Please define what is, and what is NOT, AC

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Re: Please define what is, and what is NOT, AC

Post by Admin on Sun Apr 28, 2013 7:23 am

PS - It's called modulation.

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Re: Please define what is, and what is NOT, AC

Post by Admin on Sat Apr 27, 2013 5:22 pm

Agreement with Ivan, 100% - your concept is sound.

Signals can alsways be considered as AC + DC. If you look at any 1/2 decent audio editing software the is a button with the function "Remove DC component". This says it all. There is DC and AC, which can be added together.

Now I will confuse a little - if you had 10v DC added to 10v (Pk-pk) AC then you would have a voltage that varies from 0v to +20v. According to your mentor, would this be AC or DC? If the DC bias was raised to +10.05v then it would be +0.05v to +20.05v which is DC (never minus). But if the DC voltage (from the solar panel when the sun dissapeared) fell to +9.95v then the waveform would vary from -0.05v to +19.95v, which by your mentor's definition now has suddenly become AC.

Naaa!

1 - if it wiggles it is AC
2 - if it wiggles and is never zero it is AC + DC
3 - If it doesn't wiggle it is DC

The only exceptiojn is: if it does not wiggle and it is neither positive nor negative then you don't have anything.

BR Harry

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Re: Please define what is, and what is NOT, AC

Post by DragonForce on Fri Apr 26, 2013 6:49 am

Thanks Ivan, I feel better now Smile

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Re: Please define what is, and what is NOT, AC

Post by Ivan on Fri Apr 26, 2013 5:59 am

Hi,
IMHO the guy that avised you has made your confusion. Forget what he told you. We can speak about any variable equally - e.g. direct and alternating component of pressure ! Rolling Eyes
Voltage resp. current resp. power often has two components: DC (called "bias" or "DC offset") and AC. The composed value may, and may not, change its polarity. Your concept seems to be OK.

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Re: Please define what is, and what is NOT, AC

Post by DragonForce on Thu Apr 25, 2013 9:47 pm

LOL Harry - I'm confused enough already. See, for the past 40 years, all the material I've read, all the things I have does in electronics, relies upon the concept that AC changes polarity - CURRENT flows first in one direction, and then the other. It matters not if the waveform is a sine wave, it matters not about amplitude. If the current flow is uni-directional (does not change direction), it is DC.

If we have a +10 volt voltage and we put an AC waveform onto it of say 10V peak to peak, then the voltage now varies from +5V (when the AC waveform is -5V) to +15V (when the AC waveform is at +5V). Now with referrence to ground, current flow does not change direction, and the voltage is still positive with respect to ground (0V)

I've asked an electronics engineer about this and he tells me that CURRENT needs to alternate (change polarity) for the waveform to be considered AC, however, VOLTAGE can alternate regardless of current flow, and that I'm getting the two confused somewhat.

As far as I am aware, a waveform that stays above ground (does not change polarity) is DC, with an ALTERNATING VOLTAGE (not CURRENT). I am trying to stay away from words such as "signal" and "DC offset" here Very Happy "Signal" (to me) implies AC, and that's not always the case, and "DC bias" (to me) implies that the voltage level is higher than the peak to peak level of the AC - again, not always the case.

So yes, i am confused, and yes I need it explaining to me - I have a feeling that a 40 year old concept I have is about to get demolished......

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Re: Please define what is, and what is NOT, AC

Post by Admin on Thu Apr 25, 2013 8:46 pm

Ok, I shall confuse things a bit now.

How about a mixture of AC and DC?

For example, the POTS system has something like 50V DC passed down a telephone line (when idle) and this DC has a function to energize relays and to arc-n-weld to prevent intermittent joints from occuring. When the telephone rings they superimpose 17Hz AC on the DC.

Couple this through a capacitor to get the AC component to ring a bell.

Simply put a relay across the line and the DC component will energize the relay, even if there is AC there.

So my answer to your question is "both" DC and AC can be added to the same circuit.

BR Harry

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Re: Please define what is, and what is NOT, AC

Post by DragonForce on Mon Apr 22, 2013 5:27 pm

Thanks Ivan, glad to see that my 40 years has taught me something Smile

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Re: Please define what is, and what is NOT, AC

Post by Ivan on Mon Apr 22, 2013 6:30 am

Hi DragonForce,
pulsating DC is still DC.
Let us be more technical: make a Fourier transformation of the value changing in time. (Generally, we may speak about voltage or current or power. I shall use current further.)
* The first cosine member is the constant DC component. If it is zero and some other component(s) are nonzero, we have pure AC current. If it is nonzero and all other sine and cosine components are zero, we have pure, non-pulsating DC.
* If the first cosine member and some other members are nonzero and their sum never crosses zero, we have a sort of pulsating DC. We can remove the DC component by a serial capacitor.
* If the first cosine member and some other members are nonzero and
their sum crosses zero in some intervals, we have biased AC. Again, we can remove the DC component by a serial capacitor.

In fact, biased AC and pulsating DC differ by the ratio of the DC and AC component only.

BR from Ivan

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Please define what is, and what is NOT, AC

Post by DragonForce on Mon Apr 22, 2013 1:23 am

Bit of a debate....

For 40 years, I've looked at AC as a current that changes polarity. If the waveform does not drop below zero volts (ie, does not change its' polarity), then it is DC. It can vary, it can change it's voltage from zero to a million volts, 10 million times in a nano-second, but as long as it does not change its' polarity, it isn't AC.

Now I'm being told different, even though Wikipedia says that AC changes polarity, else it's pulsating DC

http://en.wikipedia.org/wiki/Pulsating_direct_current

Does anybody have an opinion on this?

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