n9xv, you were doing alright till you came to this bit.

"...The RMS value is an "AC" value not a "DC" value...".

The RMS value of an AC waveform is the equivalent DC voltage which would produce the same power. So it could no more be described as AC than DC.

re ".....The DC/average value of 110-Volts (calculated) will produce the same heating effects as the 120-VAC (actual voltage)".

Surely, the differences between RMS and mean DC voltages after rectification are irrelavant to the question posed initially, and can only tend to confuse.

 

David, no problem man. Consider yourself bought a cup of coffee, or beer if you prefer.

In the feild of engineering, the truth does'nt lie. thats really all I was trying to get accross. I know some people reading this will say calculation, smalculation, bla bla bla! But when it comes to engineering/de-engineering one must respect the truth, otherwise its all guess work! - "handshake"

Pebe: Surely, the differences between RMS and mean DC voltages after rectification are irrelavant to the question posed initially, and can only tend to confuse.

RMS and DC are revalant if accuracy means anything to anyone. Once again, RMS is an AC quantity - NOT a DC quantity. Anytime you here the term RMS it is denoting an AC value. People hear that "equivalent heating effect" phrase and missinterpret it to mean that .707 X Peak = DC value. Thats wrong and ther is a difference. I know it sounds like splitting hairs but sometimes in the feild of engineering you split hairs. What good can result by introducing error after error?
I once worked for an outfit that repaired medical equipment, fluid-flow monitors, temp monitors etc. and in that feild accuracy could mean the difference between life and death. Obviously no one will die over the power supply scenario we are discussing here but the point is the same.

Speaking of dieing, has anybody checked on Buster lately? - - - Hang in there buddy were all pullin for ya!

 

Originally posted by n9xv@Feb 23 2005, 06:56 AM
David, no problem man. Consider yourself bought a cup of coffee, or beer if you prefer.

[post=5540]Quoted post[/post]​

Thank you. The next beer I have will be in CT. I travel across later today to spend some weeks there. ..... and I thought I would be leaving the snow behind.

 

Originally posted by n9xv@Feb 23 2005, 05:56 AM

So RMS is an AC value? Let’s look at how the term is derived.
An infinite number of voltage samples is taken throughout a waveform and each sample is Squared. The Mean value of all the squared samples is found, and the square Root is taken of the mean value, to give the Root Mean Squared value.

Squaring either positive or negative numbers yields a positive result, so the polarity of the original waveform is lost and the mean value will also be positive. The square root of a positive number can be positive or negative, but by convention is considered positive. In any event, it is unidirectional and constant.

So can you explain to me how that value can be called AC? And talking of engineering accuracy, can you tell me why an alternating voltage should be termed AC and not AV? Traditional but illogical, isn’t it?

Now to my point about irrelevancy. Take another look at the original posting. Buster was asking why there was ripple and how best it should be measured. He didn’t say there was a capacitor in circuit but it is reasonably to assume that there was – he wouldn't get such different readings from different DMMs if a large DC was not present.

The only reasons I can think of why anyone would be concerned with ripple are to ensure troughs did not go below the dropout voltage of a regulator, or minimum operating voltage of some other device, or is a potential source of hum in an amplifier. The question of RMS versus mean doesn’t come into it. It’s irrelevant to the topic.

 

Pebe: So RMS is an AC value? Let’s look at how the term is derived.
An infinite number of voltage samples is taken throughout a waveform and each sample is Squared. The Mean value of all the squared samples is found, and the square Root is taken of the mean value, to give the Root Mean Squared value.

Exactly!!!

You just reenforced my point that RMS IS an AC value.

Listen to what you are saying;

Pebe: An infinite number of voltage samples is taken throughout a waveform and each sample is Squared. The Mean value of all the squared samples is found, and the square Root is taken of the mean value, to give the Root Mean Squared value.

How or why would you take samples of a DC quantity and square the result.

As you stated; An infinite number of voltage samples is taken throughout a waveform. Its not like soup of the day and choose your waveform, its a sinewave form. The only sinewave forms I've ever encountered are AC sinewave forms.

The very definition of RMS defines RMS as an AC quantity. But dont take my word for it. Here's a textbook definition (The Illistrated Dictionary of Electronics 6th edition; author Stan Gibilisco, published by McGraw Hill ISBN# 0-8306-4396-6) Which states:

FYI: When you look up RMS value it directs you to see effective value.

Effective value - The root-mean-square-value of an AC quantity. The effective value an alternating current produces in a pure resistance has the same heating effect the equivalent direct current does. Also called RMS value.

RMS is the effective value of the sinewave. That is, its the part of the sinewave that does "usefull" work. The portion of the sinewave above the RMS value is not significant in terms of equivalent power.

Again the wall outlet is rated at 120-VAC RMS. Thats not 120-VDC.

My stereo is rated at 400-watts RMS (200-watts RMS /channel). The music booming in my speakers is AC not DC.

I'am an amatuer radio operator (callsign N9XV) I can transmit from 1.8-MHz up through 300-GHz at 1500-watts RMS which is the legal limit as defined by the FCC.
The radiation from my antenna is RF, radio frequency (which is AC) not DC.

I'am running out of examples.

The thing about AC Vs AV. The term AC is used because its the current that does the work in any circuit, AC or DC. In other words, the term is implied with respect to that which produces usefull work i.e. the current.

 

Exactly!!!

You just reinforced my point that RMS IS an AC value.

You are taking my comments out of context to suit your argument. Why no comment on my second paragraph?

How or why would you take samples of a DC quantity and square the result.

I assumed you would know. But DC quantity??? I can’t see where I said that.

Effective value - The root-mean-square-value of an AC quantity. The effective value an alternating current produces in a pure resistance has the same heating effect the equivalent direct current does. Also called RMS value.

You have left out quotation marks, but I assume that is the quote from McGraw Hill in its entirety. It refers to RMS as the ‘VALUE of an AC quantity’ that equates to a DC quantity – not to whether it is AC or DC.

Here is another quote – from this forum’s site.

“The qualifier "RMS" stands for Root Mean Square, the algorithm used to obtain the DC equivalent value from points on a graph (essentially, the procedure consists of squaring all the positive and negative points on a waveform graph, averaging those squared values, then taking the square root of that average to obtain the final answer). Sometimes the alternative terms equivalent or DC equivalent are used instead of "RMS," but the quantity and principle are both the same”.

Perhaps it’s better to think of the term as a factor to be applied to a waveform.

The portion of the sinewave above the RMS value is not significant in terms of equivalent power.

I’m afraid that’s left me cold.

As for the rest of your examples, I've been a long time in this business and would prefer not to be patronised.

My stereo is rated at 400-watts RMS

I won’t open another can of worms with that one. I’ll leave you with a link that deals with it.

http://jnaudin.free.fr/meg/ths720p.htm

 

You are taking my comments out of context to suit your argument. Why no comment on my second paragraph?

Your second paragraph is mathematically true. Its just a more in depth explanation as to why it reenforces my point that RMS is an AC value.

I assumed you would know. But DC quantity??? I can’t see where I said that.

You did'nt say anything about DC, I did. I was trying to make the point that all those voltage samples were from an AC sinewave and that it had nothing to to with DC (in terms of the samplings).

n9xv (via McGraw Hill) Effective value - The root-mean-square-value of an AC quantity. The effective value an alternating current produces in a pure resistance has the same heating effect the equivalent direct current does. Also called RMS value.

[/QUOTE]Pebe: You have left out quotation marks, but I assume that is the quote from McGraw Hill in its entirety. It refers to RMS as the ‘VALUE of an AC quantity’ that equates to a DC quantity – not to whether it is AC or DC.

The effective value of an "alternating current" (not a direct current) produces in a pure resistance has the same heating effect the equivalent direct current does. Also called the RMS value. (RMS is not a DC value but has the same heating effect as the "equivalent" DC does. I dont know how this could be any more clearer.

? this forum: “The qualifier "RMS" stands for Root Mean Square, the algorithm used to obtain the DC equivalent value from points on a graph (essentially, the procedure consists of squaring all the positive and negative points on a waveform graph, averaging those squared values, then taking the square root of that average to obtain the final answer). Sometimes the alternative terms equivalent or DC equivalent are used instead of "RMS," but the quantity and principle are both the same”.

He's right on untill this part;

? this forum: Sometimes the alternative terms equivalent or DC equivalent are used instead of "RMS," but the quantity and principle are both the same”.

The "principle" is the same in that RMS produces the same heating effect as the equivalent DC but the quantities are not the same. Once again, many people have a loose off the cuff interpretation of that.

n9xv: The portion of the sinewave above the RMS value is not significant in terms of equivalent power.

Pebe: I’m afraid that’s left me cold.

If it were significant (in terms of equivalent power) than the term RMS would not exist.

Pebe: As for the rest of your examples, I've been a long time in this business and would prefer not to be patronised.

Not sure I understand that?

n9xv: My stereo is rated at 400-watts RMS.

Pebe: I won’t open another can of worms with that one. I’ll leave you with a link that deals with it.

http://jnaudin.free.fr/meg/ths720p.htm

Thats an interesting site. The information is good as long as its interpreted properly. The RMS term is most accurately applied to a pure siusoidal waveform. Which is the case with 60-Hz power circuits. If is not a pure sinewave than you have to look at the peak-to-peak values of voltage and current to derive a value called PEP or peak envelope power. The upper-most-point/lower-most-point of voltage & current as viewed on a scope. My statement about the 400-watt RMS rating was to reflect that the 400-watts RMS was an AC term. the 400-watt rating is a bunch of "crap" in reallity due to the way its calculated. Its just a rating (errored as it may be). When you specify power in an AC circuit it is universally understood to be RMS watts (unless otherwise specified i.e. peak power, peak-to-peak/PEP power). In RF work, the term PEP is used to indicate transmitter power.
PEP is what you would see on a scope display, but the typicall analog wattmeter will read the "effective" or RMS value. The more high-end "peak reading" meters actually have a correction circuit in them and will actually indicate the PEP value on the analog meter.

The statement "Power is power" is true only in the context that the RMS/effective value produces the same heating effects as the equivalent DC/average value.

An example of PEP or peak power is an airborn WX radar system. A common radar sending unit operating from 24-VDC (aircraft voltage) can transmitt peak pulses in the 10 to 20-GHz range at power levels from 10 to 50-Kilowatts. They can do that because the period of the pulse is of such short durration.

I am understanding you to say that RMS and DC are the same thing in terms of
AC quantity or DC quantity. If this is true than why is'nt a common DC power supply rated at in terms of RMS. If you walk in to a computer rebuild/shop etc. and tell them you are building a computer and you need a DC power supply rated at 200-watts RMS, you'll get some rather strange reactions from the technical staff.

 

Originally posted by n9xv@Feb 23 2005, 09:01 PM

Your second paragraph is mathematically true. Its just a more in depth explanation as to why it reenforces my point that RMS is an AC value.

I could not have put it any clearer. If you think it reinforces your point then we obviously speak different languages.

Not sure I understand that?

Do you think telling me that wall sockets and loudspeakers are fed with AC and not DC, is not being patronising?

This discussion is getting nowhere fast, so I'll bow out and leave you to have the last word.

[post=5562]Quoted post[/post]​

 

I could not have put it any clearer. If you think it reinforces your point then we obviously speak different languages.

Do you think telling me that wall sockets and loudspeakers are fed with AC and not DC, is not being patronising?

This discussion is getting nowhere fast, so I'll bow out and leave you to have the last word.

[post=5562]Quoted post[/post]​

[post=5566]Quoted post[/post]​

[/quote]

Guys,

I have come to this conclusion; The reason there is an AC component in a DC rectified output is mainly because the rectified output is not true DC. This pulsating DC is simply an approximation of true DC. True DC would be that of a battery or even a piezo.....I think.When you rectify AC you inherently end up with a portion of the original AC input present in the DC output i.e. the AC component superimposed on the DC bias. Being that it is an AC component it should be referenced in terms of a RMS value. Thanks for your help.

 

Basically you got the right idea Buster. The ripple/pulsating DC is the residual "left overs" of the origional rectified AC. Sorry for being a thread hog :unsure: . But I find this forum very interesting and if I might say, more professional than some other forums I've visited.

If I can make a note to Pebe here;

My intent was certain not to offend! I can accept a fundamental disagreement. Sorry if I've made any enimies here but I am of the mind - search for the truth no matter where it lies.

I'am a little surprised that no one else had anything to add to this discussion but ok.

Respectfully,
n9xv

 

Originally posted by n9xv@Feb 24 2005, 05:26 AM
If I can make a note to Pebe here;

My intent was certain not to offend!......

[post=5580]Quoted post[/post]​

Fair enough. Honours even.

 

With reference to the term "r.m.s. power".

Another relevant article is available here http://www.davidbridgen.com/watts.htm

 

Very good article David! I hope I can get it printed while my printer is still working.
I was looking around on your website too!. Pretty cool stuff. I have'nt had time to really digest much yet. I spent the last couple of days cleaning the hard drive and dumping out all the "garbage" thats been on this machine for some time. After about 15-minutes of trying to pull up your web page, I reallized I was spelling it wrong :lol: .