Which material has high enough ohmage to resist lightning?

[Bob Myers]

AC -- by definition -- requires that the current reverse its flow.

A changing voltage isn't necessarily AC. Its only AC if the current
changes direction. Otherwise its still DC.

w_tom, unfortuately, brought in a concept that you are clearly
not ready to deal with. Mathematically, ANY function which
changes with time - which clearly covers any electrical waveform
that is not absolutely constant forever (which, if you want to be
extremely pedantic about it, is what "DC" really "should" mean)
may be expressed as the sum of a series of sinusoids. In some
cases (a "perfect" square wave or impulse are both good examples)
you need an infinite number of sinusoids to precisely equal the
desired function, but the math DOES work out. And in a practical,
real-world sense, if you filter an impulse you can in fact isolate
"real AC," even though you can also say that the impulse appears
to be going in "only one direction." This is why, for instance, you
can hear a seemingly "DC" discharge (lightning, or even a simple
static spark that results from shuffling your feet across the carpet)
on a radio.

A DC impulse is a impulse that -- for a relatively short duration --
increases its strength of flow toward one direction and then stops,
without reversing its flow.

It would certainly seem to be just that, but it works out to be
somewhat more complicated than it would first appear. You'll
need a pretty good grounding in calculus before you'll be able to
see why this must be so.


Bob M.

 

[Bob Myers]

Good idea. I remember reading about a small (relatively speaking)
Tesla coil that was supposed to be able to produce a few million
volts. It was in a large horizontal oil bath.

He didn't say petroleum, he said petrollium. It appears in
the periodic table in Group 42B, Subgroup 3.1416, Genus
fantasticum, ZIP code 999999.3, right between cavorite and
dilithium. It is one of the primary constituents of thiotimoline.

Bob M.

 

[Radium]

w_tom, unfortuately, brought in a concept that you are clearly
not ready to deal with.

Mathematically, ANY function which
changes with time - which clearly covers any electrical waveform
that is not absolutely constant forever (which, if you want to be
extremely pedantic about it, is what "DC" really "should" mean)
may be expressed as the sum of a series of sinusoids.

DC isn't necessarily constant but it is still DC.

Heard of pulsed DC?

In some
cases (a "perfect" square wave or impulse are both good examples)
you need an infinite number of sinusoids to precisely equal the
desired function, but the math DOES work out.

And in a practical,
real-world sense, if you filter an impulse you can in fact isolate
"real AC," even though you can also say that the impulse appears
to be going in "only one direction." This is why, for instance, you
can hear a seemingly "DC" discharge (lightning, or even a simple
static spark that results from shuffling your feet across the carpet)
on a radio.

Yes. Static can be heard on the radio. However, static is still DC. It
is a DC "spike".

Lightning is an example of a massive DC spike. Sure it is anything but
constant, however, its polarity does not reverse so it does not qualify
as AC.

 

[Homer J Simpson]

The term DC impulse is a contradiction or oxymoron. Either it is an
impulse - a sum of many frequencies - or it is DC.

One can have a large DC transmission with a small superimposed AC element.

 

[w_tom]

One can have a large DC transmission with a small superimposed AC element.

Then you don't have an impulse or a spike. That DC contains noise
that we commonly refer to as AC ripple. And the charts of frequency
verses energy (voltage, current, power, whatever) do not look anything
like that chart for an impulse.

There is no way around the science - an example of why science so
differs from junk science reasoning.

Impules are sums of many frequencies - AC waves. It was demonstrated
by how we measure a circuit's frequency response. It was demonstrated
by what is taught in math and in a first year engineering book (IOW
should be understood by anyone with sufficient mathematics training).
It was demonstrated by what a radio receiver detects.

It is also why voltages such as lightning require comprehension of
reactance - not just resistance.

If noise is received by a radio, then the noise is not DC. A radio
is tuned to a frequency. Only that frequency - an AC signal - is
received. That noise created by a 'so called' DC impulse' is AC energy
at a particular frequency. DC impulse is a classic oxymoron.

How does a radio transmitter create AC? Feed a DC current into a
non-linear circuit to create AC currents.

How did the Kettering ignition create 20,000 volts from 12 volts DC
by only opening and closing a switch? Same principle. DC currents did
not cross an ignition coil or pass through the condenser. Only AC did.
Same principle.

The 'so called' DC impulse is AC. A narrower impulse means more
energy in higher (AC) frequencies. But then that is basic science, a
principle taught in mathematics, and why resistance alone is not
sufficient to discuss material response to lightning.

 

[Tim Williams]

dilithium.

Heh, as I recall, alkali gasses -- at least at lower temperatures -- are
diatomic, so dilithium is merely vaporized lithium.

Tim

 

[Radium]

DC can also have frequencies. Think about it. A sine-wave current that
changes from zero to peak voltage and then back to zero.

If the DC current goes from zero to peak and then back to zero in one
second, then the DC current has a frequency of 1 hz.

DC can have other waveforms too. Such as square, impulse, triangle,
sawtooth, etc.

Then you don't have an impulse or a spike. That DC contains noise
that we commonly refer to as AC ripple. And the charts of frequency
verses energy (voltage, current, power, whatever) do not look anything
like that chart for an impulse.

There is no way around the science - an example of why science so
differs from junk science reasoning.

Impules are sums of many frequencies - AC waves. It was demonstrated
by how we measure a circuit's frequency response. It was demonstrated
by what is taught in math and in a first year engineering book (IOW
should be understood by anyone with sufficient mathematics training).
It was demonstrated by what a radio receiver detects.

It is also why voltages such as lightning require comprehension of
reactance - not just resistance.

If noise is received by a radio, then the noise is not DC.

Noise can be DC. DC just means that the current does not reverse it
polarity.

A radio
is tuned to a frequency. Only that frequency - an AC signal - is
received.

What about a changing DC current? That would also be received.

That noise created by a 'so called' DC impulse' is AC energy
at a particular frequency. DC impulse is a classic oxymoron.

Not really. Its only AC if its the current reverses its flow.

How does a radio transmitter create AC? Feed a DC current into a
non-linear circuit to create AC currents.

How did the Kettering ignition create 20,000 volts from 12 volts DC
by only opening and closing a switch? Same principle. DC currents did
not cross an ignition coil or pass through the condenser. Only AC did.
Same principle.

The 'so called' DC impulse is AC. A narrower impulse means more
energy in higher (AC) frequencies.

So, by your definition, if a battery powered deviced is switched on and
off, the change in current result from the on/off is action qualifies
as AC?

Think of current as a line on a graph, as long as the line stays on or
above the x-coordinate, it is not AC current.

 

[Tim Williams]

DC can also have frequencies.

DC is zero Hz. Face it, even if there were a current of some magnitude
running since the Big Bang, it would still be some 2 x 10^-18 Hz, not zero.

In practical terms, since (outside of simulation) we can't have zero Hz, we
settle for 'small enough' frequencies.

Note that a circuit which responds instantaneously (within perhaps a
fraction of a microsecond in practical terms) and is "DC coupled" (as
commonly defined) responds basically evenly to all signals as it would
respond to DC, even some arbirary frequency that may otherwise be
considered, say, RF.

For practical purposes, "DC" has a flexible meaning. A more precise
definition might be, the potential or current averaged over some arbitrary
period of time, particularly if the value has stabilized (e.g., having
turned on a power supply).

Think about it. A sine-wave current that
changes from zero to peak voltage and then back to zero.

Take for instance a perfect second order distortion, i.e., sin^2(x) = 1/2 +
sin(2x). No, bias *IS* DC, and, added to it, a sine wave. But it's not
like "the DC is varying", that's bull and you know it.

If the DC current goes from zero to peak and then back to zero in one
second, then the DC current has a frequency of 1 hz.

Direct current current [sic] has no frequency, DC is DC. The integral of
that pulse over any arbitrary time period always gives a DC component, no
matter what.

However, the value and distribution of the frequencies produced in that
pulse depend on repettition rate (if any), waveshape and so on.

DC can have other waveforms too. Such as square, impulse, triangle,
sawtooth, etc.

No, those are all determined by harmonics. They can all ride on a DC bias,
but DC a waveform is NOT.

So, by your definition, if a battery powered deviced is switched on and
off, the change in current result from the on/off is action qualifies
as AC?

Put a capacitor in series with your observed voltage. Where there's a
derivative (such a circuit is called a differentiator), there must be AC,
because a constant value has a derivative of zero.

Think of current as a line on a graph, as long as the line stays on or
above the x-coordinate, it is not AC current.

^^^^^^
Should read "has a DC"..."component."

Tim

 

[Bob Myers]

DC isn't necessarily constant but it is still DC.

Heard of pulsed DC?

Like I said, this is getting into concepts that you are not
ready to deal with.

"Pulsed DC" DOES have AC components; it can't
help but have. Mathematically, as I said, ANY time-varying
waveform is the sum of a series of sinusoids. I can't
show you why this is so without getting into some math
that you have not demonstrated any knowledge of at
all, and for that matter would be difficult to convey via
this medium. But it's true nonetheless.

Lightning is an example of a massive DC spike. Sure it is anything but
constant, however, its polarity does not reverse so it does not qualify
as AC.

Not in the sense you're thinking of, perhaps, but there is
most definitely a very large number of "AC" components
involved.

Again, the best practical demonstration of this is the detection
of lightning or other "sparks" via radio. Your radio isn't
at all sensitive to DC, is it? If lightning is ONLY "DC," then
where is all the radio-frequency noise coming from?

Bob M.

 

[Bob Myers]

If the DC current goes from zero to peak and then back to zero in one
second, then the DC current has a frequency of 1 hz.

DC can have other waveforms too. Such as square, impulse, triangle,
sawtooth, etc.

But in that case, it is NOT pure DC, which is the whole point.

Imagine that you see a waveform which is in the form of a
square wave with a peak value of 10V and a minimum of 0V
- what you would no doubt at this point refer to as a "pulsed
DC" waveform. How is describing it in this manner any different,
in ANY practical or theoretical sense, from describing it as
5V of "pure" DC summed with a 5V (zero-to-peak) "purely AC square
wave?" And once you understand that the "AC square wave"
part of that can also be viewed as the summation of a series
of sine waves, you'll see that anything you would call "pulsed
DC" most definitely does have AC components.

Bob M.

 

[Radium]

But in that case, it is NOT pure DC, which is the whole point.

Imagine that you see a waveform which is in the form of a
square wave with a peak value of 10V and a minimum of 0V
- what you would no doubt at this point refer to as a "pulsed
DC" waveform. How is describing it in this manner any different,
in ANY practical or theoretical sense, from describing it as
5V of "pure" DC summed with a 5V (zero-to-peak) "purely AC square
wave?" And once you understand that the "AC square wave"
part of that can also be viewed as the summation of a series
of sine waves, you'll see that anything you would call "pulsed
DC" most definitely does have AC components.

Bob M.

You are implying that DC is a current whose amperage remains constant.

From what you are saying, there is no such things as DC.

If DC is current that remains constant [which, by your definition
does], then DC is physically-impossible because there is always a
change in amperage. The change maybe EXTREMELY small [e.g. 10^ -10,000
attoamp] but nonetheless, there is change. When a device reads a DC
currnt of say "1 amp", this "1 amp" maybe slightly -- at an
infinitisemly small level -- fluctuating, this fluctuation [no matter
how trivial] indicates a change in current, hence it is actually AC.

In addition, a DC device with an on/off switch implies that there is
change in amperage when the switch is move from off to on [or visa
versa].

So from your definition of DC, DC does not exist in real universe. From
what you're saying, any electric current that exists, is always
alternating to some extent.

Is my perception of what you're saying correct?

 

[Radium]

DC is zero Hz. Face it, even if there were a current of some magnitude
running since the Big Bang, it would still be some 2 x 10^-18 Hz, not zero.

So, IOW, "direct current" is impossible. Its "direct" because humans
perceive it as flowing only in one direction but in physical reality,
there is some variation in current which make its alternating current.
Right?

In practical terms, since (outside of simulation) we can't have zero Hz, we
settle for 'small enough' frequencies.

So "direct current" is physically impossible?

Note that a circuit which responds instantaneously (within perhaps a
fraction of a microsecond in practical terms) and is "DC coupled" (as
commonly defined) responds basically evenly to all signals as it would
respond to DC, even some arbirary frequency that may otherwise be
considered, say, RF.

It seems like "alternating current" is defined as any current that does
not remain completely constant. Since no current can be direct, it must
always be alternating -- to some extent. Even if it is current from a
battery

For practical purposes, "DC" has a flexible meaning. A more precise
definition might be, the potential or current averaged over some arbitrary
period of time, particularly if the value has stabilized (e.g., having
turned on a power supply).
Okay.


Take for instance a perfect second order distortion, i.e., sin^2(x) = 1/2 +
sin(2x). No, bias *IS* DC, and, added to it, a sine wave. But it's not
like "the DC is varying", that's bull and you know it.

All right.

If the DC current goes from zero to peak and then back to zero in one
second, then the DC current has a frequency of 1 hz.

Direct current current [sic] has no frequency, DC is DC. The integral of
that pulse over any arbitrary time period always gives a DC component, no
matter what.

Once again, I come to the conclusion -- from your statements -- that DC
does not exist in physical reality. Mathematically its possible, but it
does not exist in the universe.

 

[Tim Williams]

It seems like "alternating current" is defined as any current that does
not remain completely constant. Since no current can be direct, it must
always be alternating -- to some extent. Even if it is current from a
battery .

Ya, a theoretical, true "DC" cannot exist, we just call it that.

Huh?

Seperate the frequencies... there's a DC part and the signal and its
harmonics, sidebands, etc.

Oh, and something else- no one has the time or accuracy or need to count
microhertz on a signal, so DC is basically anything sufficiently "down
there". Like I said, it depends...

Tim

 

[John Larkin]

It seems like "alternating current" is defined as any current that does
not remain completely constant. Since no current can be direct, it must
always be alternating -- to some extent. Even if it is current from a
battery

There are no hard definitions of AC and DC. They mean different things
in power systems as opposed to a signals environment, and different
things to different people.

In a signals environment, one usually considers a voltage to have two
components, as observed over some time interval. The averaged value is
usually considered to be the DC component, and what's left is the AC
part.

John

 

[Bob Myers]

You are implying that DC is a current whose amperage remains constant.

First, in DC everything remains constant. And no, there is no
such thing as true "DC" in the absolute strictest sense. However,
for just about any practical application, we can treat a lot of things
as though they WERE "DC," so it's still a very useful concept.

So from your definition of DC, DC does not exist in real universe. From
what you're saying, any electric current that exists, is always
alternating to some extent.

A more practical way of saying this is that any transient -
anytime something changes vs. time - involves AC components.

Bob M.

 

[Tim Williams]

A more practical way of saying this is that any transient -
anytime something changes vs. time - involves AC components.

The AC components are definetly of more prime interest.

I think a good example might be something like this.
http://webpages.charter.net/dawill/Images/Line Discharge Pulse Gen.gif
The MOSFET is used as a switch, to dump the pulse line, which, being a piece
of open coax cable, is but a capacitor. This causes a voltage across the
output until the line discharges. When the charge is gone, however, the
integral of that pulse caused a nonzero current in the inductance of the
"coupling transformer" (which is coax wound around a toroid, so that the
gate convieniently recieves the same drive no matter what voltage the MOSFET
is at), which causes the output voltage to drop below zero for some time.
The diode was added to clean up this little bit of ugliness.

In this circuit, the inductance finds the DC value, while whatever the
output is connected to is more interested in the AC components (especially
harmonics beyond 20MHz, as the rise time was around 10ns for a simple
mockup).

Tim

 

[Alan B]

DC isn't necessarily constant but it is still DC.

DC *is* necessarily constant. The mathematics show this.

Heard of pulsed DC?

"Pulsed DC" is a term meant to describe a waveform that switches between
two distinct states, and does not cross the zero-voltage plane. The term
should not be taken to imply that the change-of-state alternations are not
an alternating current.

 

[[email protected]]

Hi:

Is there any material that has enough electrical-resistance to block
off high-voltage such as that of lightning? Lightning is around a
million volts.

Another question. Which material has the strongest electrical
resistance of all?


Thanks,

Radium

Radium,

I haven't read this entire thread, but...

If lightning can jump from the clouds to the ground, how do you think
it will not simply jump around/across/past the resistive material that
you intend to use to try to "block" it?

e.g. I hear rubber is a pretty-good insulator (high resistance). So,
do you think that if you wear rubber-soled shoes, then lightning that
jumps a mile to get to you can't jump another half-inch to get from you
to the ground??

OK. Maybe one good way to make something relatively-safe from lightning
would be to make an electrical path that DIVERTS IT AROUND whatever you
want to protect. I.E. Use a very low-resistance heavy metal box, or
cage, connected to ground, surrounding the thing to be protected. But
you'll still have to wonder whether it's fail-safe. And you'd still
have to deal with any effects of the strong fields/currents that would
be produced.

- Tom Gootee

"He who lives in a glass house should not invite he who is without
sin."