EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: Hobgoblin612 on January 19, 2016, 12:38:58 pm
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Hi all,
I am building a wireless sensor network and I need a base station/ collector node. The antenna and RX/uC needs to be mounted outside on on a high tower (An old hot water tank stand. Metal and about 8m tall) and have a serial connection to a PC inside the house.
My current plan is to use RS485 over Cat5 cable and use the remaining 3 pairs to send 5v from the PC to power it like so:
(http://i68.tinypic.com/9biwav.png)
I want to make sure that the PC is protected from nearby lightning strikes and general static from the antenna/Cat5 cable.
Is there anything I can do apart from grounding the antenna?
I have considered using isolated RS485 converters and an isolated 5v DC-DC module to galvanicly isolate the whole thing but I'm not sure this would do much at the extremely high voltages involved?
Thanks, Timothy.
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Do use isolated converters regardless of what other protection is applied to the antenna. That will protect you from low level transients that wouldn't necessarily be caught by the high power protection at the antenna, and also avoid potential ground loop problems.
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How about using POF for the data and putting a bunch of hipot isolated DC/DC modules in series for the power?
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The easiest way to protect your receiver front end from damage from static discharge and nearby lightning strikes is to use an antenna that presents a DC short at the connector.
Protecting against direct lightning hits gets really expensive.
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Not much you can do to protect from a direct or even close strike, but you could use isolators to protect from general static and such. I would also power the receiver from a seperate supply (like a 5v phone charger) rather than your PC which eliminates that path.
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Thanks for your suggestions guys,
I think I will use an isolated driver and a separate wall wart. If I use a double insulated power supply do I still need a DC-DC as well?
Thanks rrinker, I had not thought of ground loop problems.
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Not much you can do to protect from a direct or even close strike, but you could use isolators to protect from general static and such.
For starters, make sure your signal wire is not the lowest resistance path to earth.
Then you could at least add spark gaps. Maybe common mode surge filtering on the supply side.
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There are relatively low cost RS-485 to fiber converters. These would provide excellent protection for your PC and then you only have to worry about power protection which can be a much easier problem.
Here's one example:
http://www.ebay.com/itm/Honeywell-Northern-Computers-Access-FC485-Fiber-Isolated-RS-485-Converter-/301504113527?hash=item46330bab77:g:1jcAAOSwk5FUxR4m (http://www.ebay.com/itm/Honeywell-Northern-Computers-Access-FC485-Fiber-Isolated-RS-485-Converter-/301504113527?hash=item46330bab77:g:1jcAAOSwk5FUxR4m)
David
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Take a look at ametur / ham radio resources there is lots of useful information to be found
http://www.arrl.org/lightning-protection (http://www.arrl.org/lightning-protection)
www.bwcelectronics.com/articles/WP30A190.pdf (http://www.bwcelectronics.com/articles/WP30A190.pdf)
www.novaris.com.au/uploads/1/Principles_of_Lightning_1.pdf (http://www.novaris.com.au/uploads/1/Principles_of_Lightning_1.pdf)
You should also check the regulations in the country you are in as there maybe specific requirements you will have to comply to.
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POut a short circuit quarter wave line on the antenna feed as a primary arrester. Add in spark gaps and GDTs to cope with the secondary level that comes out of the short circuit.
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+1 on asking the HAMs
Seems they already have material on the topic.
https://www.youtube.com/watch?v=Luy8XP8O390 (https://www.youtube.com/watch?v=Luy8XP8O390)
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I would also look at isolating the actual USB with an off the shelf, quality, USB optoisolator. I have had one on a scanner which was hit by lightning, and my computer lives on.
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It might be worth making the base station solar powered and using WiFi or fibre back to the PC. Done right, that might even survive a strike to a lightning rod on the tower, and 40m of separation means your PC would be no worse off than if the tower was struck now. If the tower has any remaining metal piping running back to the house, that should be ripped out as it could direct a large ground current into the house earthing system if the tower was struck.
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It might be worth making the base station solar powered and using WiFi or fibre back to the PC. Done right, that might even survive a strike to a lightning rod on the tower, and 40m of separation means your PC would be no worse off than if the tower was struck now. If the tower has any remaining metal piping running back to the house, that should be ripped out as it could direct a large ground current into the house earthing system if the tower was struck.
It'd probably be pretty cheap to use a raspberry pi for that part of it.