From: firstname.lastname@example.org (Tom Bruhns)
Subject: Re: My CDS cells are shorting out! (part two)
Date: 23 Nov 2002 21:50:20 -0800
NNTP-Posting-Date: 24 Nov 2002 05:50:20 GMT
Interestingly, I found your original postings on the topic a couple
weeks ago, because I, too, was having to replace a CdS cell...the
second time. I was wondering what the failure mechanisms for the
cells are. I don't doubt that moisture in them is not a good thing,
but I had replaced the original one (encased in moulded clear plastic)
with a hermetically-sealed one, and it failed after about three years.
Both have failed with a low resistance when dark, down in the
few-hundred-ohms range. A good one will show literally gigohms in the
dark. I've replaced it again, and again with a hermetically-sealed
one, but I wonder if it, too, will die in a while. These sealed ones
are in cases like a TO-8 transistor case (similar to a TO-5, but
larger diameter), and have a clear glass cover, with gold-plated leads
sealed in glass-to-metal feedthroughs in the base. The failed unit
showed no signs of moisture ingress.
One of the things I've wondered about is if there is a problem with a
continuous DC voltage across the cell. I suspect the circuit (in a
commercial landscape lighting system) for mine runs it with DC. Could
it be that they don't tolerate DC bias? I know that some sensors are
eventually destroyed by DC; I believe some of the humidity sensors are
that way. My web search didn't turn up anything about CdS cell
failure mechanisms that was really definitive, but I'll bet someone
knows. If that is the problem, it's not difficult to arrange for AC
"KILOWATT" wrote in message news:...
> Hello to you all! It is to my despair that i have
> to post again to report that i'm still havin problems
> with cds cells. At lot more details in pics at the
> link below but here's the short story. It's a circuit i
> did in 1994 to switch on the exterior lighting in our
> backyard at night. (Some of you may remember my original
> posting and the web page i've created to show what
> happened to my cells.) Everything went fine 'till 1998
> when i found that the CDS cell (wich is outside,of course)
> was shorted. I'm on the net since summer 2000 so since
> then a few ones worked with me on newsgroups to find a
> solution. Changed 3 times the cds cell since then, with
> improvements over time but the problem isn't solved.
> We discussed three major point that would have to be checked:
> 1- humidity
> 2- sunlight that focus on the photocell during the day and
> slowly "burning" it.
> 3- Damaging overvoltages could be induced in the cable and
> then zapping slowly,with time,my photocell since the cable
> for it is hidden for about 25' inside pvc electrical conduit for
> our 120/240Volts garage's power line and exterior lighting.
> Point #2 and 3 are no longer viable as procedure as been
> taken in late 2000 to solve that. And after seeing the pics of the
> shorted cells in the web page, many ones argued that since
> the beggining the problem has always originated from humidity.
> In may 2002 when i had(again) to replace a defective cell, i took
> some measures against humidity but i think it was the wrong
> measures, so here i am again! Please take the time to
> see the details at the link below. Thanks very much in advance
> for any new comments/suggestions!