hello there,

The introduction is:
I can imagine, in my mind's eye, AC current, being generated
due to the spinning a coil of magnet wire around a permanent magnet. The speed
of rotation might be 60 turns per second, so i presume that gives an AC frequency
of 60 cycles per second.
And i can imagine, that with a few diodes a rectifier might be constructed, to
provide an AC to DC current conversion. But that the rectified DC current is apt
to still have the peaks of the AC current embedded in it.

And my questions are:
1. Does the DC current coming from a battery have a frequency ?
2. Does DC current coming from a rectified AC source have a frequency ?
3. In general, does DC current have a frequency or does it depend on the
source of the DC current ?

Thank you to anyone who supplies answers, perspectives and further clues.

Sally

 

Not making much sense... What does DC current have to do with AC current being generated?

 

I think the basic answer to your question is that you can think of a pure DC signal as being an AC signal that happens to have a frequency of zero -- or, equivalently, an infinitely long period.

 

DC is what the name implies as in Direct Current, It has a uni-directional (one way) current flow.

And my questions are:
1. Does the DC current coming from a battery have a frequency ?
2. Does DC current coming from a rectified AC source have a frequency ?
3. In general, does DC current have a frequency or does it depend on the
source of the DC current ?

1. No
2.No, not if it is cleaned up and filtered, then it is DC
3. No, DC current as defined as DC has no frequency.

Ron

 

A rectified AC signal is DC with an AC ripple.
Typically this is filtered to bring this ripple voltage below an acceptable level for the circuits using the DC.
The ripple has an AC frequency component, depending upon the mains frequency and the whether it's a half-wave or full-wave rectifier, but the DC has a frequency of zero.

 

1. Does the DC current coming from a battery have a frequency ?

DC is generally thought to not have a frequency.

2. Does DC current coming from a rectified AC source have a frequency ?

By definition, DC means Direct Current which does not alternate. In the case of rectified AC, it could be called pulsating DC; with a frequency of 1 or 2x the AC, depending on rectification method.

3. In general, does DC current have a frequency or does it depend on the
source of the DC current ?

In general, no frequency.

It's generally considered boorish to edit a post after someone has already responded.

 

I will offer an opposite answer to those given above.

All signals, DC or AC, have frequency.
Even the signal from a battery has frequency.
The next question is what is the measured frequency?

I am sure that over time, the voltage measured from a battery will show variations, perhaps diurnal variation.

In essence, every electrical signal can be transformed into a frequency spectrum using the Fourier Transformation. Any measurable frequency component that is not 0Hz constitutes frequency.

 

I can imagine, in my mind's eye, AC current, being generated
due to the spinning a coil of magnet wire around a permanent magnet. The speed
of rotation might be 60 turns per second, so i presume that gives an AC frequency
of 60 cycles per second.
And i can imagine, that with a few diodes a rectifier might be constructed, to
provide an AC to DC current conversion. But that the rectified DC current is apt
to still have the peaks of the AC current embedded in it.

And my questions are:
1. Does the DC current coming from a battery have a frequency ?
2. Does DC current coming from a rectified AC source have a frequency ?
3. In general, does DC current have a frequency or does it depend on the
source of the DC current ?

'

If a single coil, this would be a single pole generator and the Ripple rectified will be 100% and when full wave rectified to DC the ripple will now be x2 the frequency (120cps) . with a 3 phase generator the ripple is less than 5% due to overlap of the 3 cycles at 120° electrical degrees separation.
Max.

 

The distinction between AC and DC comes up here every few months or so. There is simply no objective line in the sand, and the distinction depends on the application and context.

It's not about any scientific distinction, it's about how words are being used. We want a simple shorthand term to communicate about some voltage or power. But the underlying details of real-world signals are not precisely described by these shorthand words.

We all have some idea about what AC is. The term "DC" is not only used to describe a pure, unvarying voltage that can only be obtained in theory. It's also applied to signals as varying as rectified AC before the filter, or the pulsing DC from a pulse-width-modulator. In many applications, referring to these bumpy signals as "DC" is meaningful. The current fluctuates in value but not in direction. If you're charging a battery, that's fine - it's DC. If you're powering an audio amplifier, you soon realize your DC supply has a large AC component that must be filtered out. Bumpy DC is not the same as, say, a battery and affects circuit design. But we don't have good shorthand terms for the various kinds of bumpy DC and we have to be more precise about the details.

 

Ahhhh yes, but how long do you have to wait after the current is applied until it becomes DC?

 

It depends on the strictness of your definition of frequency. Can dc voltage or dc current have frequency.....yes...absolutely. Does dc current have frequency?.....IF it doesn't.......it still is full of different rates.

So yes it could, and even when it doesn't....it still has different rates.

Example one.......a dc current that varies in amplitude. It doesn't change direction......just amplitude.

Like a rectifier output.......I would consider that frequency. I don't look at frequency as an alternating cycle....although it may possess one. Current is rate of flow.......frequency is the change in that rate.

Example two. Let's say you have a steady dc current of 1 amp. That means 1 coulomb of electrons, pass a plane in one second. The problem is that the electrons passing thru the plane........all are not incident. They come thru at different angles......giving different rates.

The steadiest current that you could imagine..........will still consist of many current rates.

Current is the result of many different velocities and angles. Current is a group of accelerations.

It will always be noisy.

 

Ahhhh yes, but how long do you have to wait after the current is applied until it becomes DC?

According to Zeno, an infinite amount of time.

It turns out that an Electrical Engineering curriculum has a time constant. When I was in school, the first quarter of EE was introduction to AC and DC circuits. By week 2 or 3 we were into transient analysis, the first real application of calculus. The time constant is 40% at 12 weeks, the percentage of the class that dropped out of school during that course.

ak

 

yeah DC has a frequency bcos the filtering of dc is not an pure..thats called not an frequency but pulse in dc.

 

I see the pedantic ones are out in force again.

 

The exact historical origin of DC is direct current.
The exact historical origin of AC is alternating current.

The electronics industry has corrupted and applied the usage of the terms DC and AC (not direct current and alternating current) to zero Hertz and non-zero Hertz respectively as in:

• DC vs AC circuit analysis
• Oscilloscope input filter functions
• Oscilloscope trigger input selection

If the current reverses direction then it is AC, alternating current, otherwise it is DC.
Yes, DC can have frequency.

 

3. No, DC current as defined as DC has no frequency.

If that is the case, then there must be a difference between an AC current that has a frequency of zero and a DC current. What is it?

Of course, this could be pure semantics since it is possible to claim that a frequency of zero is the same as having no frequency -- just as a man that has no money can be thought of as the same as a man that has zero dollars.

Still, a signal that can be written in the form of an AC signal is generally understood to be reasonably described as an AC signal, otherwise you have to start putting all kinds of unnecessary caveats on things. Plus, AC analysis applies to DC circuits by simply setting the frequency in the analysis equal to zero.

 

The exact historical origin of DC is direct current.
The exact historical origin of AC is alternating current.

The electronics industry has corrupted and applied the usage of the terms DC and AC (not direct current and alternating current) to zero Hertz and non-zero Hertz respectively as in:

• DC vs AC circuit analysis
• Oscilloscope input filter functions
• Oscilloscope trigger input selection

If the current reverses direction then it is AC, alternating current, otherwise it is DC.
Yes, DC can have frequency.

Even right as these terms were coined they typically referred to time-varying voltages more often than not. An AC generator was still called an AC generator even if the current it produced never changed direction.

 

Hi,

The simple answer is that we already have generalizations for this kind of thought about signals.

The generalization in it's simplest form is the Fourier Series which works with continuous signals of any kind.

The outcome of this is that we characterize a signal based on it's frequency spectrum, of which DC is considered zero. We end up with a description that includes whatever components that happen to be in that wave, usually (but not always) based on the wave's lowest non zero frequency (such as 60 Hz). If there is a DC component then there is a DC component, which will be a constant. There may also be phase shifts that go with each frequency as well.

The components of a continuous signal can be found by doing a Fourier Series analysis. This will show both the frequency components and the DC component.

An example is a full wave rectified sine. The frequency components are all even harmonics of the fundamental, and there is a non zero DC component. The fundamental in this case is usually considered to be the lowest frequency of origin rather than the lowest in the signal itself, although there are variations here.

 

Hi,

The simple answer is that we already have generalizations for this kind of thought about signals.

The generalization in it's simplest form is the Fourier Series which works with continuous signals of any kind.

I can fully agree with that.

The outcome of this is that we characterize a signal based on it's frequency spectrum, of which DC is considered zero. We end up with a description that includes whatever components that happen to be in that wave, usually (but no always) based on the wave's lowest non zero frequency (such as 60 Hz). If there is a DC component then there is a DC component, which will be a constant. There may also be phase shifts that go with each frequency as well.

An example is a full wave rectified sine. The frequency components are all even harmonics of the fundamental, and there is a non zero DC component.

How did you ever come to equate zero Hertz = DC when DC stands for direct current?

 

I think we can, for the most part, lay the question to rest by referencing Edison and Tesla.