Gas Discaharge Tube...

LA6WNA

Hi again folks.
I`m looking for something to protect my RS-485 line between my house and my power-station. It`s abt 150m TP line. Lightning did burn 3 SN75176 (rs-485 chip) for me last week, and I`m trying to find a way to make the line more resistant to overvoltages / lightning spikes. Any of you out there that have any experience with GDT like this ???:
http://www.ebay.com/itm/Gas-Discharge-Tube-GDT-SL1021-SL1021B230RF-Littelfuse-/110610948377?pt=LH_DefaultDomain_2&hash=item19c0ed2119#ht_2727wt_1361
Is it many types of this GDT`s?? This one is marked 100V to 1 kV, but the text says that it is suitable for telecom lines.
A RS-485 line has max 5v between the wires, and I guess the differecial inputs on the chip does`nt stand any much higher voltages than this.
If this GDT starts discharge at abt 100v, then I guess the chip is broken long before that. Or am I wrong...? I may have missed something..
Any rec`s for what`s the best solution for RS-485 lines???

kwinn

You are right. The chip will be blown long before the GDT starts to conduct, and for the B230RF the minimum discharge voltage is 184V. They are really intended for the power line, not logic level signal lines.

Best idea would be to put them underground. Using optoisolators and a floating power supply between the house and power station might work.

LA6WNA

I`ve ordered some of this TVS diodes: http://www.ebay.com/itm/390854453898?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2648#ht_2815wt_894
Thought I would connect one of this from each wire and to earth/ground, and do this in both ends right before the chip input,
Do you think that could help a lttle bit?
My guess is that it could`nt do anything wrong.

kwinn

It might help, and certainly will not do any harm. At least those diodes will start to conduct at a voltage where the 75176 has a chance of surviving.

Tracy Allen

TVS diodes could definitely help. That said, the SN75176 is 1980s technology. Today's RS485 transceivers are more robust.

For example, the LT1785 (5V) and LTC2862 (3V) have +/- 60V fault protection, 15kV ESD protection, and +/-25V common mode range. And to better calm your paranoia, the LT1535 provides 2500V of isolation and breaks the ground loop completely.

LoopyByteloose

LA6WNA wrote: »

I`ve ordered some of this TVS diodes: http://www.ebay.com/itm/390854453898?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2648#ht_2815wt_894
Thought I would connect one of this from each wire and to earth/ground, and do this in both ends right before the chip input,
Do you think that could help a lttle bit?
My guess is that it could`nt do anything wrong.

Any little bit helps, but nothing is a 100% guarantee. So it is best to have a last line of defence be an RS485 module that can be easily and quickly replaced when damaged. The idea is that when the interface burns up, it protect the next-in-line device from any damage. If the baud rates are not too high, opto-isolation would not be a bad idea at that juncture.

Best bet is to include the Gas Discharge Tube to take the bulk of a lightning strike, then the TVS to trap the portion that gets through, and opto-isolation betwen the RS485 chip and the microcontroller in case the jolt gets that far downline. This is not the only scheme, but it is a multiple lines of defence that really works best.

And nothing will prevent the actual wire from burning up from a lightning strike if it finds a source to ground. You might consider having a second line ready to be switched over to complete with the the RS485 drivers in case you go down and need to revive immediate communication. Redundancy will be very helpful.

There are commercial devices that include both the RS485 and the DC power supply. http://www.l-com.com/surge-protector-rs422-rs485-rs232-aisg-lightning-surge-protectors

http://www.lightningsafety.com/nlsi_lhm/Data-Communications-and-Lightning-Protection.pdf

stamptrol

Another consideration is to replace the hardwire communications with a wireless link.

Otherwise, optical isolation is the best bet. The usual failure is not so much the lightning strike but rather where the ground current goes. If you can have only one ground, say at the powerhouse, you can minimize high ground fault currents and make the system more resilient.

Cheers,

LoopyByteloose

Well if you look carefully at the photos of the commercial solutions in the links I provided, you will see the Gas Discharge Tubes on the backs of the printed circuit boards and what might be TVS devices as well. So you can either spend the big bucks or DIY.

I made a point of including a commercial provider of devices that openly admits that nothing will always work. So it is up to you to decide how much protection to buy and how much protection to build. In any event, you will need a good ground connection .. like driving a copper rod into the soil.

Tracy Allen

You have to understand that the "ground" is an extensive two-dimensional conductor. When lightning hits the earth, its current spreads out and creates large potential differences from place to place, say between the "ground" at your equipment in a house and the "ground" at the equipment in the barn. If you pound in a ground rod at both ends, whatever conductors lie between them become the path of least resistance. That goes whether the conductors are buried or hanging in the air.

The stark reality of this came home to me when installing ag weather equipment in Nicaragua. Driving around in an old truck without windows during an aguacero. That means downpour, literally "water horn". Huge pillars of lightning striking the earth right and left. The intensity of which we hardly ever see in California, and I don't remember anything like it from growing up in Denver CO.

LoopyByteloose

About 3 weeks after I first arrived in Taiwan, I had to rush across town in my first ever typhoon.

Nearing my destination, the streets were flooded so I was carrying my shoes under my umbrella with my wallet in one of them and wading barefoot through waste high water when I experienced a flash like a solar flare and a huge boom.

The hair on the backs of my arms were standing up and all the car horns of parked cars were honking. It seems I was just passing by a telephone company radio tower that took a direct hit. I could smell the ozone in the air, so I know it was close.

+++++++
Traveling the country side a few hours north of Chiang Mai in Thailand, we got caught in similar torrential downpours on dirt roads. We luckily didn't get stuck, but the experience was fun and made me appreciate that the trip wasn't just marching in and out of hotels and on and off of tour buses. We had been visiting a relative of my host's wife that lived in a small village that are rather isolated. The roads are washed out for about six months of the year, but they seem to like the isolation and independence. If you really had to go somewhere, you could always take the elephant to get through.

LA6WNA

Yeah man.., that where a lot of experience out there with you guys. Thank you so much for sharing, and good advices.

Tracy; I was`nt aware of those LT1785. Sounds like a much better chip than those SN75176 that I use. Do you know if LT1785 are pin-compatible with SN75176? I have them mounted in soccets, so changing chip is pretty simple. If the`re compatible, I may look for those chips and replace my old once.

Loopy; Yes, I sure know about those commercial devices and they cost from almost nothing to well over plenty... I guess you get what you pay for. However, this is not a critical dataline for controlling the powerstation. I`ts only for monitoring the system. RS-485 system is driven from its own cog in the Propeller, and a broken connection at this line will not affect the rest of the system that runs the powerstation. (as long as the Prop itself is up running..) I also looked at those optoinsulators, but the big point for me is to make things myself in my own way. I`ve already learned alot from this project, both programming in Spin and other things related to electronics. It is the learning and understanding itself that drives me to make things at my own. Othervice I just could have bought all that I needed to make this running, but that would`nt be a percent as fun....

stamptrol; I sure have thought about get this data home via wireless system. My weather station, my pool and my cabin sends all data home to main station via PMR`s and modem. Thats a good solution, I think. They`ve all been running for 2-3 years, almost fault-free, and I newer had troubble with the lightning at that system. The thing is, when I have my "power-station" page up at my TV-set, the RS485 talks to the powerstation as long as the page is up. This makes a great "live" monitor for the turbine, generator etc.. The communication is two-way (Main station asks and slave answers at each datapacket) and this takes longer time doing the same things over radio and modems. Therefore I prefere 485 at this communication. When I do`nt have my power-station-page up at my TV, the communication at the line happens only twice pr minute. This is for updating the Spinneret webserver.

Well, now I`ve learned alot again from you guys. I appreciate all the comments and that you take time to share your knowledge. My guess is that this forum is the best in the world for
"think-outside-of-the-box-guys" like us. Thanks again.

:thumb::thumb::thumb:

Sapphire

Peter,

Same pinout for the 8-pin DIP package.

http://www.linear.com/product/LT1785

LA6WNA

Same pinout for the 8-pin DIP package.

http://www.linear.com/product/LT1785

Thanks alot for the info, Sapphire. I`ve ordered some of those now, so I can try them.

Guess I still gonna hang the TVS-diodes on the line when I get them. I may also connect a GDT at the grounded end off the line as an extra discharger. Things can`t be worse, so every small improvement will help.

kwinn

LA6WNA wrote: »

Thanks alot for the info, Sapphire. I`ve ordered some of those now, so I can try them.

Guess I still gonna hang the TVS-diodes on the line when I get them. I may also connect a GDT at the grounded end off the line as an extra discharger. Things can`t be worse, so every small improvement will help.

I hope that works out for you, but just in case it does not here is a circuit I have used a few times with good results.

You will need two circuits if you need full or half duplex bi-directional data, and it can be wired so half duplex only needs one pair of wires. It provides 7.5KV isolation.

The isolated power supply can be between 3 and 16V. I have used 4.8V(battery) & 12V using one of the high isolation transformers. For battery operation make sure it is connected so the idle state is no current in the loop.

The resistor limits current to the 10 to 20mA range. I used 10mA for battery operation and 20mA when the loop was powered from the AC line.

LoopyByteloose

CANbus actually uses RS485 in industrial settings where they can ill afford a loss of control (like petro-chemical plants). So you might Google around for some CANbus board schematics or Application Note related to lightning on the internet and see how they provide added protection to the RS485 ports. That is where I have seen some really fancy adaptations.

I believe that the AirBus380 uses CANbus for its 'fly-by-wire'. I am sure that they have some roboust lightning protection, which also includes a second set of wires to switch over to in case of a complete failure.

http://www.holtic.com/news/766-holt-integrated-circuits-releases-guidelines-for-lightning-protection-of-arinc-825-can-components.aspx

http://www.analog.com/static/imported-files/application_notes/AN-770.pdf

Tracy Allen

An April 2014 article from Linear Tech tells about their fault tolerant RS485 transceivers. On page 13 you will find a summary of real world hazards they face, and also a circuit diagram figure 9 with part numbers for additional protection that brings the surge tolerance up to much higher levels than the chip alone can withstand. The focus of the article is the LTC2862, but everything they say applies also to the LT1785.

The additional multilayer protection does in fact start off with a gas discharge tube, but it also includes a Bourns thyristor and a TBU (fast solid state resettable fuse) and an MOV to protect the TBU.

KMyers

In my experience in CATV systems I had one system that used C-COR amps. Each port was protected by a gas discharge tube (Simmons as I recall). This was in the late 70's early 80's. Do they work? Yes and no. At the time they stated they could take 10 hits before failing. When they fail then the amplifier would let the magic smoke out. They also generally failed as a short which caused power problems that could short out 60 volt 15 amp power supplies.

Problem was you could never tell if it was the first hit or the tenth hit. Always fun climbing poles during thunder lightning storms. The next version of actives from C-COR did away with the tubes and basically just beefed up the caps and inductors on the ports. Bottom line I guess im my experience is do all you can. ground, bond triac discharge but lightning will always find a way in....

My 2 cents worth

LA6WNA

Bottom line I guess im my experience is do all you can. ground, bond triac discharge but lightning will always find a way in....

That`s for sure... always. You can protect the system, protect the protection...and so on, but one day; BANG, the magic blue smoke comes out anyway.

The additional multilayer protection does in fact start off with a gas discharge tube, but it also includes a Bourns thyristor and a TBU (fast solid state resettable fuse) and an MOV to protect the TBU.

This seems to be the nearest what I thought could be a good solution. Guess the GDT`s can do the heaviest job of the discharging, and then other components takes the rest of it. So now I got a statement of my theory of not using only GDT alone. I`ll go for the GDT in combination with TVS-diodes, in additional to change the chips to LT1785. Then we can discuss futher on afther the next thunderstorm..,..

If the smoke stil come out too often, I may look nearer at kwinn`s opto-isolator solution. Seems to be a tolerant system. I eventually has to make a half duplex system at 2 wires. Guess that could be done. But refering to my first quote here, I sure know that the optoelectronics also contains "hidden smoke".

KMyers

Good luck keeping the smoke in!

LoopyByteloose

Time for fiber optics. You can get a fiber optic modem for $160USD.... maybe less or DIY
And you get a big increase in range if that is what you need.. 9 miles or so... maybe 12.4 miles

The best deal I could find is at 'The Fiberstore' ==> a pair of modems for less that $100USD, cable is additional.
http://www.fiberstore.com/a-pair-of-fiber-modems-industrial-rs-422-to-fiber-converter-multi-mode-double-fiber-1310nm-sc-st-fc-2km-p-17013.html

http://www.bb-elec.com/Products/Serial-Connectivity/Serial-Converters/Serial-to-Fiber-Optic-Converters.aspx

http://www.serialcomm.com/fiber_optic_converters/media_converter/rs232_rs485_rs422_to_fiber_optic_sm/rs232_rs485_rs422_to_fiber_optic_sm.product_general_info.aspx

http://www.eks-engel.de/en/products/rs485-fiber-optic-converter/

http://www.garrettcom.com/links_serial_485.htm

KMyers

Fiber is GREAT until it breaks. Locating and splicing is a royal pain!

LA6WNA

Time for fiber optics. You can get a fiber optic modem for $160USD.... maybe less or DIY
And you get a big increase in range if that is what you need.. 9 miles or so... maybe 12.4 miles

WOW Loopy... You`ve done a great research. Fiber optic cable has`nt even been in my mind for this project, but I sure would love to try it. Guess there is gonna be a cost of the cable, but the modems was`nt that expecive. It could be an alternative way to do it, if the smoke come back... No impulses from the environment around the cable at all. No need for GDT and TVS at that line. Sounds great.

BUT Ken; I absolute agree with you. Terminating such cables is really a pain, at my opinion. I`ve done it som times long time ago, with a data network. Was`nt that fun, I remember..,.
There is maybe some easier systems nowdays to terminate them, but I will leave that idea for now anyway.

BTW tonight it`s raining, so now I hope my powerstation starts spinning again. The creek has been dry for weeks now, and that is really unusual conditions here. Same happened back in 1997, when I first built the powerstation. The water in the creek compleatly disappeard in the best summer weather ever...
Maybe I should restore the station again once more... I sure love the sunny days we`ve had so far this summer. Lightning or not..., the whole station is just for fun. That said, it gives me about 5000kWh /year of free electric power, so the fun is worth NOK 5000,-/year.(US $850,-) Can buy a lot of Parallax equipment for that.....:thumb:

LoopyByteloose

FiberStore seems unbeatable on prices and appears to be going after being the world leader for these products. A Pair of modems can be gotten for less that half what others are selling, though the distances may not reach as far.

Fiber optic cable from FiberStore seems to be about 0.10USD per foot or 0.30USD per meter roughly. And that is for outdoor cable with a wire backbone for support and multiple strands for full-duplex -- More likely useful for full-duplex RS422 than the half-duplex RS485.

The debate about the cable breaking is somewhat dubious as you have the same problem with any wire. If it does break, it is easiest to first evaluate the connections at each end. If that is not the issue, pull a new cable to quickly resolve the issue. After all we are talking 150 feet here, not miles and miles.

Wire or fiber optic versus wireless is a question of having complete privacy. Anything that is wireless can be snooped and jambed. And electrical storms affect communications as well.

I suppose if the fiber optic's wire backbone is not grounded to anything, it poses no lightning hazard. But I suspect there is other cable that doesn't have any wire in it that can be hung pole to pole or laid underground.

LA6WNA

Ended up with this solution so far. There`s a GDT first at the input end, then 2x100mA fuses and at last 2xTVS diodes at 6,8V
I`m gonna try this for now.