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FourierXFM t1_ixd0o2n wrote

Reply to comment by VulfSki in How exactly is the "direction" of the flow of power measured in a alternating current system? by Landhund

I have never heard it that way. Alternate means to change direction. The power is not changing direction.

I have never heard someone call a DC voltage with a high frequency ripple as "alternating", even though that is kind of the same as what you're talking about.

People do say AC power a lot, but AC means alternating current, which is true. Not alternating power, which is not true (again, all assuming pf = 1).

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I_dont_have_a_waifu t1_ixd7lhq wrote

See, in that situation, I'd say that there was a DC and AC component of the signal.

The AC component is the high frequency ripple, with an average value of zero. Then the total signal is the AC signal added to the DC signal.

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VulfSki t1_ixd9umy wrote

Yes. I think in their example tho they are thinking about power supplies that rectify and the smooth ac into a useful DC signal. And on practical terms many people consider the ripple negligible and call it a DC power supply.

But I think they are coming from a pretty simplified explanation and then the added nuances don't exactly work with the simplified explanation

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I_dont_have_a_waifu t1_ixda5h0 wrote

You're right, and it's not often that I would bother thinking much about the AC ripple on a power supply like that. I think the only I really gave it much thought was when I was designing rectifiers, and switching converters back in power electronics.

Otherwise I'd just throw a filter on it and call it a day.

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VulfSki t1_ixd9hpq wrote

Then you learned it wrong. (Or just a simplified explanation) I have a bachelor's in EE, the standard convention for the terms is that any signal that is not DC is considered AC.

You can for example take any AC voltage and add a DC offset which then makes it so that the entire wave is positive through an entire period.

Nothing has changed about the signal, other than you have shifted the wave enough to no longer be negative, this is most certainly NOT a DC signal.

But it is entirely positive.

Take amplifiers for example as well, the transiistors in the output stage of a class A amplifier are biased in a way that the entire signal is positive. This obviously is not a DC signal.

The issue with thinking of AC signals as requiring to be both positive and negative is that all the meaningful conventions fall apart when you consider signals that are not centered around zero .

An ac signal with a DC offset will pass through capacitors, and then lose the DC component. The same way an AC signal, with a DC offset will not see an inductor as short.

What you refer to as having a little bit of ripple tells me you're thinking strictly in terms of power.

So yes when people rectify a signal and then try to smooth it to convert from AC to DC you can accept a small ripple in the signal. yes I definitely see how someone would call that a DC signal because that is what you are looking for in that case. And the ripple is small enough to not cause an issue. But of course depends on how precise your power needs to be.

So alternating having charge flows back and forth is not entirely wrong. But it's just overly simplified and in EE we consider any signal that it's not DC to be AC. Because that is a more useful convention in terms of how the laws of physics govern electro-magnetics. Because you can have alternating current signals that are entirely positive (or negative)

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FourierXFM t1_ixdbot3 wrote

>Then you learned it wrong. (Or just a simplified explanation) I have a bachelor's in EE, the standard convention for the terms is that any signal that is not DC is considered AC.

I don't mean to get into a pissing contest, but you're being rude, so I will. I have a masters in EE with a specialty in power electronics focusing on AC/DC conversion. I promise I did not learn it wrong.

Alternating means back and forth, or positive and negative. A full bridge rectifier with no capacitive filter at the end is still called DC even though it's oscillating up and down.

At some point of ripple you would be more right to say it's DC with an AC component, but nobody in industry calls that alternating power... because it's not alternating.

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VulfSki t1_ixdcxxb wrote

Yeah that's not what I'm talking about at all.

Yes I said it makes sense to consider that DC power. I never said otherwise there.

You seem to have misread my comment. What I said was that as a matter of convention, anything that isnt DC we called AC.

The phrase flowing back and forth can mean a number of things. My point was that it doesn't need to be negative to be considered AC.

When I work on power electronics, and still do, we often refer to the ripple at the output of the supply as an AC ripple as part of the DC. But yes of course we would never consider that an AC power supply. Of course we would still call that DC power. Sure.

Just to be clear. I said anything that isn't DC we refer to as AC. And you picked one very specific example to say "no everyone I talked to calls a DC power supply DC even if it has a ripple" which yeah or course they do. But that's not really related the point I was making

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