[TowerTalk] my long lightning story (was RE: lightning strike)

[Jim Lux]

-----Original Message-----
>From: Dick Green WC1M <wc1m at msn.com>
>Sent: May 30, 2008 12:32 AM
>To: "'Roger (K8RI)'" <K8RI-on-TowerTalk at tm.net>, 'Skip K3CC' <k3cc at verizon.net>
>Cc: towertalk at contesting.com
>Subject: [TowerTalk] my long lightning story (was RE:  lightning strike)

<snip>
>Besides, Polyphaser says the ground systems won't "see" each other if
>separated by more than 75' because of the inductance of the wire (some
>disagree with this.)

75 feet is about 25 meters, so the inductance is around 25 microhenries.  At 1 MHz (which is a nice round number for the spectral peak in a lightning strike), the impedance is 25*6.28, or about 150-170 ohms, which is pretty high compared to the resistance of a single ground rod referred to "infinity" (typically in the few tens of ohms range).


>
>Remarkably, most of the equipment in the entertainment center survived,
>including a brand-new plasma TV, a Dish HD receiver, a tube pre-amp, a phono
>amplifier, a turntable,  two solid-state audio amplifiers, an FM tuner, a
>DVR, and several other pieces of gear. Only the CD player and HD media
>processor were damaged. 


Modern consumer equipment is actually pretty tough for transients.  To a certain extent, the whole "surge suppressor" business in high end consumer A/V is in the same class as those green marking pens they used to sell for "reducing internal reflections at the edge of your audio CD".


>
>Unfortunately, the SteppIR and Green Heron controllers had delicate
>semiconductors directly attached to the control lines, and I suspect the
>MOVs didn't start conducting until the max voltage level of those
>semiconductors was exceeded. Remember those CMOS NAND gates in my old
>antenna switching system? They were the only semiconductors directly
>attached to the control lines, and they were the only parts that
>consistently failed during storms, despite being on surge suppressors. 


And this is the typical design flaw of hobby and ham gear that grows up from being an interesting project in QST or other magazines.. microprocessor pins directly connected to the outside world.  It's easy, less expensive, etc., but wouldn't generally be good practice in anything that has to be ESD tested (which is to say, most consumer equipment, these days).  

No disrespect to the mfrs of the products.. they're nifty and cool, you get a high function piece of gear at low cost and short time to market, at the expense of the occasional failure.


>
>However, after this experience, I'm not convinced this is enough. It's one
>thing to rely on MOVs to protect switches, relays and the old analog rotor
>controllers. But it's not clear to me that the semiconductors in more modern
>equipment can stand any reasonable level of voltage above their operating
>voltage. It may not be possible to find a MOV that will conduct at a voltage
>less than what it takes to kill the semiconductor without getting tripped by
>operating voltage spikes. If the device happens to be connected to other
>devices through a communications link, a lot of damage can occur if one of
>the semiconductors breaks down.

You CAN do semiconductor interfaces that are quite tough.. Hard to to very high speed interfaces (why USB and 1394 aren't typically protected).  But the MOV approach probably isn't the right one.  Typically you do galvanic isolation (for common mode voltages) and some sort of fast clamp to the supply rails with a series resistor to limit the clamp current.

Spend the $20 on the book by Standler on Overvoltage Protection.  Published by Dover press

http://www.amazon.com/Protection-Electronic-Circuits-Overvoltages-Standler/dp/0486425525 is one source