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Lightning protection

:: EPE Chat Zone ≠:: ≠Radio Bygones Message Board :: » EPE Forum Archives 2007-2009 » Archive through 20 April, 2008 » Lightning protection « Previous Next »

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echase
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Username: echase

Post Number: 90
Registered: 07-2007

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Posted on Tuesday, 08 April, 2008 - 03:26 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Like to have your views on my design for a lightning suppression circuit. It goes in the cable between a roof mounted 5 volt low current sensor and a PIC based device in the house which is reading data from the sensor. If lightning strikes the sensor it will destroy the whole lot, but a near miss may be protected by this circuit:-

A T shaped circuit that has R1 of 330ohm (low wattage) in series from sensor cable to cathode of a P6KE15A transient suppressor diode. http://www.rapidonline.com/netalogue/specs/47-5440e.pdf
The anode of the diode goes to sensor and PIC earth. Then R2 of 330 ohm from cathode to PIC.

The R1 resistor acts like current limiter / slow fuse although itís probable that under a severe strike it will flash over and act like a short circuit instead of as a resistor. The P6KE15A is like a zener with a high tolerance to current pulses so dissipates most of the current. It also acts like a normal diode limiting reverse voltage to around 0.6V. Finally whatever excess voltage gets past the diode (15V or so) is limited by R2 and the normal protection diode on input to PIC (there are small diodes internal to PIC between each input and the +ve and Ėve supplies clamping the input to 5.6V or Ė 0.6V.

Not sure if a lightning strike should be considered as a positive or negative voltage flash or both. Anyone know? Presumably even if predominantly one way, inductance and capacitance in the circuit is going to cause ringing so cause large spikes of the opposite polarity too.

The inductance, resistance and capacitance of the long and thin input cable also hopefully limits the current. Indeed it may melt and short or blow open circuit which all helps protect downstream.

Someone pointed out to me in a previous post here that a spark gap is ideal as well, but these are expensive and large so I do not want to use them.

Now my main question here is that I am worried about the response speed of the P6KE15A diode so was thinking of adding a fast recovery diode such as IN4936 http://www.supplyframe.com/partsearchservlet/partdetail/1589659/EICSemi/1N4936G in parallel with it. This also clamps the reverse voltage to 0.6V, but I am not sure if itís needed and whether itís really is any faster than the P6KE15A. Is it? Especially as itís no use for forward voltage protection.

As itís a 3 terminal sensor these is a second set of identical components protecting the + supply line.

Ideally the P6KE would be a 5V device but the reverse leakage of these is too high and would affect the sensor accuracy.
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zeitghost
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Username: zeitghost

Post Number: 1107
Registered: 01-2006

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Posted on Tuesday, 08 April, 2008 - 04:20 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

In the event of a direct lightning strike, it won't matter much what protection components you have because the entire thing will become plasma.


I read somewhere that most lightning strikes are negative, but the positive strikes are much worse when they do happen.
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echase
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Username: echase

Post Number: 93
Registered: 07-2007

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Posted on Tuesday, 08 April, 2008 - 05:17 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I had a lightning strike on or near my roof mounted TV aerial and it killed the aerial amplifier but nothing else. There was nothing visibly charred in the amp so it canít have been too serious. This is the kind of strike I am aiming to protect against with my circuit.
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terrym
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Username: terrym

Post Number: 591
Registered: 05-2005

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Posted on Wednesday, 09 April, 2008 - 03:48 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Here's a link to an IEEE document that might help, it has references to other sites at the end:

http://www.lightningsafety.com/nlsi_lhm/IEEE_Guide.pdf

TM
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alan_stepney
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Username: alan_stepney

Post Number: 38
Registered: 01-2008

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Posted on Wednesday, 09 April, 2008 - 09:02 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

In my limited experience, the best lightning protection is to provide a VERY low resistance path to earth (outside of the building) and to link any device to that with a circuit that will ensure that the lightning goes that way to earth rather than through the equipment concerned.
(There is a British Standard for the circuitry on the input of TV sets for that purpose.)

The only strike I have witnessed, or rather the aftermath of one, hit the tv aerial, went down the cable, the TV exploded, and the strike went across the room to a radiator, and from that to the earthed incoming water mains.

En route there were burn marks showing that it had jumped over 10ft across the room.

Took the roof off the house, destroyed the local substation, and the house had to be rebuilt.

With that level of strike, any electronic device will be toast!

Hence a lightning rod serves to diffuse the lightning during its build-up phase and before it becomes (too) harmful.
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zeitghost
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Username: zeitghost

Post Number: 1108
Registered: 01-2006

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Posted on Wednesday, 09 April, 2008 - 09:26 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Yup.

2 inch wide copper strip down to a buried & effective earth rod/mesh.

Spikey thing at the top to spray charge at the cloud.
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obiwan
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Username: obiwan

Post Number: 2226
Registered: 12-2005

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Posted on Wednesday, 09 April, 2008 - 02:29 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

You want to use gas tubes for one. They're small ceramic tubes filled, well, with a gas! duh.... they use them in phone protection circuits and they carry the most current and work the best, trust me, I tested thousands of them and built equipment to test them, simulated lightning....

They also have "spark gaps" as a back up/secondary source of protection.

Most of the tubes will have a center prong for ground, and one small tube to either side of that, and each end cap will have another prong. the metal is mounted on the sides of the ceramic tubes in such a way that the two prongs (center and one side) each form a spark gap. So there's two spark gaps, one for each tube. You can protect two wires with that.

In addition, I would use something a bit faster acting too. Something replaceable.

When the lightning strikes, you need something to hold that voltage at a reasonable level until the heavy duty stuff kicks in. Normally, the gas tubes are high voltage and just a bit slow. Fine for telephones, but not the best for regular electronics.

I would put that further away from the inside stuff, like just at the inside to the house, the wall. Then, inside your indoor electronics, I'd put some more protection, lower level stuff. But fast.

Remember, you're going to kick a current down that wire, so even if you do limit the voltage on it, it's still going to try to go higher because of the inductance.

I would do it in at least three stages. The outdoor with gas tubes (you want this outdoors because it can get messy). Heavy duty inside, inside your box maybe, and some more, maybe just some diodes right at the sensor input.
Do Not Hit The Fly That Lands On The Tigers Head.

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