What do you think of my lightning protection circuit? - Page 1
Forum section: electronics / projects

author: AlanR
author_rank: Regular Contributor
timestamp: February 25, 2014, 08:45:21 am
content: I whipped up this circuit to protect my isolated DC-DC convertor. Don't really care if lightning destroys all my security cameras that will be outdoors. Just want to put importance of protecting the DC-DC converter from lightning entering onto the output side. Other side of DC-DC converter is powered by a battery that is switched in when power failure occurs. It passed the microwave oven transformer torture test, but that is just a baby compared to lightning's few billion volts at a couple 100's of thousands of amps. You guys think this will work out well against lightning strikes that might hit one of my cameras that in turn will send the surge down the DC power buss line?

author: Rerouter
author_rank: Super Contributor
timestamp: February 25, 2014, 09:01:51 am
content: lightning is millions of volts or thousands of amps, while searching its high voltage above ground, then it finds a path to ground, the whole column becomes a plasma and it turns to thousands of amps, in both directions,why in the world are you using a 1000V spark gap? aim lower than that, the lower the voltage the faster the arc is quenched, equally you want to give your output capacitance half a chance,

author: AlanR
author_rank: Regular Contributor
timestamp: February 25, 2014, 09:14:30 am
content: Quote from: Rerouter on February 25, 2014, 09:01:51 amI put the inductors there to hold back the voltage spike that is about a few microseconds in duration for a lightning strike from getting that high on the transient voltage suppressor side because inductors will delay a sudden change in voltage and the resistance of the traces and the fuses is enough to get a decent L/R time constant. It will also help the spark gaps fire since they have a quite a bit of a delay for voltage rise time. I could not find any decent low voltage spark gaps. So, I built with what I could find in stock. It works fairly well, but I need moooaaaaaaaaaaaaarrrrrrrr voltage potential with high current.Got to get some destruction testing done. I like to build my personal stuff to survive out of this world events, but if this was a consumer product I would not bother going to such extremes.

author: Rerouter
author_rank: Super Contributor
timestamp: February 25, 2014, 09:26:06 am
content: get an automotive ignition coil and hook it up, that's going to be one of your best tests,a lightning strike can be 10's of surges in reversing polarity, just like the ringing of one

author: Richard Head
author_rank: Frequent Contributor
timestamp: February 25, 2014, 09:29:48 am
content: AlanI think the general idea is correct but would be a bit concerned about the Tranzorbs failing. The inductor may well saturate before the pulse is blocked by the spark gap.More importantly however is the grounding of the chassis of the DC-DC converter. What happens in practice is that the potential of the ground at the load side (DC-DC side) will probably differ significantly from the ground potential at the source (batteries I expect) side. This difference in potential is often high enough to cause an arc to develop between the DC-DC PCB ground and the DC-DC chassis. What I'm saying is that there is a common mode jump in potential that causes the equipmemt to fail. This is often the cause of power supply failures following a lightning strike.This can be difficult to solve with a DC-DC converter as an isolation transformer cannot be used between source and load as is done in AC applications.Dick

author: AlanR
author_rank: Regular Contributor
timestamp: February 25, 2014, 09:46:05 am
content: Quote from: Richard Head on February 25, 2014, 09:29:48 amThe case ground on the DC-DC converter is not internally connected to any components inside the DC-DC convertor. The output negative rail is also isolated from the input negative side. This is a transformer based DC-DC convertor. The fuses are fast blow types that will blow much quicker than before the inductor cores' saturation is reached I believe. The core material is N30. Of course I need to do some real world testing with much higher voltage potential than with MOTs. May have to go visit a lab that does HV testing. Or build my own high voltage source for testing.What core material would you suggest that is better than N30 for staying out of saturation limits? I could add some pulse rated resistors that will come to about few tenths of an ohm if that will help also.

author: VK5RC
author_rank: Unknown
timestamp: February 25, 2014, 10:10:06 am
content: Aren't there a lot of other problems with lightning? If you collect a decent rike, the 'ground' may be elevated several hundreds of volts, the EM pulse can similarly induce high voltage in low capacitance circuits /wires,not only the mains but also within electronic devices, often lightning won't flow down a straight wire, the high current will see the inductance of a straight wire as an impedance and often jump to another.At my old house, we lost 7 telephones to lightning (one at a time!) over 20years.I would go for a simple anti voltage pulse strategy and either unplug in a storm or bulid a cheap disposable power supply!I still NEVER leave electronic gear that I care about plugged in if not in use.

author: Richard Head
author_rank: Frequent Contributor
timestamp: February 25, 2014, 10:15:48 am
content: AlanRegarding the inductor core, I would definately use a large airgap. No airgap means it will saturate with very little current through it. For blocking common-mode jumps a current compensated or common mode inductor is used.The material loss is almost irrelevant as the surge is a one-off event. What is probably more important is how it is wound. I would try to reduce the inter-winding capacitance (increase self-resonant freq) to avoid the steep edged surge from jumping across the inductor.I know the DC-DC case is not internally connected to the electronics! What I'm saying is that the high voltage between the grounds can be so high as to jump across the airgap between the PCB ground and the chassis.The fuses are fast blow types that will blow much quicker than before the inductor cores' saturation is reached I believe.Not necessarily. If the surge voltage is huge (as it very often is) the time required to saturate the inductor can be very low. In fact, even fast blow fuses take a couple of milliseconds at best to open.I have done surge testing at our national electrical test facility and they use a HUGE bank of capacitors charged to a couple of KV. They couple the capacitors to the equipment by means of a triggerd spark gap. The entire energy of the cap bank is then dumped into the poor equipment. It's a brutal, but realistic test. They also have a pulse forming network to tr to emulate a 8/20us current pulse.

author: SeanB
author_rank: Super Contributor
timestamp: February 25, 2014, 06:12:53 pm
content: First any direct strike will destroy everything plugged in. No getting away from that. Plugged in, or with a mains lead attached and switched off it will be toast. What I have found to help is the following.Every camera has a grounding lead connected straight down from the camera to a ground rod directly under the camera. Connect to the case and mounting bracket.If you have a metal roof ground it as well next to the camera.Inside ground the case of the equipment to a ground wire going directly out to a ground rod. Connect every metal case direct to this wire. If you are using a TV as a monitor ground the antenna connection to this as well, using a F connector barrel and nut to attach a 10mm lug. If you have surge arrestors they all need a short direct link to this line as well.All earth cables 2.5mm or larger copper wire, earth rods are driven in to full length in ground and wire is attached to the end with a hose clamp and this joint is covered in grease and wrapped in a plastic tape to protect from rust.This means that nearby strikes do not induce massive voltage differences in the cables and kill the electronics. It will do nothing if you have a direct hit, that needs a lightning ground that has been set up to be lower resistance to earth than the mains ground. That is difficult as a lightning earth is considered to pass if the resistance is under 8R, while a mains earth must be under 1R. In a direct strike the mains earth will handle the majority of the strike energy, as there is enough voltage drop on the lightning earth for it to arc over even through a half meter of brick and pass a massive current into the mains earth.

author: calzap
author_rank: Frequent Contributor
timestamp: February 25, 2014, 06:39:19 pm
content: Agree with SeanB. A direct strike on an object that is connected by a conductor to sensitive electronic circuits will fry the circuits regardless of any protective devices; it's just a matter of how badly they're damaged. Best bets are to disconnect the circuits when lightning is likely (not always possible) or to have high, grounded conductors (lightning rods, metal roofs, flagpoles, etc.) that are more likely to attract a strike. However, protective circuits can help with the EMP of a close strike or surges from a distant strike on power or communication lines .Mike in California

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: February 26, 2014, 05:05:28 am
content: Quote from: AlanR on February 25, 2014, 09:14:30 amHow about a neon lamp?

author: AlanR
author_rank: Regular Contributor
timestamp: February 26, 2014, 10:00:22 am
content: Quote from: SeanB on February 25, 2014, 06:12:53 pmIf you mean by direct strike as in a leader bolt comes down from the clouds and the streamer bolts comes out of my 12 VDC wires, then I have FAR worse issues to worry about!! Like fire, because the poor 16 AWG cabling will vaporize and catch the wood studs on fire! I definitely know that my circuit will not survive that. I would like to meet or exceed specification UL1449 for my personal circuit. I suppose I could just send off my circuit to them, but may cost a lot.Rather than use a Benjamin Franklin style lightning rod, I will look into a type of lightning rod that looks like a stove pipe cleaner, but the bristles are made of stainless steel that protrude in all directions from the rod.The cameras are wireless. Only 12 VDC power is needed. So no TV connection to cameras.Good points.Quote from: CaptnYellowShirt on February 26, 2014, 05:05:28 amA neon lamp it is not intended to be used in such way as are gas discharge tubes. They could fail from the transient event.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: February 26, 2014, 12:35:15 pm
content: If you want direct strike protection, the best you can have is one of these.http://www.lpi.com.au/ProductsServices/DirectStrike.aspxthe bulb shaped terminals.They have a number of capacitive plates around the terminal that have a much higher chance of triggering a leader and actually capturing the strike than even an array of the the traditional Franklin rods.The spike on one of the bulb shaped terminals doesn't make any difference.Most buildings only need one of these terminals.Then you run straight to ground no bends at all if you can help it and keep stuff at least a few metres away from the down conductor has it can have MegaVolts.You can bond with the electrical earth at ground but not before, and obviously nothing else must be bonded with the down conductor.A good earth critical too.With the surge circuit you don't want the inductor to saturate so definitely at least some gap in the core and not to many turns.You can also have a capacitor X2 on the secondary side next to the MOV. Is it a MOV?

author: AlanR
author_rank: Regular Contributor
timestamp: February 26, 2014, 03:00:47 pm
content: Quote from: HackedFridgeMagnet on February 26, 2014, 12:35:15 pmThe inductor I am using is N30 material that has a saturation rating of 0.380 Teslas that occurs about 100 amps per meter.http://www.epcos.com/blob/528848/download/3/pdf-n30.pdfModel B64290L0674X830 that can be seen here:http://www.epcos.com/inf/80/db/fer_07/r_30.pdfI suppose it still needs to be air gaped.Those are not MOVS. Those are transient voltage suppressors with the two back to back that go to ground. This circuit is meant for the output with the surge condition occurring as back feed into where the surge suppression begins at the sparks gaps and then follow right from there. In other words I am protecting the output section of the DC-DC convertor from back feed onto the 12 volt DC buss that will power all the cameras I have.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: February 26, 2014, 10:42:28 pm
content: QuoteSorry should have really looked back at you diagram.I never really understood the way to specify inductors for current surges.Knowing you can have a 100kA surge or more it seems I big call to get as much inductance as you can but not to saturate. It is really at what level of surge does it saturate.I think these type of descisions tend to involve a fair bit of testing rather than straight equations.Without an air gap you haven't got a chance. Once it saturates it becomes an air core for surge purposes anyway.Surge coils I have seen tend to be straight cored ferrites for low current ratings with 10 - 20 windings and a figure 8 laminated transformer cores with even less windings for higher currents.

author: kc9qvl
author_rank: Regular Contributor
timestamp: February 27, 2014, 03:17:59 am
content: Radio towers get hit everyday with lightning and the equipment survives fine. Here is a bit about groundinghttp://www.w8ji.com/ground_systems.htm