Forums

Lighning protection

Started by Phil Hobbs January 27, 2016
On 01/28/2016 10:15 AM, Joe Hey wrote:
> On Thu, 28 Jan 2016 10:03:51 -0500 > Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: > > }snip{ > >> Roight. I'll probably use another of those 150 uF coupled inductors > ^ > Shouldn't that be 'micrometer'? ;-)
Nah, these ones are about 10 feet tall. 10000H, 150 uF. ;) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
On 28/01/2016 02:25, Phil Hobbs wrote:
> So I'm looking at lightning protection for the mezzanine units of the > cotton spark detection system. > > I have a copy of Standtler's book, "Protection of electronic circuits > from overvoltage", which is useful but a bit out of date (1989). (He > doesn't know about HV depletion MOSFETs or polyfuses, for instance.) > > It looks as though I can protect the isolated RS485 pair and the > power/common pair with a cascade consisting of > > (3-terminal spark gap) 500v 1a depl MOS > 0-------*-----------* *--*-----*-----*--* *-------*-----(Iso RS485) > A | | V | | | V | | > | ---------- | --------- V > | -------* | *------- /---/ bidirectional > | | | | A TVS > | *-----*-----* | > V | > -----GND | > A | > 0-------* .... similar.... .... * ... > B > > Lighter-weight lines are easier to protect, of course--Ethernet just > uses transformers. > > Any wisdom about other ways of proceeding? > > Thanks > > Phil Hobbs >
Check out Bourns TBU's and associated parts rather than faffing about with depletion MOSFETS.
On Thu, 28 Jan 2016 10:35:40 -0500
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

> On 01/28/2016 10:15 AM, Joe Hey wrote: > > On Thu, 28 Jan 2016 10:03:51 -0500 > > Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: > > > > }snip{ > > > >> Roight. I'll probably use another of those 150 uF coupled > >> inductors > > ^ > > Shouldn't that be 'micrometer'? ;-) > > Nah, these ones are about 10 feet tall. 10000H, 150 uF. ;)
Hah :) joe
> Cheers > > Phil Hobbs >
> One of my rules is that no pin of a semiconductor should > have a direct path out to the world.
thats been one of my rules as well I express it this way Don't connect copper to silicon without carbon in-between. M
On 2016-01-28 07:03, Phil Hobbs wrote:
> On 01/27/2016 09:48 PM, Joerg wrote: >> On 2016-01-27 18:25, Phil Hobbs wrote: >>> So I'm looking at lightning protection for the mezzanine units of >>> the cotton spark detection system. >>> >>> I have a copy of Standtler's book, "Protection of electronic >>> circuits from overvoltage", which is useful but a bit out of date >>> (1989). (He doesn't know about HV depletion MOSFETs or polyfuses, >>> for instance.) >>> >>> It looks as though I can protect the isolated RS485 pair and the >>> power/common pair with a cascade consisting of >>> >>> (3-terminal spark gap) 500v 1a depl MOS 0-------*-----------* >>> *--*-----*-----*--* *-------*-----(Iso RS485) A | >>> | V | | | V | | | ---------- | >>> --------- V | -------* | *------- /---/ >>> bidirectional | | | | A >>> TVS | *-----*-----* | V >>> | -----GND | A >>> | 0-------* .... similar.... .... * ... B >>> >>> Lighter-weight lines are easier to protect, of course--Ethernet >>> just uses transformers. >>> >>> Any wisdom about other ways of proceeding? >>> >> >> Instead of the depletion mode FETs which are kind of expensive people >> often use PTC. > > The spark gaps can pass brief peaks as high as 600V before firing, which > would tend to make a mess of whatever's on the other side of the PTC, > though. It needs to drive double-terminated twisted pair (58 ohms-ish), > so the resistance has to be pretty small, maybe 10 ohms per side at > most. The FETs I had in mind are IXYS IXTY08N50D2, 500V, 5 ohms, about > 78 cents @ 1ku.
That is a small fortune. If you want to spend this much there may be better solutions but you'll have to inquire about capacitances: http://www.bourns.com/docs/Product-Datasheets/TBU-CA.pdf
> ... They might avalanche a bit with really bad transients, > but should survive. Whether they're worth the bother, I'm not sure yet. > I can probably reduce the ON resistance by enhancing the gates, but > that would require a bunch more parts to ensure that when a transient > hits, the gate enhancement goes away without punching through the oxide > in the process. > > For the test board I'll probably put a few things in parallel and try > them out. Junior Tesla time. ;) > > The trouble is that a lot of the places where these gizmos are used > don't conform to any sort of electrical code we'd recognize over here. > Things like ductwork being part of the ground return for big > squirrel-cage motors, so I can't put a good number on the required > transient resistance. I'm guessing 5kA, 20/1000 us plus maybe an amp of > follow-on current. >
Installation in China. Me: "Why did they disconnect PE for the whole machine?" ... "Because some other electronics in there didn't work well enough with PE so they snipped it".
>> Be careful with the TVS. Those things have a large capacitance and if >> your data rate is too high you'd see marginal signal integrity or it >> won't work at all. One way (if available) is to use a helper voltage >> that is bypassed and limited. > > Right, thanks. I'll probably put the TVS across the supply and put > diodes to there. It's 200 kb/s RS485 (RZ initially, for convenience), > so I have a bit of leeway on the reactance. >
200kbit/sec is fast enough to start watching capacitances. No way this would work dirctly with a TVS. Provide lots of capacitance on the supplies and then check whether all teh parts can stomach the upper tolerance limit for the TVS when it comes on. Usually they can't and then you'll need a shunt regulator bleeder. I had to do that a lot. Engineers first thought that was voodoo but not anymore after the field failures almost vanished. Then, provide at least a small resistance between the clamp diodes and the IC signal pins unless the IC can withstand 3-4V more than you clamp. This is to avoid the on-chip ESD/substrate diodes being burdened with the lion's share of the jolt. Good RS485 transceivers can stomach a lot more though. [...] -- Regards, Joerg http://www.analogconsultants.com/
On 2016-01-28 08:37, Joe Hey wrote:
> On Thu, 28 Jan 2016 10:35:40 -0500 > Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 01/28/2016 10:15 AM, Joe Hey wrote: >>> On Thu, 28 Jan 2016 10:03:51 -0500 >>> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>> }snip{ >>> >>>> Roight. I'll probably use another of those 150 uF coupled >>>> inductors >>> ^ >>> Shouldn't that be 'micrometer'? ;-) >> >> Nah, these ones are about 10 feet tall. 10000H, 150 uF. ;) > > Hah :) >
With capacitance or Leyden flasks, shouldn't it be liters? Oh wait, litres, of course. -- SCNR, Joerg http://www.analogconsultants.com/
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
> So I'm looking at lightning protection for the mezzanine units of the > cotton spark detection system. > > I have a copy of Standtler's book, "Protection of electronic circuits > from overvoltage", which is useful but a bit out of date (1989). (He > doesn't know about HV depletion MOSFETs or polyfuses, for instance.) > > It looks as though I can protect the isolated RS485 pair and the > power/common pair with a cascade consisting of > > (3-terminal spark gap) 500v 1a depl MOS > 0-------*-----------* *--*-----*-----*--* *-------*-----(Iso RS485) > A | | V | | | V | | > | ---------- | --------- V > | -------* | *------- /---/ bidirectional > | | | | A TVS > | *-----*-----* | > V | > -----GND | > A | > 0-------* .... similar.... .... * ... > B > > Lighter-weight lines are easier to protect, of course--Ethernet just > uses transformers. > > Any wisdom about other ways of proceeding?
fiber media converter. They make them for serial as well as ethernet. Complete isolation between end points, and distance limiations for serial and interference is no longer a problem. The only issue is price and where to plug in the power packs for each side although they do make rack mount concentrators as well.
On Thu, 28 Jan 2016 10:34:32 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>> You might check the relevent sections of IEC61000 and IEEE/ANSI C62 to >> figure out just what level of transient or surge immunity that you >> intend and what exposure level is targeted. > >I wish. The design requirement is "It has to work through repeated >thunderstorms in an old wood/steel/cinderblock/mud/thatch/whatever >building in the hills of Bangladesh, with 200 metres of rusty steel >ductwork that may be carrying mains current, and very few grounds."
I have been working with industrial communication systems for decades also in tropics with daily thunderstorms. I have used the thumb of rules: 1.) you can use non-isolated RS-232 for equipments in the same room 2.) use galvanic isolation (2 kV) RS-422/485 in the same building 3.) use fibres between buildings After my customers also understood these rules, I haven't heard complaints by the end users in the tropics Rule #1 for lightning protection: There is no such thing as a ground potential, the best that you can get is equipotential bonding !!
On Wed, 27 Jan 2016 21:25:14 -0500, Phil Hobbs wrote:

> So I'm looking at lightning protection for the mezzanine units of the > cotton spark detection system. > > I have a copy of Standtler's book, "Protection of electronic circuits > from overvoltage", which is useful but a bit out of date (1989). (He > doesn't know about HV depletion MOSFETs or polyfuses, for instance.) > > It looks as though I can protect the isolated RS485 pair and the > power/common pair with a cascade consisting of > > (3-terminal spark gap) 500v 1a depl MOS > 0-------*-----------* *--*-----*-----*--* *-------*-----(Iso RS485) > A | | V | | | V | | > | ---------- | --------- V | > -------* | *------- /---/ bidirectional | > | | | A TVS | > *-----*-----* | > V | > -----GND | > A | > 0-------* .... similar.... .... * ... > B > > Lighter-weight lines are easier to protect, of course--Ethernet just > uses transformers. > > Any wisdom about other ways of proceeding? > > Thanks > > Phil Hobbs
Contact Jeff Dunnihoo, Pragma Design, Inc. He lives and breathes ESD www.pragma-design.com jeffhoo@pragma-design.com He can also test your final boards to make sure they are not going to die in the monsoon. -- Chisolm Republic of Texas
On 01/28/2016 01:26 PM, upsidedown@downunder.com wrote:
> On Thu, 28 Jan 2016 10:34:32 -0500, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >>> You might check the relevent sections of IEC61000 and IEEE/ANSI C62 to >>> figure out just what level of transient or surge immunity that you >>> intend and what exposure level is targeted. >> >> I wish. The design requirement is "It has to work through repeated >> thunderstorms in an old wood/steel/cinderblock/mud/thatch/whatever >> building in the hills of Bangladesh, with 200 metres of rusty steel >> ductwork that may be carrying mains current, and very few grounds." > > I have been working with industrial communication systems for decades > also in tropics with daily thunderstorms. I have used the thumb of > rules: > > 1.) you can use non-isolated RS-232 for equipments in the same room > 2.) use galvanic isolation (2 kV) RS-422/485 in the same building > 3.) use fibres between buildings > > After my customers also understood these rules, I haven't heard > complaints by the end users in the tropics > > Rule #1 for lightning protection: There is no such thing as a ground > potential, the best that you can get is equipotential bonding !! >
Thanks, #2 is more or less where I was going--isolated power, isolated RS485. The system has a number of sensor pods powered by a small concentrator box nearby (bolted to the same duct, cables 6 feet maximum, no isolation). The isolation link goes from the concentrator to the control panel on the ground floor, up to probably 500 feet away. The customer doesn't want the box ground connected to the long wiring, which is right, but also doesn't want it connected to the board ground, which ISTM is wrong, both for safety and EMC. Ground everything early and often, except when you can't, and then filter the daylights out of it. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net