some cheap ones last forever, some expensive ones dies quickly, and vice-versa. Horses for courses... -Adam On 3/31/07, Peter Todd <spam_OUTpeteTakeThisOuTpetertodd.ca> wrote: {Quote hidden}> I have a few projects that I'm using cds-type photocell resistors in > them. Specifically they are all digikey #PDV-P9001-ND in a simple > voltage divider circuit. I'm running them to spec, 1ma max-current etc. > They've all gotta last at least 10 years. My definition of failure would > be their specs drifting by more than about 30% In the applications they > are at worst completely uncovered and *may* be subject to UV light, but > would generally be used in a standard office environment. > > I mentioned this to someone who said that photocells are unreliable and > won't last more than a year or two. He said they all fail due to light > exposure within a few years. > > Seems pretty dubious to me, photocells are used in streetlamps after > all, and the datasheets don't mention anything worrying, but... What do > you guys think? > > -- > http://www.petertodd.ca > -

At 04:03 PM 3/31/2007, Peter Todd wrote: >I have a few projects that I'm using cds-type photocell resistors in >them. Specifically they are all digikey #PDV-P9001-ND > >I mentioned this to someone who said that photocells are unreliable and >won't last more than a year or two. He said they all fail due to light >exposure within a few years. He may in fact be thinking of early failure in street-lamp usage. Early street-lamp photo-cell units were notorious for failing after only a few years. My limited experience with these failures is that the failures were caused by excessive power dissipation within the photo-cell while driving the relay coil in low to moderate light levels. Seemed to be much more of a problem in northern areas because of the slow change from bright to dim and vice-versa. As far as I know, those problems don't exist on modern street-lamp photo-cell controllers. I do think that you may be running your photo-cells a little hot if you are operating them at their maximum rated current. Depends upon the voltage across the cell. Hope that helps! dwayne -- Dwayne Reid <.....dwaynerKILLspam@spam@planet.eon.net> Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 22 years of Engineering Innovation (1984 - 2006) .-. .-. .-. .-. .-. .-. .-. .-. .-. .- `-' `-' `-' `-' `-' `-' `-' `-' `-' Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email.

> > Early street-lamp photo-cell units were notorious for failing after > only a few years. My limited experience with these failures is that > the failures were caused by excessive power dissipation within the > photo-cell while driving the relay coil in low to moderate light > levels. Seemed to be much more of a problem in northern areas > because of the slow change from bright to dim and vice-versa. I took one apart, and was rather surprised to see about a 1" diameter CDS cell, and a relay, in series. No electronics at all.. I was wondering about dissipation, but I assumed they had such things under control. It certainly couldn't get any simpler :)

>> > I mentioned this to someone who said that photocells are unreliable >> > and won't last more than a year or two. He said they all fail due to >> > light exposure within a few years >> >> Sound pretty pessimistic. If you're really worried about UV then just >> about any plastic will block it. Cut up a sunglass lens, everyone's got >> a busted pair of sunnies in the drawer > >Thanks, that's good to hear. As for the guy I heard it from... Well, >suffice to say I've had my suspicions that he can't stand being anything >but the smartest guy in the room, even when the subject is electrial >engineering and he's a Java programmer... But, wanted to see what you >lot thought first. Cds cells used to be used in camera light meters. They suffered form 2 problems - 1. they have a long time constant going from bright to dark, to get the last bit of sensitivity out of them at the dark end required a quite respectable time out of bright light (several tens of seconds typically). 2. they suffered from failure to reach a high enough resistance at the dark end after a time. This was a specific failure mode for some, and the only solution was to replace the cell. The reason for failure was not known by us (my father used to repair cameras) but could have been some form of contamination during manufacture, or just plain bad manufacturing process. I just remember my father did replace a fair number of them over time. More recently photodiodes have proven more popular due to better linearity with light levels, smaller devices for fitting into ever more cramped camera bodies and better overall sensitivity. I don't know that there is any inherent failure mode in them, but from experience as given in point 2, there may be some form of chemical migration or similar problem if exposed to high enough light levels.