Which material has high enough ohmage to resist lightning?

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.

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

Heard of pulsed DC?

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.

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.

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

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

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.

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

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

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

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.

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).

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.

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.

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

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.

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

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

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.

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.

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.

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?

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?

So "direct current" is physically impossible?

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

Okay.

All right.

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.

Okay.

Sorry.

Okay.

Huh?

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

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

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

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

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.

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

Bob M.

The AC components are definetly of more prime interest.

I think a good example might be something like this.

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

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

On 2 Sep 2006 10:43:16 -0700, in message , "Radium" scribed:

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

"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.

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."