Lightning and Watchdogs.
Sunflower
Posts: 48
Lightning and Watchdogs.
Will the Stamp be more resistant to the EMF of nearby lightning when Vss is not coupled to the earth?
And,
I would like a have a relay drop out from Stamp failure (power loss, lightning, acts of God, etc.) and drive a motor from backup power to its limit switch for safe system shutdown. Is there a watchdog system, like a 555 timer, that would respond when the Stamp stops pulsating the watchdog input?
Will the Stamp be more resistant to the EMF of nearby lightning when Vss is not coupled to the earth?
And,
I would like a have a relay drop out from Stamp failure (power loss, lightning, acts of God, etc.) and drive a motor from backup power to its limit switch for safe system shutdown. Is there a watchdog system, like a 555 timer, that would respond when the Stamp stops pulsating the watchdog input?
Comments
We have a 'single point' grounding system in stalled. So, the idea is , if something gets hit, ALL the equipment chassis will rise to the same potential and then all funnel down a single ground point.
It's the differential potential that can blow things up. So far it has worked....but we always joked, "How do you know it works well...." ; "Well, when it stops working!!"
There are some HUGE strikes....you might think of them as Rogue Waves. They're there, but just rare.
We also have long runs of cables that go through conduits under airport runways. We put in MOVs and Gas type surge protectors at 2 ends: right at the main junction panel near the sensor and then back where the cable enters the building/box where the logging system might be.
These Gas-type protectors are reusable, where an MOV isn't! The gassed arrestors have an inert gas in a capsule with two metal posts passing up out of to attach wires to.
The gnd wire goes on one terminal and your wires go on the other. There is no electrical connection between gnd and your wires...YET! When an electrical potential of Xvolts (X being what your particular arrestor is rated for) then the inert gas ionizes and becomes conductive (sounds strange....and I might've used the wrong word). This large potential that would normally kill your equipment then spikes across to the 2nd terminal via the ionized gas and is then grounded out. You lose data for a period but should save your sensor/equipment!
Will the stamp take a direct hit? NO! lol But you can do a lot to protect it! Remember that lightning takes the easiest path (like we all wish to do) and putting your stamp in a metal box that is grounding will protect it from EMF's but you also need to isolate/protect the wires that would run in to that box! (read up on Faraday cages -- you can safely run wiring down the center of those tall towers...when the tower takes a lightning strike, it energizes that center column/cage which protects the cables but the EMF would be grand I suppose...someone else can chime in and explain it better).
So....in the end, you won't know it works until a big bertha strike comes and kills your setup!
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Steve
http://ca.geocities.com/steve.brady@rogers.com/index.html
"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."
Vss Ground -- the negative side of the battery, so Vdd is positive.
Earth Ground -- actually a connection to the dirt -- I'm not being funny here.
Safety Ground -- Very similar to Earth Ground.
AC Ground -- the third wire of a typical 3-wire "grounded outlet"
AC Neutral -- the 'neutral' wire of a typical outlet. AC 'hot' goes from +115 to -115 volts, while this wire sits at neutral.
IF the outlet is wired correctly. If it's not wired correctly, then THIS wire goes from +115 to -115 while the
OTHER wire sits at neutral.
The first ground is the Vss -- tied to the negative side of the battery. There's no real reason to tie this ground to 'Earth', 'Safety', or AC Ground.
We traced it back to ground loops! We had hooked up all the "drains" (cable shields) and grounds and we were bringing in the transmission line noise and it was going through the grounding up to our sensor and giving erroneous readings!
Disconnected the drain from the sensor head (so that the drain was only connected at the near end) and it was fine!
So, you may want to ground everything out to protect yourself and be 'safe'...but you might find you create other problems too!
It's an art!! Not a science! [noparse];)[/noparse]
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Steve
http://ca.geocities.com/steve.brady@rogers.com/index.html
"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."
A lot depends on what your system is connected to, meaning power lines, phone lines, and sensor and actuator cables. If it is simply a system that sits on a table, powered by a battery, there is little need to worry about the grounding for lightning, unless there is also a noise issue.
Look at your system as an antenna, and if that antenna covers a wide area (which includes power and phone line connections as well as sensor and actuator cables), then it stands ready to receive electromagnetic pulses (high frequency) as well as simple capacitive and inductive coupling (low frequency). There might be catastrophic events from time to time, like a direct strike, so be it, but the more frequent events are much more manageable. The object of lightning protection is to keep those from appearing as differential signals across your circuit. For example, if you have a sensor cables extending to an anemometer 50 feet up on a tower, and also a soil moisture sensors out 100 feet north and 100 feet south, the electrical field gradient can be tremendous when lightning strikes even at a distant point. The voltage measured between two stakes driven into the ground can be in the hundreds of volts. The voltage difference appears on the signal inputs of your system and can burn right across. The potentials on the power and phone lines can also be different. The recourse, as others have already pointed out, is to funnel ESD currents off to a single ground point, usually through a shield or conduit that shields the whole system. You give the ESD and easy path that is not through your Stamp!
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Tracy Allen
www.emesystems.com
The application is control for a field of glass mirror solar dishes (for concentrator photovoltaic cells / steam). In the past, fields of heliostats reflecting to central towers had reliability issues with lightning from wires connecting all to a central pc. To bypass this reliability issue each dish will have its own micro with no interconnecting wires, including a pv and battery system for each dish. The dish does have motors on the frame, and temperature, pressure, position sensors. Your help has saved much engineering time, and cost.
Because of this Parallax group I have been able to do the basics rather than hire and manage others, a much more difficult process. And thanks to Parallax for this most excellent enabling technology. We are way ahead of schedule and are now ready to install the system. Thanks to all!
Douglas Wood.