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Old 06-08-2009, 07:28 PM
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Armagdn03 Armagdn03 is offline
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Originally Posted by jibbguy View Post

While AC "Impedance" can only be calculated, and is a Variable dependent on the Frequency of the signal and many other factors such as material composition and specific design of the components (yet it is still a very "real" phenomena for sure); DC "Resistance" of course can be accurately measured and is generally a Constant (...Except sometimes for "temperature", lol).

So did you mean the "R" of the coils?

But it does not really matter at all what the R of a coil is, as the only interest here is when the AC signal is applied. Any two coils could possibly be of differing DC Resistance; yet can still Resonate at the same F (.. and when coupled with the same value Capacitor)... It depends entirely on how they were made: The number of turns, the distance between turns on the spool, the wire gauge, the wire's metal composition, the core, and of course the phase of the moon lol ).

When we make two coils ourselves, we do it the same way, with the same materials, core, and turn spacing... So the DC "R" can then matter as an indicator.

But i do remember having to hand-match auto-transformers before when working on old oscilloscope power supplies: Even though they looked & Ohm'ed identical (same P/N), they can still be different enough somehow to cause problems in a tightly balanced circuit (ahh, the bad-old days of Analog and continuous supplies.. I used to dream of getting revenge on the engineers lol).

But i fully agree with the rest of your post, and that the Murray stuff is fascinating.

What ever happened to this guy? Does anyone know what is he doing these days?
Hello, and thanks for the reply

Actually, im not 100% sure what part of what I said you are referring to but... I would define impedance as anything that impedes current flow, and many texts agree with this. There is ohmic impedance, and the complex impedance of Inductors and Capacitors, which do not dissipate heat the same as ohmic resistance, however the unit of measurement in both cases is the ohm. And in both cases, if the variables are taken into account correctly apply to ohms law, where impedance is the fulcrum of change.

You cannot interchange the use of complex impedance and ohmic impedance, however the "umbrella" word impedance can mean either or both.

As to your statement:

"But it does not really matter at all what the R of a coil is, as the only interest here is when the AC signal is applied"

I would not agree with this. Perhaps a little mind experiment will clear the air on this one.

Imagine two capacitors, both identical, filled to the same voltage. Each is on the verge of discharging into an inductor. One will discharge into an inductor of .000000001 ohms, the other will discharge into an inductor with a resistance of 1,000,000,000 ohms.

One cap will discharge rather quickly, the other will take a very long time.

One inductor will have a large amperage flow, and because of this, will be able to return much of its stored kinetic energy to the cap from whence it came.

One inductor will have an incredibly minute amperage flow, spread over a long time (much longer than the inductors time constant) and when the energy in the cap is zero, and the inductor collapses to return its energy, which is minuscule. One would have a very difficult time indeed trying to measure how much the inductor charges the cap on this collapse cycle.

(you might look up the coefficient of restitution)

Reality falls between the extremes, but its useful to look at them none the less.

The crap hits the fan when people read what we are discussing trying to make heads or tales of what were talking about. Our scientific terms seem to deviate from what we intuitively know to be true. A stiff spring has a great resistance to movement, however it doesn't meant that it dissipates more energy than its less stiff buddy. This we know intuitively, but to make the connection into other disciplines like electrical engineering is difficult, because so few spend their time connecting the dots.

There is a lot of error in what I said when strictly applied. There is never zero impedance, there is never infinite impedance, etc.... I simply outlined the extremes so that people who were not classically trained engineers or physicists could understand the concepts, and hopefully start understanding what the words are without having to run to a dictionary over and over.

Its almost like were still living in the tower of Babble............