On (04 Jan 04) Bob Lawrence wrote to Roy Mcneill...


 RM> a nearby lightning strike knocked out our power for a while
 RM> (according to our neighbour, we were all out). The cables to
 RM> the sprinkler solenoids must have acted as antennae

 BL>  Ha, ha! Told you so!

 RM> or maybe our 240V earth was sufficiently different from the
 RM> sprinkler controller earth, because when I got home both of the
 RM> serial ports on my pooter were dead, and the RS232 signal from
 RM> Drip (the sprinkler controller) was iffy. I replaced the
 RM> MC1488/1489 RS232 chips in Drip, but the com ports on the
 RM> pooter were still dead.

 BL>  How long is it since you built DRIP? It must be five years! And I
 BL> told you so!

Nine years, at least. That's the oldest file I can find.

 BL>  As I said then... a good way to isolate long antenna leads from
 BL> microchips is an ordinary resistor (they're all spiral cut nowadays)
 BL> with a physically small capacitor to ground at the micro end. What
 BL> value resistor and capacitor depends on the speed of DRIP (or
 BL> whatever), and the slower the better. Ideally I used 10K and 2.2uF
 BL> electro, but on a telephone dialler I dropped that to 2.2K and 0.1uF.
 BL> Little electros are really good at absorbing sparks. The blue 0.1uF
 BL> only last for ten sparks or so.

The serial interface runs at 2400 baud.  I think I'd prefer a series
resistor and a pair of back-to-back shunt [zener-in-series-with-a-
diode]'s to local ground.  This should bother the serial data a bit
less.

 BL>  It's important to establish a GROUND on the printed board, so that
 BL> if "EARTH" jumps up as you suspect, then the micro and
the isolating
 BL> capacitors also jump up. You have to watch any other connections you
 BL> make to ground. During a lightning strike there is *NO* ground. It all
 BL> jumps up in the air by ten of thousands of volts and there is no
 BL> way to prevent it... you have to make sure there is an isolating
 BL> resistor to limit the energy that flows into the micro, and a
 BL> capacitor to tie the micro grounds together.

Yes. I'm familiar with lightning protection in radio huts on
mountain tops.  Step 1 is to install a good ground just outside the
hut.  After that, connect that ground to a metal plate that all the
coax cables pass through (through lightning protection modules that
absorb high voltages on the coax inner conductors).  Then, inside
the building, connect that metal plate to the mounting racks of the
radios, and to the 240V electrical ground.  Outside the building,
install a ground at the foot of the antenna tower (sometimes the
tower foundations will be adequate, but not always) and connect the
coax shields to this earth as they leave the tower to go to the hut.
Make sure that the highest point of the antenna tower assembly is
the tower itself, and not an antenna.

Protection for Drip would be similar.  At the Drip end, series
resistors and bidirectional zeners to Drip's local ground.  At the
PC end, ditto, although defining the "local ground" could be fun - I
think I'd use the pc's grounded metal case, rather than the "common"
connection in the RS232 cable.

 BL>  I've never actually done it with a computer box, but it you just
 BL> wired the capacitors on the back and let the resistors hang out it
 BL> should be okay.

 RM> A couple of days later, Win98 failed, and would only start in
 RM> Safe mode. WinXP would only get as far as its 1st advertisement
 RM> page. I took the pooter to a surgery, and it got a motherboard
 RM> transplant.

 BL>  Yair... that's the problem. Once the spark getsinto the board, there
 BL> is no way to know where it will end up. You need the resistor to limit
 BL> the energy (it actually sparks over inside), and a capacitor to absorb
 BL> the sharp edges of the spark. Even 2.2K and no capacitor makes a huge
 BL> difference.

 RM> Now its video system is different, and the monitor doesn't want
 RM> to know about Win98 screen resolutions greater than 800x600 -
 RM> it comes up with a little message that says "Attention:
 RM> 67K/74Hz is out of range" (or 68K/84Hz, 64K/59Hz, etc) XP runs
 RM> ok at 1024x768, but no higher. So now I'll need a new monitor
 RM> as well. Drat.

 BL>  That *really* sucks! Every motherboard comes with its own video, and
 BL> none of then run the old monitor. Have you tried reinstalling the
 BL> video drivers that came with the old board? The monitors are usually
 BL> standard...

Turns out the new video drivers had an "enhanced" option that was
turned on by default. Turning it off fixed the problem.

Cheers

--- PPoint 1.88