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Where is mean voltage used?

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EPAIII:
Mean and average are pretty much the same thing. If you take the arithmetic average of any pure, sinusoidal waveform, it will be zero because there will be a negative instantaneous value for every positive instantaneous value and vice-versa. That is exactly the same thing for mean and average because those two words mean the same thing.

Now, if you rectify that AC waveform and all the values are positive, the situation changes. The mean or average value is not half way between zero and the instantaneous, maximum, peak value. If you perform the integration over the positive going, half sine wave, you will get an average or mean value that is 0.637 times the peak value. That is in the nature of the shape of the sine wave.

From a practical point of view, the mean or average value of a rectified, AC waveform is the value that a mechanical meter (moving coil mechanism) will respond to. This factor is built into almost every analog VOM (Volt Ohm Meter). Of course, there is an almost constant Voltage loss in a bridge rectifier which effects every AC Voltage scale on those meters. The fact that it is a constant amount of lost Voltage means that it has a greater effect on a low Voltage, AC scale than on a higher Voltage one. This is why most VOMs will have separate scales for 0 to 10 VAC and 0 to 10 VDC while higher AC and DC Voltages will be read on the same scales.

As others have said, RMS values are often considered more useful because they represent the amount of energy present or work that can be done. And it is easier to build RMS scales into digital devices than analog ones. True RMS scales are present only on the most expensive analog meters due to the cost of the circuitry needed. But in digital meters the true RMS value can be calculated with math from the individual, instantaneous samples. The common and less expensive analog meters were simply given an additional correction factor to translate their average readings into the equivalent RMS values when the waveform is a pure sine wave. That, of course meant that they were inaccurate when other waveforms were being read.

So the answer to the OP's question of where is mean Voltage used, it is primarily used in analog VOMs. And it is used there by necessity because getting a true RMS value with analog circuitry would be prohibitively expensive in most of those meters.




--- Quote from: Zero999 on November 09, 2023, 09:04:13 pm ---The mean voltage is just the DC component, which should be zero for the mains. If it's non-zero, then you have a problem.

--- End quote ---

EPAIII:
Not necessarily true. If a scope's gain is set to display a rectified sine wave full scale (zero at the very bottom and the peaks at the very top) and then the DC offset is set so that the average or mean value is at the center line (normally used for zero), then the negative peaks will be driven below the bottom of the screen and not be seen and the positive peaks will be below the top.

In order to display any AC signal, rectified or not, from peak to peak and using the full vertical space of the screen, you need to set the DC offset to the value that is half way between the positive and negative peaks.




--- Quote from: coppercone2 on November 09, 2023, 11:05:54 pm ---the point of average voltage is to determine a DC offset or a average DC offset . at least on a scope. to see for instance what bias level your coupling capacitor is at.

--- End quote ---

EPAIII:
I also find the remarks about a low pass filter somewhat concerning. There are many types of low pass filters so I don't want to get into any arguments about specific types. But I don't think that simply saying "after a low pass filter" is always true.

Most power supplies have a (bridge) rectifier followed by some filter capacitors and, since the output is assumed to be primarily DC, those capacitors must be called a low pass filter. But many power supply designs, dare I say most, do not operate at the average or mean Voltage nor at the RMS Voltage. They usually are designed to operate at higher Voltage levels by having capacitors that will maintain a higher percentage of the peak Voltage for the discharge interval between the half sine waves. In fact, in theory those filter capacitors would remain at the peak value if the current being drawn were equal to zero: each peak would charge them to that level and with no load current there would be no draining current to discharge them.

So, while some low pass filters may indeed only pass a DC level equal to the average or mean value, not all low pass filters operate in that manner. And I will leave it to others to name and give examples of low pass filters that do, as a design rule, output only the average/mean Voltage level.

ejeffrey:

--- Quote from: EPAIII on November 10, 2023, 05:19:17 am ---I also find the remarks about a low pass filter somewhat concerning. There are many types of low pass filters so I don't want to get into any arguments about specific types. But I don't think that simply saying "after a low pass filter" is always true.

Most power supplies have a (bridge) rectifier followed by some filter capacitors and, since the output is assumed to be primarily DC, those capacitors must be called a low pass filter.

--- End quote ---

It's not really about the filter (capacitors and possibly inductors and resistors) but the source (diode bridge) which has a nonlinear impedance.  If you look at a buck converter, if you are in continuous condition mode (either the low or high switch always conducting) the filter output is just the mean voltage.  But when it goes into discontinuous mode and the inductor current drops to zero the low switch turns off and is high impedance then you get the same behavior as a filtered bridge rectifier where it's no longer an average.

SiliconWizard:
Just a note to add to what's been already said: in practical use, one must define over which period of time we consider the "mean". If you use a low-pass filter, for instance (assuming that it's the proper approach in your given context), it gives you some kind of moving average (over a period proportional to its time constant). So, short point being, when we talk about mean (or average) voltage (or current), we very often talk about a moving average, in practice.

One way to get a 'true' average from a precise start point to a precise end point would be to use an integrator, reset it at the start point, let it run until the end point, stop it. And divide the accumulated value by the duration. Making sure the integrator is sized properly so as not to saturate till the end point.

But in practice we usually "settle" for a moving average. (Or a moving RMS if RMS is what we're after.)

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