I strung a fiber optic cable between my gas well (200') and the house. The PIC at the well has fiber optic transmitter and receiver on board. I was concerned with cross talk from the 120 volt AC in the buried conduit. Once in the house, I have a B & B electronics fiber optic to RS-485 converter to a PLC. The 120 volt is protected by 20 amp GFCI in the house and when lightning strikes near, the GFCI may open. The ground is only at the house main breaker and the neutral is grounded there also. M. Adam Davis wrote: > The long cables are going to be the bane of your efforts, I'm afraid. > Were it me I'd be very tempted to instead build a datalogger for each > well and get rid of the cables entirely. Bury the electronics box > near or inside the well. > > When lightning is onvolved, however, there's really just not much you > can do. Most lightning protection is designed to prevent fires, > damage, and death - not to keep the electronics safe. Lightning > proofing equipment has been studied at length, however, by radio > engineers - in the US we have Ham radio hobbyists ( http://ARRL.org ) > and their electronics book covers a lot of information on antenna > grounding and circuit protection. There are probably other books that > cover it in more depth. I expect you'll find many on the list that > can help as well. > > If you can't move to one datalogger per well, you might consider > replacing the cables with fiber optic cables, this would require > additional circuitry and power at the wells, though, and wouldn't be > much less work than simple having a datalogger at each well. > > Typical surge circuitry at the microcontroller consists of a > powersupply that can absorb huge surges, and diodes from each input to > the power rails (positive and negative) - this conducts the current to > the power supply which handles the surge, and the inputs never go > above the diode's ratings for conduction. This, with the GDT, should > take care of most of the problem, but lightning is still rough enough > that you still may not survive - the mosfet idea you have is good, but > they'll be destroyed and you'll lose data anyway when a strike hits. > You might also consider optocouplers for the role of isolation. > > But getting rid of the cables and using fiber optics, radio, or an > individual logger per well is likely going to be more robust than > trying to protect the circuits you already have. > > -Adam > > > On Thu, May 28, 2009 at 12:16 PM, Electron wrote: > >> Hi all, >> right now I am totally demoralized, but I want this to be the day >> I learn something and improve myself (I'm a self-taught hobbyst, >> not an engineer, but with very high goals and made some really >> advanced designs, and right now I am working (as I posted some time >> ago) on a insect-like robot system solar-powered and permanently >> connected to the Internet via GPRS, thanks which each robot swaps >> informations with the others and help each other to reach a common >> goal, e.g. to go from point A to point B of the forest (I own an >> enduro bike (KTM 300 XC-W) and I know some places in the forest >> where no human has been before) on a territory where just having a >> GPS (like each robot has) is not sufficient, as you find crevasses >> here and there, or hills that are too step for the weak robots to >> climb, so the explorators keep in touch will each other, sharing >> information, asking for help (it's my duty to get in in this case) >> and to work together to reach the goal.. while everything gets >> monitored into my home server). >> >> Anyway, the story that is causing me trouble belongs to the past: >> >> I have designed a datalogger for my grandpa, to monitor the levels >> of his wells, temperature, humidity and so on, but expecially the >> level of about twenty wells. I know nothing about geology, but my >> grandpa is a geologist, and as the zone is landsliding he wanted >> to find a correlation between the landslide and the levels (or >> variations of) water in each well. Things that he has been doing >> when he wasn't retired, and are not enough of interest to the local >> government to get funds for. So he asked my help and I've been glad >> to come in this project and do what I could, the dataloggers. >> >> Anyhow, I designed the datalogger, built 5 of them, and placed them >> around the territory. The most tricky part was that the piezometers >> could run hundreds of meters of cable before reaching a datalogger, >> and there could be several piezometers (all interfaced via 4-20 mA) >> at opposite positions each other. >> >> This has made me afraid of lightnings (I imagined that the two very >> distant piezometers reaching the central datalogger may carry very >> high tension), so I decided to add a lightning protection thereafter, >> lightning protection is a matter about which I knew even less than >> about geology (I'm much more for digital electronics). Moreover, the >> decision was taken on the field, else I'd have the time to study it >> well and choose the right components. When I went there to install >> the dataloggers, I thought that the sensors would be all placed very >> near to them, I discovered too late that in most cases this wouldn't >> be possible at all. >> >> The dataloggers do NOT have isolated channels, when my dsPIC turns >> a piezometer on to read and record the data (every 60 minutes, else >> the piezometers are always powered off to save batteries), in my >> datalogger the A2982SLW IC powers the + side of the sensor (placed >> hundreds meters away), and the return current goes, through a high >> precision (further calibrated via software) shunt and a 10K limiting >> resistor, straight into one of the 7 channels of the ADC, the ADS1243. >> >> Since all piezometers are galvanically isolated each other, I never >> though this could pose any problem.. of course until I heard that the >> piezometers had to be installed hundreds of meters away each other. :( >> >> I knew that in case a lighting hit a datalogger, a sensor or a cable, >> that part of the system wouldn't survive, but anyhow I installed the >> best thing I found there at a local shop, gas discharge tubes, those >> with 3 terminals (I connected, at the datalogger side, the + and - >> wires to the GDT left/right pins, and I connected the central GDT pin >> to earth through a 1.5 meters copper pile vertically buried into the >> terrain. The bad note is that I couldn't find any GDT rated at less >> than 200V.. if I had found one at, say, 30V, I'd have been pretty >> confident that it would have served its scope well, but as I had to >> install 200V ones, I had to hope that the extra tension induced by >> a lightning had to rise very very fast to get the GDT in condution >> immediately, and had to fall equally fast, to avoid consequences from >> the 30..200V unprotected range. >> >> Sorry if I use wrong terminology, I'm used to the Italian language, >> as it's my native tongue, and maybe the translation I get in my >> dictionary is not technically correct. >> >> All dataloggers, like I'm used to do with electronics I've to leave >> on environmentally harsh places (whereas the IP66 box) or that could >> be possibly stolen, were all potted into ROBNOR PX700K epoxy resin: >> http://it.farnell.com/jsp/search/productdetail.jsp?sku=537330&_requestid=341872 >> >> The system has worked wonderfully for days, the system was calibrated >> to gain further precision (although the shunts were IIRC less than 1%) >> and all verifies made by measuring manually the depth of the wells >> reported/proven that the system was extremely accurate. >> >> After 3 days, one even with a violent temporal, I left the system >> perfectly running with a sense of satisfaction, although the lighting >> protection was deeply worring me anyhow, although it had survived at >> least for the first (even violent) temporal. >> >> I have left my dataloggers for 6 months alone, my grandpa country >> is in another, distant region, and because of some problems of my >> own I couldn't go back there until now, although I had planned to >> go back after 15 or 30 days maximum. :( >> >> Well, yesterday I returned there, and I found 3 out of 5 dataloggers >> dead. :( >> >> One has the tantalum cap burned, a clear sign (to me that designed >> it) that the input channel brought high tension through the A2982SLW >> to the main power, and thus burned the tantalum cap (and the rest). >> The other two had no tangible sign of damage, but all is buried into >> the epoxy as I said. >> >> Of the two dataloggers that survived, one had some channels nonworking, >> but it's probably due either to the piezometer broken or a broken cable, >> unfortunately I had to take the dataloggers and bring them back home to >> my lab, and I didn't have the time (or the mind) to verify if the sensors >> are ok, still there, etc.. >> >> But two data loggers survived.. and the firmware worked flawlessly (not >> even one WDT reset was necessary, etc..). >> >> Sorry for the long post.. here my questions go: >> >> 1) is there any way I can dissolve this epoxy resin now that I could >> learn something by seeing which components burned? (the only visible >> one is the tantalum cap accross the batteries, just in one datalogger). >> I used ROBNOR PX700K epoxy resin: >> http://it.farnell.com/jsp/search/productdetail.jsp?sku=537330&_requestid=341872 >> >> 2) as although I love my grandpa I don't have the time, expecially now, >> to redesign everything from scratch, but only to improve on the existing >> design and rebuild 3 new dataloggers, I'd also like to keep the existing >> PCB's I have if possible. This means that the datalogger remains, in >> substance, hundreds of meters of cables that run, through a shunt and a >> 10K resistor, directly into each of the 7 channels ADC pins. A weak design, >> BUT could it work if I improve on the external lighting protection instead? >> >> 3) the GDT's.. even now that I looked into a suitable, low voltage model, >> I cannot find any GDT that goes down to ~30V. Needless to say, adding low >> voltage tranzorbs/transils to a 200V GDT doesn't make me feel safe, as they >> could prevent the GDT's from triggering, and/or fail short, or work only once. >> The best I came as idea so far is to keep the datalogger as is, and to design >> an additional pass-through for each channel, independently powered, where >> a mosfet senses voltage coming from the datalogger (thus if the channel is >> on) and thus activates a relay that puts the channel truly online (through >> those hundreds of meters of cable). This way, except for about 150ms each >> hour, the datalogger channels would remain galvanically isolated (relays >> are rated even 5000V if I'm not mistaken) from the cables and piezometers >> and even more from each other, and (unless I get a lightning strike into >> that short 150 ms window) the 200V GDT's would be certainly adequate to >> protect the relays. Do you think this can work as a pass-through solution? >> >> 4) Should I add GDT's (and copper piles) even near the sensors, or at the >> datalogger side it is enough? >> >> 5) I wish the cables were externally conductive (shield) but they are not. >> Should the cables's be, too, connected to GDT's and copper piles every, say, >> 100 meters? Or, again, surge protection (to earth) at the datalogger side >> is enough? >> >> Anything obvious that I miss? I have Googled for 2 days and nights, read >> a lot about surge protection and lighting arrestors, but I still have the >> feeling that I don't understand this matter. >> >> And to quote someone else, the problem is that just because you survive a >> lightning strike today doesn't mean that it was the same condition as the >> lightning strike that took out everything last month. It takes a long time >> to prove a solution in the field especially if it's not raining, and here >> where I could test it, it doesn't rain often, especially in this season. >> >> The dataloggers haven't costed me much in terms of money, they have costed >> in terms of time but it was the pretext to learn a lot of things I hadn't >> had a chance to look indepth before (dsPIC, SPI, and more).. what really >> makes me sorry is that my grandpa, who's not really young, has lost >> 6 precious months worth of data because of my technical limits, and he >> has always had the highest regards for my achievements. >> >> Thank you very much for your time. >> Mario >> >> -- >> http://www.piclist.com PIC/SX FAQ & list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist >> >> -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist