# Ac and DC definition

Discussion in 'General Electronics Chat' started by Papabravo, Nov 6, 2009.

1. ### Papabravo Thread Starter Expert

Feb 24, 2006
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A minor, but important point. A bridge rectifier by itself does not produce anything like DC. It produces a sequence of positive half cycles at a frequency of 120 Hz., or a period of 8.33 milliseconds. The addition of a filter capacitor holds the peak value of the half cycle sinusoids for approximately 8.33 milliseconds until the leading edge of the next peak comes along. Even with a filter capacitor you won't have DC, but you will have AC with a DC offset.

Do people just not know about transformers or are they hell bent on being slam dunks for a Darwin Award?

2. ### SgtWookie Expert

Jul 17, 2007
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I beg to differ with you here. You will have DC with 120Hz ripple on it; the amplitude of the ripple being a function of the size of the filter capacitor and the load current. The ripple isn't really AC, as it's not very sinusoidal in nature - more like a ramp with a slow rise time.

Please be kind to our n00bs; many are simply unaware that they must use a transformer for safety reasons. It is not their fault if they don't know this; nobody has taught them that fact yet.

It's very obvious to "old hands" or even moderately experienced experimenters that transformers are required. We antiques just need to give our n00bs a gentle nudge in the right direction, so they get the chance to live through their experiments.

3. ### Papabravo Thread Starter Expert

Feb 24, 2006
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"You will have DC with 120Hz ripple on it..." is a distinction without a difference, but I'll stipulate that your characterization is also correct.

"The ripple isn't really AC, as it's not very sinusoidal in nature - more like a ramp with a slow rise time." Here we have a problem since nothing in the definition of "AC" requires it to be sinusoidal. Would you argue that the output of a bridge rectifier is not "AC"? Most of us understand "DC" to mean "does not vary with time" which means that everything else that "does vary with time" must be AC. You may argue that "AC" means sinusoidal, but then we need another term in the taxonomy to describe periodic, non-sinusoidal waveforms. I'll let the other Philadelphia Lawyers weigh in on this one, but I think I'm on pretty solid ground.

4. ### beenthere Retired Moderator

Apr 20, 2004
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I had it that a signal/waveform was AC if it crossed the zero axis. It could resemble anything and remain DC as long as it stayed all positive or negative.

5. ### Papabravo Thread Starter Expert

Feb 24, 2006
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So by your definition if I add +7VDC to sin(377t) what do I have? Is it AC or DC or the as yet unnamed third variant? Is it AC with a DC offset or is is DC with some ripple or ???. Says me, zero crossings are irrelevant -- it is AC. We got ourselves a sure fire debate here!

6. ### SgtWookie Expert

Jul 17, 2007
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OK, so you could block the DC with a large cap, and wind up with an AC output.

Or, you could bypass the AC component to ground, or remove it entirely with a pi filter - and wind up with DC.

But we're really getting off topic here. Guess it's my fault; I more or less started it.

Hope our OP gets that 1:1 isolation transformer and tries again.

Apr 20, 2004
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8. ### Papabravo Thread Starter Expert

Feb 24, 2006
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I guess I would question the autority of the book on this point. Is there a reference that agrees with the book? There is something vaguely unsettling about considering the magnitude of the DC offset as the determining factor. So you're saying that:

100 mV + 120sin(377t) is still an AC signal, but

200 V + 120sin(377t) is a DC signal.

Sorry but that fails the common sense test. I still think "amplitude depends on time" is the more consisten view.

9. ### SgtWookie Expert

Jul 17, 2007
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Merriam-Webster defines "alternating current" as:
Main Entry: alternating current
Function: noun
Date: 1839
: an electric current that reverses its direction at regularly recurring intervals —abbreviation AC

Princeton University defines "alternating current" as:
S: (n) alternating current, AC, alternating electric current (an electric current that reverses direction sinusoidally) "In the US most household current is AC at 60 cycles per second"

Wikitictionary defines it as:
an electric current in which the direction of flow of the electrons reverses periodically having an average of zero, with positive and negative values (with a frequency of 50 Hz in Europe, 60 Hz in the US, 400 Hz for airport lighting, and some others)

The United States Navy, in the NEETS course, Module 2, Page 1-1, under the heading "AC AND DC", states:
"Alternating current is current which constantly changes in amplitude, and which reverses direction at regular intervals."
The NEETS modules are available here:
http://www.phy.davidson.edu/instrumentation/NEETS.htm

There are many more entries on alternating current.

10. ### beenthere Retired Moderator

Apr 20, 2004
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Of the two vaguely descriptive labels, the "direct" one is the worst. How does one interpret a "direct" flow of current? "Unidirectional" trippeth not so lightly off the tongue, though, although it is more literally correct.

Alternating current, though, is reasonably labeled. The direction of current flow alternates according to the changing polarity of applied voltage.

11. ### beenthere Retired Moderator

Apr 20, 2004
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By the way - that was another nice hijack. These posts came out of this thread - http://forum.allaboutcircuits.com/showthread.php?t=29810

I thought that they might divert attention away from the grave dangers of using rectified line voltage, so I gave them a separate thread.

12. ### studiot AAC Fanatic!

Nov 9, 2007
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I agree.

Any unipolar signal whatsover is DC. It only becomes AC if it changes polarity.

There is nothing in the definition of DC to say that it has to be constant, which often confuses folks.

So you can have slowly or quickly varying DC.

13. ### Papabravo Thread Starter Expert

Feb 24, 2006
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After some further research it turns out that the term "Pulsating Direct Current" can be applied to a waveform that is not constant, but also does not change sign, and thus direction. This resolves my original question about weather there were two terms to describe current (DC & AC), or more. I don't know if you all will be happy with "pulsating DC", but I'm good with it.

Back to our original question. If we accept the above definition then a bridge rectifer produces "pulsating DC". Adding a filter capacitor smoooths it out into another form of "pulsating DC"

14. ### studiot AAC Fanatic!

Nov 9, 2007
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Yes I've heard of pulsating DC before.

Yes that applies to a bridge rectifier output.

But there are other types of varying DC.

Pulsating DC actually reaches zero (but does not cross), hence the term pulse or pulsating.

15. ### BillB3857 Senior Member

Feb 28, 2009
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I thought that was what PWM was all about.......Create the effect of a variable level of DC by controlling the duty cycle of pulsated DC

16. ### ELECTRONERD Senior Member

May 26, 2009
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Pulsating DC? That sure is an interesting term. I thought that any complex waveform would be regarded as AC and that any straight line above a certain point on the y axis would be regarded as DC. Even though adapters have a slight pulses from the positive AC peaks, it is still known as DC.

Austin

17. ### Thav Member

Oct 13, 2009
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I consider pure DC as a signal with only frequency content at 0. If it has a DC component and other frequency components, it's just that. I'll usually describe it in terms of what it's most like, DC with 10kHz ripple or 120V 60Hz with a 10V offset for instance. Pure AC to me is any signal with no DC component.

Saying some DC + AC waveform is DC only because it doesn't change polarity doesn't really sit well with me.

18. ### beenthere Retired Moderator

Apr 20, 2004
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Notice the rectified waveforms in the Ebook - http://www.allaboutcircuits.com/vol_3/chpt_3/4.html

They are example of pulsating DC. The level falls to 0, and than returns to some peak value. It is DC because the voltage does not change polarity.

Pulse width modulation is able to control a motor's speed by the averaging effect of current pulses in the coils. The waveform is DC, yes, but it is a square wave with a variable duty cycle. See the Ebook link for pulsating DC. Notice the duty cycle is fixed, and the waveform is not square.

Last edited: Nov 10, 2009
19. ### ELECTRONERD Senior Member

May 26, 2009
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Yes I recognize that waveform due to my power supply project I'm building. I did not, however, realize it was known as "pulsating DC". Its a good thing to recognize, so thanks.

Austin

20. ### studiot AAC Fanatic!

Nov 9, 2007
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This view is useful in certain limited circumstances.

However is is not tenable as a definition.

The exponentially rising (falling) voltage across a capacitor connected to a battery is most certainly simple Direct Current, however complex analysis shows it also contains frequency components.
It is simply exponentially varying DC.

It is not therefore inconceivable to think of DC varying in other ways.

One other such way is periodically. This is then periodically varying DC.

To qualify for the term 'alternating' something must actually alternate. Wookie's Wiki reference gives the current flow direction, which is OK so long as there is actually a current flowing.
However as you can have voltage (potential) with no current it is better to go with the voltage definition of alternating polarity.

In fact this definition can be expanded still further, which a frequency frequency based definition cannot, to include polyphase signals, where the phases alternate.

Don't forget the origin of the definitions from electricity generating machines relating to different arrangements of the slip rings on the commutator.