DC Wave Questions

Pure DC, or something close to it, is actually pretty rare stuff.

Even on battery power plants, which are extremely good filters, there is some AC on the leads of just about anything powered from the battery unless either the battery or the load is all but embedded in the other.

For example, the 48 volt battery plants that telephone companies have, use some rather large cables to supply voltage to equipment. Yet a filter is required at every fuse bay to decouple the AC noise on the supply cable from the equipment in the bay. Even then, the supply lines have an astounding amount of AC noise on them.

That was particularly true back in the days of mechanical switches, when a telco switch was filled with "DC" switched lines that had mechanical contacts, and most of the loads being switched were inductive.

There is even more of the same going on in modern digital switching systems, minus the inductive kick, but those are filtered much more effectively because unlike the old mechanical monster, these new ones will malfunction themselves if the noise isn't filtered out.

And is pretty cold - ask Thevenin.

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So, tell me, what is the vapor pressure of Cadmium at 20 degrees C?

--- Nickel-cadmium usually refers to the metals used in fabricating a family of secondary cells used in redchargeable batteries, while cadmium, by itself, was once used to plate mechanical fasteners. It has dropped out of favor and its use may now be prohibited for that purpose due to its toxicity and effect on the environment.

-- John Fields Professional Circuit Designer

So, the first Fourier term is always zero. Got it.

Damn, this thread will hit 200 posts soon. The less the content, the bigger the thread.

John

No, the "intrinsic" _meaning_ (in case you'd care to check the original post) of "boiling" or "freezing" doesn't change, no matter what the temperature and pressure you're applying.

You're confusing _meaning_ with _parameters_. I have seen water freeze and boil simultaneously in a single container. Anybody can do this - just get a decent bell jar and a good vacuum pump. The "boiling point" certainly changes with ambient pressure, but that does not change the intrinsic nature of _what boiling is_. I think this might have been Mike Berger's point here.

Thanks, Rich

He has tutorials? You've got to be kidding.

John

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I don't know. Look it up.

Takes a nice little vaccuum pump to do it though.

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I think he is confusing boiling with sputtering.

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I agree with your examples of DC power supplies and AC noise. Been there, done that.

Defining how may angels can dance on a DC power cable without having to redefine it is pointless, however. Everyone I knew in the telco industry had good, workable terms for the cause of the need for filters, not only at the FB, but at the equipment rack too; it was noise, trash, crap. spikes....., but the 48V and 130V "power" were always DC and we knew the noise had to be dealt with as AC riding the DC. No other esoteric, mindless definitions are needed even though the terms AC and DC may be misnomers. They are historic and work very well.

Don

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I had a feeling that something like this would be his response.

The CRC handbook indicates that the vapor pressure of cadmium is about 10^-12 torr at room temperature (20 degrees). This is better than the vacuum at the moon. The best vacuum pumps available today can't hit that in a bell jar, much less 40 years ago.

No way did he "...get ni-cad to boil at room temperature (by) simply reduc(ing) the pressure to something *significantly* below its vapor pressure. We did it knowingly with gold too once..."

Gold has a vapor pressure of 10^-11 torr at about 800 degrees. I don't think gold will boil even in interstellar space (10^-17 torr) at 20 degrees. It *will* evaporate, though; so will tungsten! Slowly!

But he says it *boiled*; he couldn't be mistaken, could he?

Don Bowey wrote: No other esoteric, mindless

The only tiny problem is that the definitions are wrong.

Just one more inconsistency in his postings. I didn't even mention the nickel component in his alleged plating because it, like gold, has a vapor pressure of about

But I think you're right. I've never heard of Nickel-cadmium plating of screws, but I used to use cad plated hardware all the time.

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Well John, it probably was cadmium plating, not ni-cad. And I'm not sure what the actual temperature was, though it certainly wasn't much above room temperature (the experiment failed before it was exposed to significant nuclear radiation, which would have provided heat).

However, the metal plating on the hardware boiled!

Here's a chart you might want to look at. Note the relative vapor pressure of cadmium compared to other metals. Then think about "a nice little vacuum pump".

My point, since it went right over your head when stated as a puzzle, is that temperature alone is not what defines when something "boils", and some materials that you wouldn't normally think of in terms of a vapor can in fact "boil". "Out-gas" might be a better term.

Amazing isn't it? We're actually witnessing a dispute over what AC and DC are? And whether such things even exist?

I went looking on the IEEE website for a standard that would define the terms. There is a standard, 100-1992 "IEEE Dictionary of Electrical and Electronics Terms" that probably has their definition of AC and DC, but it isn't available on the web. I wonder if anybody participating in this thread has access to it?

Even though I couldn't find the dictionary referred to above, I did look at a number of their standards, and they are quite happy to use the terms AC and DC. I guess they don't realize that those terms are "Total and utter horseshit". Someone should tell them!