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Could have been a fire!

5 or 6 years ago I put a Harbor Freight driveway sensor at the stairway to my business to wake me, um, alert me that I have a customer.

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Rather than change batteries I built a power supply, wall wart, L7809CV and L7805CV. I scratched out simple traces on a Copper clad pcb. I mounted this in a clear plastic box I had on hand. Yesterday, I was alerted with a single weak beep, which is strange because it usually has 3 strong beeps. I checked for a customer, no customer, no birds, sometimes they trigger it. I happened to look up on the wall where the circuitry sets and there was a orange glow coming from the clear plastic box. My first thought, why did I put an LED in that? Then, I didn't put an LED in that. I immediately climbed up to the outlet near the ceiling and unplugged it. When I disassembled the unit, I noted condensation on the walls of the clear plastic container. The PCB had black growth between the input and ground leads of the L7809 and across the gap between traces. I think the glow was coming from the growth between the L7809 leads. This morning I cleaned all the corrosion from the PCB and parts, Powered the circuit and I have 9 volts and 5 volts as I should. The condensation was caused by water coming in an outdoor AC outlet through the conduit and dripping into the clear plastic box. This precipitated the corrosion growth and when it completed the circuit, started to glow the pretty orange color. I opened up the near by PVC conduit Tee connector and water dribbled out. So I have two repairs, the customer sensor power supply and water proofing the outdoor outlet. Glad I was there when it happened and not my wife. btw, this is on my boat.

Mikek

Reply to
amdx
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Good that you mention that, before I'd suggested to repair the roof. :)

joe

Reply to
joe hey

Hmm, VERY interesting that you got a glow from a 9 V circuit, presumably with quite limited current available (wall wart or such). Glad you caught it before it did something bad.

Jon

Reply to
Jon Elson

Good analysis. You were lucky it didn't start a fire. Smoke alarms on boats are problematic:

Some random thoughts. Out of curiosity, was the PCB mounted horizontally, where the water could puddle, or vertically where it would rip off one edge? In the marine radio biz, we tried to design everything with vertically mounted PCB's.

We also had problems with mold, moss, mildew, and "the creeping crud" growing inside the radio. I've seen some that came back from the south seas that looked like a biology experiment gone awry. We did some experimentation with conformal coatings, which sometimes included a fungicide additive. Methinks the wax based conformal coatings would be best because they are easy to remove or retouch with solvent or heat. Don't forget to mask the connectors and controls before spraying.

Getting carbonized mold to glow (incandescence) takes a bit of power which was probably coming from the unregulated side of the 7809. Methinks the xformer might have been rather warm. Did this thing have a line fuse? Thermal fuse in the xformer? Thin trace on the PCB? UL sticker?

Incidentally, my office doesn't have a door bell or motion detector. Instead, I have a small sign on the door that says "Yell for Service". That's been good enough for the last 25 years.

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558
Reply to
Jeff Liebermann

I recall designing electronics for the DoD to meet a "fungus" spec. Seems in some parts of the world many materials that appear to be inert are food for fungi.

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Rick
Reply to
rickman

It was vertical, not cause I knew better, just hung it from a run of conduit and that's how it end up.

I think that is correct.

I don't recall the transformer as being warm, but I think I might have caught it just as it happened. I assume my wall wart is thermally protected. UL? all right, I'll climb up and look! Yes UL, Panasonic KX-A11.

Notice these are gentle use, very soft electrons were used.

Yes, but I get a little extra warning when the sensor at the stairway beeps. It gives my time to get my head off the pillow and get myself turned so I don't have to use my lower back to rise. :-) Or, time to get the laptop put down, I don't always sleep! Mikek

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Reply to
amdx

Heh. And then it gets mounted wherever there's an available space, probably rotated 90 degrees to maximise the chance of failure :P

Reply to
Clifford Heath

Usually marine stuff is well veltilated, to avoid condensation. Might consider that and some conformal coating.

Cheers

Reply to
Martin Riddle

Reminds me of some ancient nightmares I would rather forget. Most porous and flexible plastics will be more than happy to host a fungus colony so that they could eat the plasticizer. Would you believe CAT5 cable?

Specs: and detail: The procedure is simple. Sterilize sample plastic parts with steam, add spores, stuff into a heated cabinet for a month, and voila, your now favorite fungus dish is served. Ummmm... Some of the radios that came back from the south Pacific looked like the test sample in the photo or worse. Painting the radios with Clorox bleach salvaged most of the parts.

Somehow, I lost my taste for blue cheese (penicillium) and mushrooms after a few months of testing. By coincidence, my flat roof chose the same time period to leak all over my walls, carpet, and bookshelves. Fairly soon, I had black mold everywhere, including behind the drywall and in the books. When summer finally arrived and I was able to dry out, I had to perform a rather expensive remodel to get rid of the black mold. It took three summers to purge the house.

Incidentally, I have a local problem. My shop gets invaded by banana slugs during the winter. Cute little slimy critters. The main attraction is the glue used to attach labels to storage boxes and Motorola service manuals. Both were systematically digested by the slugs. It was quite a shock walking into the shop, and finding all the labels gone. I also have a fair collection of household chemicals in cans but with no labels. To the best of my limited knowledge, there are no ASTM specs for banana slug resistance, yet.

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558
Reply to
Jeff Liebermann

Moisture in my boat is not a normal problem. The interior space is

8'x 8'x 8' and I run a 6000 btu air conditioner 24/7 in the summer, heat when needed in the winter. This happened because of a driving rain storm that pushed water into an AC outlet, through the conduit and dripped on my circuit. Kind of an oddity, the boat has been in the water 13 years and I never noticed any water ingress from that outlet before. I was on the boat the day of the storm, at times I wasn't sure I wanted to be. Mikek
Reply to
amdx

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