Ken wrote:
"w_tom" wrote in message
Hence, fitting a surge suppressor (only) in a domestic situation will help
if the strike is some distance away - the further the better. It won't help
much is the strike is on the phone wires leading from your local pole to
your house!
First sensible statement.
If you think that you're at real risk, you can buy commercial units (for
rather more than 69p) which contain a spark gap, a series inductor and a
parallel surge protector. These, if connected to a GOOD earth, will filter
out all but the biggest surges. They are available for power, phone, TV
aerial and data lines.
Yes.. And what is on the mains side of most equipment?
Eitherer a whacking great transformer with plenty of (leakage)
inductance and coupled into at least an RFI cap and usually a bloody big
electrolytic....or a RFI circuit comprising er - would you believe
series inductors and capacitors across, and to ground?
Presumably you wouldn't.
The spark gaoop and or varistors are teh only thing missing, BUT
remember that youi DON'T jget kilovolt surges on teh mains as its is
down convreted by a thwacking big transformer somewhere up your street.
THAT is what has the surge proection in it, aand its built like a brick
(or iron) ****house.
You get a smaller surge. Easly enough to be taken care of by a bit of
RFO filtering and/or teh regulation in teh PSU itself.
insult snipped
It is routine to suffer direct strikes without damage even
in regions that have more serious lightning storms.
True, but these systems are designed with lightning strikes in mind.
It isn't routne for anything to survive direct strikes except stuff
specifically designed for it, which a 69p surge arrestor is not.
Howevr, unlike you guys, I have actually worked servicing kit in the
worst storm area on earth. Southern Africa.
But the
basic concepts must be understood. Plug-in protectors avoid
all discussion about the most critical function in protection
to sell their ineffective but so profitable products.
These devices are NOT inneffective. The worst you can say is that their
effectiveness is limited.
Yah well no fine. Limited to exploding just before teh rest of what they
are conected to explodes.
No pluse that is tamed by a tuppeny ha'penny surge arerestor will even
cause a modern PSU to blink. If the strike is near and big enough to
trouble it, the surge arressor will be long gone.
There are limits to all forms of lightning
protection. If lightning actually strikes your TV aerial or phone/mains
incomer, then it is almost impossible to avoid some damage - even if it's
only a zapped surge filter!
Indeed. But they tedn to blow open and pass te crap straight through.
Why are those CMOS devices, rated only for tens of
volts, not damaged by million volt lightning strikes?
Most devices in transmitter output stages (and receiver input stages) are,
in fact, rated at hundreds of volts, not tens, but I see the point! They
are not damaged because the designers put in a LOT of effort to fit surge
filtering and suppression between the semiconductors and the exposed bits of
metal.
Actuyally you use opto isolators. CMOS is totally and utterly vulnerable
to even a few tens of volts pickup on lines in or near storms. Soldered
in the chips enough times to tell you that fora fact.
If you want to use CMOS, so it in a metal grounded box with opto
isolators on ALL inputs and outputs.
Transmitters may well be valves still. Valves take lightning strikes
well,. being a sort of controlled lightning strike themselves
With a lot of complec protection even a strke on an antenna can be
reduced to something the power stages can handle, but its damned
expenisve, and only worth it if the call out fee to replace yet another
blown head amp up a mast is too high.,..
..
The worst design scenario is the nuclear emp, where you have to protect
against pulses with a risetime of 50kV per millisecond - this edge is so
fast that it often manages to couple itself round standard protection and
fry sensitive electronics, even inside sealed metal boxes...
Quite right too.
K