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from overvoltage", which is useful but a bit out of date (1989).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
--
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
Lighning protection
Started by ●January 27, 2016
Reply by ●January 27, 20162016-01-27
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 from overvoltage", which is useful but a bit out of date (1989).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. 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. You could, for example, run a small current through a TVS or zener to keep it up at xx volts and then run a diode from the signal line into that. Same for the negative-going side. That way there is only the small capacitance of two diode which remain reverse-biased at normal operation. A common mode choke can't hurt either. Mostly that's needed anyhow to pass EMC. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●January 27, 20162016-01-27
On Wed, 27 Jan 2016 21:25:14 -0500, 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? > >Thanks > >Phil HobbsI would humbly suggest an RC right after the spark gap, maybe instead of the fets. One of my rules is that no pin of a semiconductor should have a direct path out to the world. The gap might let through a lot of spike before it ionizes. And I suppose it could fire and launch a picosecond edge along the trace to the fet. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●January 28, 20162016-01-28
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> writes:> 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 > > Any wisdom about other ways of proceeding?I'd add common mode choke + C after the GDT to slow down the hit. Use a sectional-wound choke instead of bifilar and you'll also get some differential-mode filtering. Lightning strike is usually manifests as a common mode signal. LC+GDT+TVS-combination with common RS485 transmitters has survived quite well on a rocky hill with plenty of close strikes and bad grounding with 200m wires. No depletion MOSFETs there. Multiple ethernet switches have died even with 10m cables on the same hill and being further away from the antenna mast receiving the strikes. Direct strike would of course be an another matter :-) -- mikko
Reply by ●January 28, 20162016-01-28
On Wed, 27 Jan 2016 21:25:14 -0500, 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? >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. For a specific level, on a specific equipment port type, there are intrinsic costs for predictable (and demonstratable) level of immunity. I'm not sure the term 'lightning protection' will be an accurate enough description of the threat level, or whether data lines can be considered alone, without other ports/packaging being considered. You'd also need to know what was being protected, at the physical and performance level. Are all RS485 isolated ports the same internal schematic? I doubt it. RL
Reply by ●January 28, 20162016-01-28
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. 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.> 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.> You could, for example, run a small current through a TVS or zener to > keep it up at xx volts and then run a diode from the signal line into > that. Same for the negative-going side. That way there is only the > small capacitance of two diode which remain reverse-biased at normal > operation. > > A common mode choke can't hurt either. Mostly that's needed anyhow to > pass EMC. >Roight. I'll probably use another of those 150 uF coupled inductors per pair. That's the value I wound up with for the flybuck--turns out that I can control the sag pretty well by putting a resistor from V_in to the feedback divider, so primary-side regulation should actually work. (Without the resistor, the coupling coefficient had to be 0.995 or more for it to have a ghost of a chance.) Thanks 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
Reply by ●January 28, 20162016-01-28
On 01/27/2016 10:42 PM, John Larkin wrote:> On Wed, 27 Jan 2016 21:25:14 -0500, 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? >> >> Thanks >> >> Phil Hobbs > > I would humbly suggest an RC right after the spark gap, maybe instead > of the fets. One of my rules is that no pin of a semiconductor should > have a direct path out to the world. The gap might let through a lot > of spike before it ionizes. And I suppose it could fire and launch a > picosecond edge along the trace to the fet.The problem is that double-terminated RS485 takes over 80 mA to run it, so the resistor would have to be pretty small, maybe 5-10 ohms per side at most. That would be about a 60-A transient, worst case, which will require a pretty beefy resistor. It would also make it difficult to use low-capacitance diodes to the supply--I'd need 1N5823s and not BAT54s. A nice Ethernet transformer that will just saturate and disconnect the silly transient would be nice. I suppose I could Manchester-encode the RS-485, but I'd like to stick to vanilla as far as possible. There are enough SCADA security vulnerabilities without putting the fire alarm on the Internet, which would be bound to happen eventually if I used Ethernet. 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
Reply by ●January 28, 20162016-01-28
On 01/28/2016 03:42 AM, Mikko OH2HVJ wrote:> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> writes: > >> 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 >> >> Any wisdom about other ways of proceeding? > > > I'd add common mode choke + C after the GDT to slow down the hit. Use > a sectional-wound choke instead of bifilar and you'll also get some > differential-mode filtering. Lightning strike is usually manifests > as a common mode signal. > > LC+GDT+TVS-combination with common RS485 transmitters has survived quite > well on a rocky hill with plenty of close strikes and bad grounding with > 200m wires. No depletion MOSFETs there. Multiple ethernet switches have > died even with 10m cables on the same hill and being further away from > the antenna mast receiving the strikes. > > Direct strike would of course be an another matter :-) > > -- > mikko > >Hi, Mikko, Thanks. I'll see your hillside and raise you a monsoon thunderstorm, long rusty steel ducts, and no electrical inspectors. ;) 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
Reply by ●January 28, 20162016-01-28
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'? ;-)
joe
> per pair. That's the value I wound up with for the flybuck--turns
> out that I can control the sag pretty well by putting a resistor from
> V_in to the feedback divider, so primary-side regulation should
> actually work. (Without the resistor, the coupling coefficient had to
> be 0.995 or more for it to have a ghost of a chance.)
>
> Thanks
>
> Phil Hobbs
>
>
>
Reply by ●January 28, 20162016-01-28
On 01/28/2016 07:55 AM, legg wrote:> On Wed, 27 Jan 2016 21:25:14 -0500, 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? >> > > 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." The standards probably have a lot of wisdom in them, though, so I'll check them out, thanks.> > For a specific level, on a specific equipment port type, there are > intrinsic costs for predictable (and demonstratable) level of > immunity.Yup. I'm going back and forth with the customer on that point. Making it really bulletproof (10 kA, 20/1000 us) looks like costing about $6 for the suppression components plus another $3 for an ISO3080 transceiver. That's a fair amount, but cheap compared to a service call way out there. Anyway, there's only one data pair to worry about.> > I'm not sure the term 'lightning protection' will be an accurate > enough description of the threat level, or whether data lines can be > considered alone, without other ports/packaging being considered. > > You'd also need to know what was being protected, at the physical and > performance level. Are all RS485 isolated ports the same internal > schematic? I doubt it.I have the luxury of designing the whole thing myself, so yes, they are. The control panel side won't be isolated, but all the far-away ones will. The remote boxes will be bolted to the ductwork, which simplifies the RF problem some. -- 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
