can small board WITHOUT enclosure but proper earthed handle transients and ESD - Page 1
Forum section: electronics / projects

author: storkbus
author_rank: Newbie
timestamp: September 10, 2017, 08:53:40 pm
content: I'm researching whether or not an environmental sensor could be used without enclosure in a EMI harsh environment.The harshest locations are some 2ft/60cm away from a lightning conductor. Separated only by roof tiles or a brick wall and some insulation like mineral wool, nothing metal like metal cladding.The building is susceptible to lighting strikes hence the lightning protection system on the building.Now my question: I want to install the sensor in between building materials. Thickness of the device is a major constraint. Can one place the pcb without enclosure in such environment without frying the board during the first thunderstorm?Some other notes:All components are SMT, so the pcb will be credit card sized.Once I get confirmation that I could use the bare pcb, designing the pcb will need some assistance as well. Laying out the protection and sizing the traces isn't that simple from what I understand.The sensor chip is quite sensitive so I rather not would want to pot the pcb but only conformal coat it. And potting will be a challenge because the sensor needs an air opening in order to sample the air. Placing some sort of breather tube on a tiny hole on a 3x3mm chip and than pot the pcb seems like a guaranteed failure on every attempt.The boards are connected via 5 to 25 meters long decent cables which have a braided shield and drain wireThe power and signal wires coming into the board will be properly protected with GDT and TVSI could step up from a 2 layer PCB to a 4 layer design. A low inductance power plane comes to mindThere's never a direct strike, like what can happen to an outside antenna, on the pcb and cable. Any transients and noise comes via EMFThe PE protective earth is a tested low impedance earthThe lighting earth is provided by multiple electrodes around surrounding the buildingThe lighting earth and PE are coupled with a spark gap device and therefore under normal conditions galvanic isolatedWires/signals are not fixed at the moment, but probably differential i2c via the PCA9600. So 3 pairs: power and gnd as a pair, a differential scl pair and sda pair. RS485 and CAN seem too expensive likewise for PROFIBUS. But in any case this is a low speed application: kHz not MHzThe cable shield is connected to PE at the datalogger inside the building.Cables will probably be soldered to the pcb's.I don't worry and care about EMI. There are no SMPS, led drivers and/or other RF generating components. And this sensor network will be deployed in my home so no commercial project so no standard complianceThis is an environmental low sampling speed sensor network not a critical application. A missed sample and/or brief service interruption is no problem.The sensors are no longer accessible after installation. Operational life should be 5 to 10 years.Side question:Probably I should place the PE/chassis ground from the cable shield on it's own layer on the pcb. And have a separate signal ground. Does that make sense?I've read some interesting topics with contributions from: T3sl4co1 and free_electron. They cover protecting the device via various means against all the threats I'm worried about but more or less always the device is in a enclosure.Many thanks for your time and interest.Changed PCA9615 to PCA9600. The PCA9600 allows higher voltage and longer distance.

author: Ian.M
author_rank: Super Contributor
timestamp: September 10, 2017, 09:41:03 pm
content: For the purposes of lightning strike protection, there will be enough voltage present that you cannot treat many materials as effective insulators. (e.g. brick or tile at normal indoor ambient moisture levels) A metallised film pouch round the board with conductive tape connecting the metallisation to chassis ground/cable screen would be cheap insurance. However if you get a side arc off the lightning conductor its game over - your whole sensor network will probably fry.I wouldn't trust I2C over any significant distance even with the PCA9615 converting it to dI2C. its only good for a +/-2.5V ground offset (common mode voltage). RS-485 is good for up to -7V to +12V of common mode voltage and some makes of transceiver extend this range by a factor of over x2. Its probably cheaper anyway to put an 8 bit MCU (either with hardware I2C and addressable UART interfaces or bit-bang them) and a RS-485 transceiver on each sensor board. Stick in a watchdog controller chip that monitors the MCU and can force-disable the RS-485 transceiver so that failed nodes drop off the bus as long as their transceiver isn't totally fried. Furthermore you only need one pair for RS-485 + another pair for the supply, so you can use significantly smaller diameter cabling and have fewer wires to terminate.For the sensor, no matter whether you pot or coat, you'll have to protect its opening - something like lightly grease the face of it, and apply a dab of neutral cure silicone caulk before potting or coating then pull the caulk off when the potting or coating is fully cured. You'll have to be very careful to avoid the grease clogging the sensor port - maybe simply wiping it with a slip of paper with a trace of silicone oil on it would transfer enough to act as a mold release without risking clogging the port. Alternatively for coating, use peelable tape, leave a long tail and use a scalpel to outline the top of the sensor before peeling the tape

author: Tomorokoshi
author_rank: Super Contributor
timestamp: September 11, 2017, 03:27:13 am
content: 1. Get a copy of Ott:http://www.wiley.com/WileyCDA/WileyTitle/productCd-0470189304.html2. Consult with an EMC lab. Is there one nearby?3. Is installation in an inaccessible area without an enclosure allowed by local building codes?4. What kind of protection is there on the power and signals going to the device?

author: storkbus
author_rank: Newbie
timestamp: September 11, 2017, 08:45:39 am
content: To all and just to clarify. This sensor network is a personal project to be installed in my own house. I live in a almost 400 years old house which is also classified a national cultural heritage. This status usually doesn't allow you to change much, especially on the outside. But it does exempt you from a lot of building code, mostly regarding energy consumption and insulation. Now how much fun running from fireplace to fireplace is, I don't have time for that nor is it very responsible is this day and age. Going from no insulation to even the smallest insulation is an infinite improvement. Since it's time for a major restoration adding some insulation would be very welcome.Butinsulation a building afterwards is a very slippery road. If one makes a mistake you'll get condensation on a point where you don't want that. By the time you find out that something is wrong the damage is usually beyond repair.The older the building the more difficult it's to insulate without risk of condensation. Just like HF RF is an art form so is building physics. So I did hire a company, which specializes in historic buildings, to analyze the building and come up with a plan. On paper their plan is sound but what if the contractor made a mistake or some bricks are more porous than the sampled ones? Condensation will take place but I can't see it because it's hidden behind one or more layers of building material. If I measure the outside and inside climate I've got two reference points. By measuring between the building materials I can see if there's a difference. If the difference is too alarming I could open up the wall or roof and assess what's going on. And hopefully prevent major damage and costs. No engineering company is guaranteeing their advice with a warranty which covers all repairs.That's why I need the sensor network. That's why I reach out to the talented members here because I simply cannot afford another consultant, the product I'm looking for doesn't exist off the shelf and building this sensor network is a lot of fun.Thank you all.

author: storkbus
author_rank: Newbie
timestamp: September 11, 2017, 08:46:53 am
content: Thank you Ian for your comments.Do I understand your first paragraph correctly that without an enclosure acting more or less as a Faraday cage the circuitry on the pcb behind the protective circuits still get fried?The building has a lightning protection system installed. I've asked two lightning protection companies what the effect of a strike on a lighting rod would be. Both answered that any conductor within a yard of a lightning conductor would pickup the EMF and experience a transient , given my building materials. Your mentioned side arc risk, was very low if I remember correctly. I think that makes sense because if there was a significant risk the roof would still catch fire.My bad, I meant the NXP PCA9600 which can drive up to 15V differentially. I initially found the PCA9615 and that seems stuck in my head. I did a breadboard test with three PCA9600's and 5M and 25M cable in between them and that worked at 12V like a charm. Since this worked I stopped looking at other solutions like RS485 because of the extra complexity and the cost of 3 pair cable easily offsets the cost of getting the mcu up and running. But I'm always open for suggestions, do you have a mcu in mind? Something like think PIC16F687? I fully agree that RS485 or more modern CAN is much more suitable for this distance and type of application. But as I wrote earlier, I dismissed them both because of the extra complexity and development costs and because my breadboard solution worked.I also looked at the DS28E17 which is a 1wire to i2c bridge. A 1wire bus could easily handle my 30meters requirement. There's even a 5V level shifter should it be needed. And I only need three wires.Some sensors have a "peel-off protective film" version. So I could coat the board and afterwards simply peel of the protective film and have a undamaged sensor. Thanks for the potting suggestions but I'll try to avoid potting at the moment. I know nothing about potting compounds but I've read that mechanical stress is very likely during curing and that's something I would like to avoid. I do have a large vacuum chamber to get the potting compound really under the components but perhaps that's too much.

author: CopperCone
author_rank: Super Contributor
timestamp: September 11, 2017, 08:47:40 am
content: so uh, why would you gamble on a circuit board if its a expensive house?i just dont understand

author: storkbus
author_rank: Newbie
timestamp: September 11, 2017, 09:10:20 am
content: @CopperCone, what's the gamble?I've hired a renowned building physics company which specializes in these old buildings and asked them for advice. They did a analysis of the building and came up with a plan. Which only allowed certain parts to be insulated. Other parts simply couldn't be insulated without risk or guaranteed damage. But as I wrote earlier no (paid) advice comes with a warranty. So as a layer of protection I want this sensor network. It will cost me some money and a lot of time but compared to the possible damage and the fun and nice graphs I think this is money well spend.Further more since this a protected cultural heritage building all plans are checked by a state committee. If they find the risk of damage to the building by your plans too great you don't get a permit. But I do have a permit.The plan on paper is sound but real life is always different. People make mistakes and samples and models might be wrong. So I fail to see what's wrong and where I'm gambling with this sensor network. It's just a extra layer of protection.Also I know my limitations. That's why I'm asking here for help regarding the protection of the boards against the EMF caused by a lightning strike.

author: storkbus
author_rank: Newbie
timestamp: September 11, 2017, 10:59:13 am
content: @Tomorokoshi, thank you for the excellent book suggestion. I've ordered a copy.The bottom of my wallet is in sight after all the building engineers sent their invoices. So I don't think a visit to lab is possible nor do I know any at this moment. The eevblog forum will probably give a better advice. Testing can only be done in a lab but since I need to have this certified I'll take my changes on existing proven (reference) designs.The accessibility point is a good one. If this was mains circuitry this wasn't allowed, code clearly states that. Afaik building code only covers the accessibility of mains not low voltage . This is a low voltage application and not connected to anything but PE earth. The datalogger is battery powered and sends it's measurements wireless. So I'll take my changes. If lightning strikes and the sensors get fried it's too bad. The EMF will not only affect my sensor network but also all regular cables (mains and ethernet) and appliances so I probably have more to worry about than just my sensors.Regarding your last question about the device protection. Since this is more or less a RS485 network, plenty of existing designs and manufacturer reference designs are available. I looked at reference designs from Bourns and Littelfuse as well as other sources. And plan to really not skimp on protection. So think GDT, surge protection MOVs and TVS diodes both for power and signal. The protection circuitry uses the cable shield as ground conductor. Since the power gnd is not connected to earth because the whole system is battery powered. I've not yet looked very carefully at this part of the problem because I want to no whether or not the sensor could be operated without (conductive) enclosure given my situation.I hope I've answered your questions sufficiently.