DCC Extras

In message , Joe Ellis writes

As a mathematician I can definitely state that you can approximate a square wave by superimposing many different sinusoidal waves one on top of the other

That has got nothing to do with it. The voltage on the overhead line equipment on National rail is 25kV, but it is still AC.

That, too, has nothing to do with it.

I'm not confusing it. As far as I am concerned, it alternates either side of 0V, so it is alternating current.

Zero stretching just superimposes DC on that AC.

Phil: AND YOU CONTINUE TO CONFUSE 'AC MAINS POWER' (of which 110Vac and

I am not responsibke for the lack of understanding of physics used by someone writing a spec that you quote

AC DOES NOT MEAN SINE WAVE - it means ALTERNATING direction

Phil: I'm not, however you are!! - confusing power (ac mains) and ac THey are NOT necessarilt the same thing!!!

Joe,

You want to put a scope across the feed to your average loudspeaker and describe the AC waveform :-) It is very rarely pure sinusoidal and, if you are listening to Heavy Metal, it is liable to be closer to square wave :-)

AC is a description that can be applied to any alternating signal whether the alternation is purely sinusoidal, or any other waveform you care to generate. You want to play around with some analogue synthesisers to see what can be achieved.

Jim.

Nor am I responsible for your inability to understand the difference between "alternating" and "bi-polar".

You didn't look at the waveforms in the last link I quoted, did you? Obviously not...

heh heh heh... that was a quote from the web site I referenced - better go argue with them.

This is the one you REALLY need to look at - these are _CLEARLY_ NOT AC waveforms!

Ah, well. It's clear you have you mind made up and no amount of hard data to the contrary will convince you otherwise. Pity.

Not that sort of offset. You're describing the system of driving one rail to a higher voltage than the other which can be detected by the decoder.

It depends how you measure it. If you measure either DCC rail from the ground of the booster then it alternates between, say, 0v and 12V. In this case, each rail is an AC signal with 6V DC offset. Obviously, a mobile decoder has no ground reference and sees DCC as a pure AC signal, but I was thinking from the other end of the system and just trying to show Mr Ellis that DCC is not "bipolar DC" whatever that means.

MBQ

It's you who is confusing the issue by using a very narrow and incorrect definition of AC.

AC does not have to be a sine wave, nor does it have to have any particular voltage or frequency.

Mains voltage sine wave at 120V is only one example of an AC waveform.

Take a look at

MBQ

In my electrical engineering courses, this was referred to as "square wave AC". The difference may be semantic.

However, mathematically, square wave AC is actually the sum of an (infinite, IIRC) series of sinusoidal waveforms at different frequencies. That is, square wave AC is simply a line which contains an infinite number of sinusoidal AC signals at the same time.

The point may seem academic, but when we're talking about capacitors, it's not...the effect on the capacitor is the same as the sum of all the affects of all of the sinusoidal AC waveforms.

If it happens that the square wave is offset by some amount (perhaps even so much that it alternates between two positive voltages), then it simply means that there is an *additional* DC signal mixed in with the AC ones.

In all cases, the capacitor is affected by the sum of all the signals that come across it - all of the AC waveforms *and* the DC offset.

Or a zero frequency AC one!

That's just bad terminology. I don't think you will find that "alternating DC" is a recognised technical term.

Consider the two outputs of a booster which both switch between 0V and, say, 12V. When output A is 0V, output B is 12V and vice-versa. Actually, you can also design a DCC booster to use ground as one output and switch the other output between +/-12V.

The track is connected between the two outputs.

When A is 0V and B is 12V the current flows one way. When A is 12V and B is 0V (or A is 0V and B is -12V) the current flows *the other way*. That is the definition of AC, the current is alternating, or changing direction.

Well, of course all the above are true, but they're all irrelevant in a discussion of whether or not DCC is an AC waveform. It may be popular to define "AC" as the "mains" power that we all get to our homes but at the level of technical detail required to understand DCC it is just plain wrong.

Which only confirm that DCC is an AC waveform.

MBQ