Alternating Current Measurement

Papabravo

Joined Feb 24, 2006
12,488
Generally speaking the answer is no. You can get meters that will measure the peak, and you can get meters that will measure the RMS (Root Mean Square). The meter you describe would need to sample the waveform for a half-cycle and integrate the samples to find the mean value. An impertinent question. Of what possible use is the mean value over half a cycle? You do know the average over a full cycle is zero -- right?
 

MikeML

Joined Oct 2, 2009
5,444
Most ballistic analog meters measure the "average" value of a half-wave rectified AC wave.
Most cheap DMMs measure the "average" value of a half-wave rectified AC wave.
My expensive Fluke DMM will measure the true RMS value of a half-wave rectified AC wave.
 
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Papabravo

Joined Feb 24, 2006
12,488
Most ballistic analog meters measure the "average" value of a half-wave rectified AC wave.
Most cheap DMMs measure the "average" value of a half-wave rectified AC wave.
My expensive Fluke DMM will measure the true RMS value of a half-wave rectified AC wave.
Not exactly what he wanted in my opinion. A half wave rectified input is at or near 0 volts for half a cycle which will of course lower the average. He wants the average of just the half cycle of interest. The TS/OP may have to clarify what it is exactly that he wants.
 

Thread Starter

Ultimate[R]

Joined Nov 22, 2015
3
the thing is, my teacher asked me why we can't use normal ammeters for ac measurement, i told him that in our country where ac frequency is 50Hz the current will change its direction 100 times in 1 sec which is too much fast for a ammeter to measure, he slapped me hard(it really hurt) and said that is not a valid reason and said that the correct answer is that ammeter measure average current which is 0 for a whole cycle in ac.So i just had a burst of curiosity and wanted to know about the above. thank you for answering the question. Tell me one more thing, was my answer that bad that i had to get slapped?
 

Papabravo

Joined Feb 24, 2006
12,488
the thing is, my teacher asked me why we can't use normal ammeters for ac measurement, i told him that in our country where ac frequency is 50Hz the current will change its direction 100 times in 1 sec which is too much fast for a ammeter to measure, he slapped me hard(it really hurt) and said that is not a valid reason and said that the correct answer is that ammeter measure average current which is 0 for a whole cycle in ac.So i just had a burst of curiosity and wanted to know about the above. thank you for answering the question. Tell me one more thing, was my answer that bad that i had to get slapped?
No, the slap was completely unjustified and unprofessional.
You didn't need to put together the sim on my account. I knew what you were talking about.
Sorry Mike I still think you are wrong in you interpretation, but who cares anyway? Certainly not me.
 

Papabravo

Joined Feb 24, 2006
12,488
No, the slap was completely unjustified and unprofessional.
Sorry Mike I still think you are wrong in you interpretation, but who cares anyway? Certainly not me.
Is there an ammeter that can measure average value of alternating current for half cycle?
BTW. The reason that we use the RMS measurement of a sine or cosine waveform is the RMS voltage times RMS current gives the same answer for power in a resistive load that integrating the waveforms over a single cycle gives. That is an enormous advantage in simplifying the calculation since the RMS value has a well known relationship to the peak value.
Also BTW, had I been your teacher, I would have commended you for your insight and your intuition. Not every notion that we come up with is correct, but that is irrelevant. What is important is that you keep thinking and coming up with insightful notions. Then proceed to test them. That is the scientific method that we should revere.
 
Meters are classified as
1. Average responding, RMS reading
2. TRMS (True RMS)

Yep, they forgot to tell you that they precision rectify the AC and multiply it my a fudge factor. The fudge factor assumes a sine wave input and makes the meter read RMS.

The RMS voltage is the equivalent amount of DC that would dissipate the sampe power across a fixed resistor.

Both meters have limitations, particularly frequency response.

Mathematically the average value of a sine wave is zero.

Teach didn't not do a good job.
 

WBahn

Joined Mar 31, 2012
24,850
the thing is, my teacher asked me why we can't use normal ammeters for ac measurement, i told him that in our country where ac frequency is 50Hz the current will change its direction 100 times in 1 sec which is too much fast for a ammeter to measure, he slapped me hard(it really hurt) and said that is not a valid reason and said that the correct answer is that ammeter measure average current which is 0 for a whole cycle in ac.So i just had a burst of curiosity and wanted to know about the above. thank you for answering the question. Tell me one more thing, was my answer that bad that i had to get slapped?
You should show your teacher that you can use a diode in series with a DC ammeter to make an AC ammeter. How does your teach claim that an AC ammeter is made?

As others have pointed out, you have to make some assumptions because what you will be measuring is the average current of a half-wave rectified signal. But, given those assumptions, you can map that measurement back to other parameters of interest, such as RMS current.
 

DickCappels

Joined Aug 21, 2008
5,933
[QUOTE="Ultimate[R], post: 925411, member: 297950" (Some text removed for clarity)
Tell me one more thing, was my answer that bad that i had to get slapped?[/QUOTE]

No, your answer was pretty good. It is your teacher who was that bad.

It seems that many teachers react poorly at exhibits of creativity.
 

WBahn

Joined Mar 31, 2012
24,850
Tell me one more thing, was my answer that bad that i had to get slapped?
No. It was a reasonable answer and has some validity to it. If the AC frequency were low enough you would be able to see the meter needle move back and forth (assuming it is of the centered-zero type, which most aren't) as it tracks the current throughout the cycle, allowing you to note the peaks.

No answer made in good faith should result in punishment of any kind. Now, if you had been insolent or flippant, that MIGHT be a different story, but that seems highly unlikely in this case. In the U.S. (and I suspect elsewhere) the teacher would have been in big trouble for hitting a student regardless of the reason.
 

Thread Starter

Ultimate[R]

Joined Nov 22, 2015
3
No. It was a reasonable answer and has some validity to it. If the AC frequency were low enough you would be able to see the meter needle move back and forth (assuming it is of the centered-zero type, which most aren't) as it tracks the current throughout the cycle, allowing you to note the peaks.

No answer made in good faith should result in punishment of any kind. Now, if you had been insolent or flippant, that MIGHT be a different story, but that seems highly unlikely in this case. In the U.S. (and I suspect elsewhere) the teacher would have been in big trouble for hitting a student regardless of the reason.
Well my teacher told the answer a day before and i didn't attend the lecture at that day so he was pissed off i guess.
Physical punishment is banned in my school too but no one follows it strictly. Also my teacher is kind of dumb(i know its not my job to judge him) but once some guy asked him on what factors does a focal length of a lens depends on and he replied, on the distance of lens from the object and the image and i almost died after hearing that.
 

wayneh

Joined Sep 9, 2010
16,114
Also my teacher is kind of dumb(i know its not my job to judge him) ...
Reconsider that thinking. You are the consumer buying an education from him. You have every right to judge him, and in many school environments you would be asked to do exactly that, to rate your teachers on a handful of criteria. The ones that get low scores are soon gone. Sometimes we have to hold our tongues, but don't be afraid to seek superior teachers.
 

gnuuser

Joined Jan 17, 2013
12
ammeter no (or at least none that I'm aware of)
oscilloscope yes
your teacher is a twit
there is absolutely no reason to strike a student!
if they cant answer the question then they are supposed to research for the correct answer!
the best teacher is the one who uses the insights and questions to learn himself so he can pass it on to his students!
 

dl324

Joined Mar 30, 2015
9,111
he slapped me hard(it really hurt) and said that is not a valid reason and said that the correct answer is that ammeter measure average current which is 0 for a whole cycle in ac.
Your teacher had no reason for slapping you and, if there were witnesses, should be reported for assault. He has no business being a teacher.
 

WBahn

Joined Mar 31, 2012
24,850
Had he made the mistake of slapping me, I just might have put him in the hospital.
You never know how that might turn out. I had a teacher (actually, I never took a class from him, just had a fair amount of incidental interaction with him) that went to break up a fight between a couple of the school's tough guys. I happened to see it because it was right outside his class room and I was on my way to class next door. One the students decided that no teacher was going to interfere with his need to put the other guy in his place, which is how I first found out that this diminutive guy was the state's reigning full-contact karate champion in his weight class. Needless to say, came as a surprise to the tough guys, too.
 

Papabravo

Joined Feb 24, 2006
12,488
You never know how that might turn out. I had a teacher (actually, I never took a class from him, just had a fair amount of incidental interaction with him) that went to break up a fight between a couple of the school's tough guys. I happened to see it because it was right outside his class room and I was on my way to class next door. One the students decided that no teacher was going to interfere with his need to put the other guy in his place, which is how I first found out that this diminutive guy was the state's reigning full-contact karate champion in his weight class. Needless to say, came as a surprise to the tough guys, too.
Well fortunately this particular proposition has never been put to the test. I think my reaction would be more instinctive than the result of careful consideration.
 
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