How about replacing the cables with RF modules? Mark 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