LDR failure over time

Terry Pinnell a écrit :

Don't know why the LDR are aging. (didn't even know they could) If they're aging naturally (i.e. not from UV or something) you could use two in a bridge configuration, with one protected from light. That way both branches are shifting the same way. Well, sort of...

I guess I'd go with a photodiode, which should be much more stable. One with glass package so as to avoid plastic aging from UV, which might well be what happens to your LDRs...

--
Thanks,
Fred.

Thanks Fred. Yes, it could well be down to UV. The LDR is mounted outside the garage door under a rain gutter. Would a simple glass screen (or pill bottle, whatever) remove that effect? IOW, no need for it to be air-tight? I've been relying on them being epoxy-sealed.

Maybe the short section of exposed wiring could be a contributor too?

I was thinking of maybe placing a second LDR, exposed roughly to the same light levels, in the lower section of the potential divider, in series with the adjusting pot. Sort of compensating for the upper LDR.

--
Terry, East Grinstead, UK

I don't think air tight would do any good.

If it's coming from UV exposure then a simple glass screen is a good idea, or better some kind of UV filter (maybe you can find some cheap UV filter lens for photographic use on ebay).

Don't think the wire is the culprit (unless you can see visible damage).

Think to that: light changes you LDR resistance. Aging changes you LDR resistance. If you set it in a configuration that compensates for aging, you'll also compensate for light variations and get no output...

I'd first try the bottle/UV filter route and see if it corrects the pb for the next few years... If not, then I'd go the glass photodiode route...

--
Thanks,
Fred.

I see street lamps all the time toggling off and then back on again, and though tempted to attribute it to my fantastic aura ;-), I have historically stated that it was the light sensor causing it to cycle, then cycle back.

Only certain lamps do it, and it was never a dusk or dawn issue. Quite dark out.

It's usually a sign the HPS bulb needs to be replaced.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

--
Hi Terry,

Here's an interesting link:

http://denethor.wlu.ca/pc300/optoisolators/analogoptoisolatorintroduction.pdf

It's about Vactrols, but I think the LDR stuff is valid in any case.

While UV may affect aging, it seems that - according to the section on
life and aging on page 37 - it ages anyway, even though the LDR in the
Vactrol is sealed up and UV can't get to it.

For a long-term solution I'd go the photodiode - comparator - relay
route.

Maybe it's taking on moisture over time.

--
John

First thing would be to keep the LDR out of direct sunlight. Second would be to use one in a hermetic package, e.g. the Silonex NSL-6510.

Third would be to use a photodiode or even a phototransistor.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

"Terry Pinnell" schreef in bericht news: snipped-for-privacy@4ax.com...

I experienced that phenomena years ago and followed several discusions about it but still found no conclusive answer about the cause of it. Possible causes mentioned were UV, moist and (too large a) DC current. Most common situation mentioned was the LDR circuit failing in open air.

petrus bitbyter

Those Vactrols are horrible with their history issues - a shame as at first sight they look useful for things like adjustable equaliser circuits with isolation.

As for Terry's LDR problem, I'd hazard a guess that you'd increase the LDR lifetime if you reduced the current through it.

Cheers

--
Syd

Since you are not running this circuit off the AC mains, an LDR is a poor choice. Try using a silicon photodiode in the reverse-biased mode, it ought to be a drop-in replacement for the LDR, you'd only have to change the setpoint with a resistor or two. or, maybe no changes at all.

Jon

Terry Pinnell wrote in news: snipped-for-privacy@4ax.com:

I would rip a solar cell from one of those cheap garden lights, or solar powered calculator, and put that in place of the ldr, then adjust for sensitivity.

yes, direct or even near direct kills them..

The UV and what ever else is in there is to much for the cell.

I've gone through this problem with my solar lights where the PR loses its value.

what I did after replacing the cells, I used white semi transparent plastic snap in plugs. You can get them at a hardware store where they keep their little nifty assorted bushings, o-rings, spacers etc.. They are used to simply plug up unused holes in metal or thin walls.

The plugs I use are nylon of natural color which makes them pass light just fine.

Something like that.

Jamie

Except that it is not always a Sodium lamp that it happens to.

This is the correct solution.

Thanks all, much appreciated.

For the time being I've just replaced the LDR in its existing circuit and position (well protected from direct sunlight). I'll improvise a glass or UV screen in front of it too. But when my workshop becomes comfortably usable I'll try the photodiode approach instead. Hopefully that will be before the next natural failure, due around Christmas.

Some 5½ years since my last regular contributions here, it's good to see some familiar names still around ;-)

--
Terry, East Grinstead, UK

I've had a few LDRs fail like this after a frost. Without investigating the problem very thoroughly I put it down to water ingress.

Replacing the LDR with a VT90N2 cured the problem.

Had exactly the same problem for years, finally replaced the LDRs with a

2-terminal phototransistor which required no circuit changes at all. The first one went in about 8 years ago, still working fine.

So, this circuit has been in operation "... a decade or three ..." and its LDR has been replaced every 9 - 12 months, or 10 to 40 times total and _now_ you ask about it?

Bob