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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 11th Oct 2018, 11:12 am   #21
Skywave
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Arrow Re: Full wave rectified RMS voltage

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Originally Posted by terrykc View Post
As RMS stands for the Root Square Mean of a sine wave, . . . .
Err, not so - according to this extract . . .

http://www.mathwords.com/r/root_mean_square.htm

No reference is made to the shape of any waveform (electrical or otherwise): deals purely with 'R.M.S' as a mathematical concept.

Al.
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Old 11th Oct 2018, 11:20 am   #22
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Default Re: Full wave rectified RMS voltage

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As RMS stands for the Root Square Mean of a sine wave
RMS stands for just Root Mean Square, any waveform will do.
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Old 11th Oct 2018, 11:22 am   #23
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Default Re: Full wave rectified RMS voltage

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Originally Posted by Skywave View Post
Quote:
Originally Posted by terrykc View Post
As RMS stands for the Root Square Mean of a sine wave, . . . .
No reference is made to the shape of any waveform (electrical or otherwise): deals purely with 'R.M.S' as a mathematical concept.
My mental model of a 'true RMS' voltmeter or ammeter is that it tells me the heating effect of the measured quantity in a resistive load. So it doesn't fully make sense to me that such a meter would ignore the DC component, which has certainly would have a heating effect!

I'm not at all sure why the manufacturers use AC coupling on true RMS AC ranges. Maybe it makes the RMS conversion easier.

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Old 11th Oct 2018, 12:21 pm   #24
G8HQP Dave
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Default Re: Full wave rectified RMS voltage

Maybe the AC coupling ought to be optional, as for oscilloscopes.

This has been an interesting thread, and a good example of the idea that what a meter does is provide raw data not measurements. To get a measurement from raw data you have to apply the correct theory.
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Old 11th Oct 2018, 12:59 pm   #25
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Default Re: Full wave rectified RMS voltage

Yes, the inclusion of the reference to a sine wave was a mistake - I'd already said that in a round about way in my earlier posts in relation to distorted 'sine waves'. Only the .707 figure is relative to a pure sign wave. I'd also mentioned the RMS voltage of a (virtual) square wave.

As Chris says, the RMS value of an AC current is the value that gives the same heating power as the equivalent DC value. In other words, if you connected two identical heaters, one to a 200V DC supply and the other to a 200V (RMS) AC supply, both would would produce exactly the same heat output.

Apologies for the confusion - I should have spotted it myself and corrected it.
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Old 11th Oct 2018, 1:07 pm   #26
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Default Re: Full wave rectified RMS voltage

So I found an article on line that says to get the "Real RMS" value for a waveform on top of a DC offset, RMS = sgqtoot((Vac^2) + (Vdc^2))

And behold it is correct using the values measured on my Fluke for DC and True RMS:
sqrt(12.43^2) + (6.49^2)) = 14V = 0.707 Vpk.
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Old 11th Oct 2018, 1:14 pm   #27
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Default Re: Full wave rectified RMS voltage

If you measure the RMS voltage directly on the transformer, and then measure on the output of a bridge rectifier feeding a purely resistive load, the two readings should be exactly the same. In practice, the rectifier drop will make a small difference, but that's a detail.

If they're not, it could be that the meter is not a 'true RMS' meter. Or, it could be that there is an internal DC-removal thingy - maybe a capacitor to just AC-couple. But although I suspect that some meters do include that, I can't help asking why?? It reduces the usefulness - for example, I couldn't use such a meter to check heater volts in a chain using a diode dropper!

Note that switching the meter to DC and measuring the rectifier output jolly well SHOULD give a slightly different reading, because then it's reading average, not RMS.
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Old 11th Oct 2018, 1:29 pm   #28
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Default Re: Full wave rectified RMS voltage

The Solartron VM1484 (analogue) true RMS voltmeter uses a Weston patented circuit and matched thermocouples with heaters in glass bulbs.
Oh - and a nuvistor as the input device...
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Old 11th Oct 2018, 2:37 pm   #29
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Default Re: Full wave rectified RMS voltage

That circuit is very interesting. It looks to me as if the resistor is AC coupled.
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Old 11th Oct 2018, 3:27 pm   #30
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Default Re: Full wave rectified RMS voltage

The circuit is very complicated - but you spotted the coupling.
The spec is from 10c/s to 10Mc/s with best accuracy 1% between 50c/s and 1Mc/s.
There are pages of % errors for different pulse trains and a page of fourier analysis which is all greek to me.
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Old 11th Oct 2018, 4:00 pm   #31
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Default Re: Full wave rectified RMS voltage

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Originally Posted by terrykc View Post
In other words, if you connected two identical heaters, one to a 200V DC supply and the other to a 200V (RMS) AC supply, both would would produce exactly the same heat output.
Although there is a formal metrology definition of the DC Volt (in terms of a Josephson Junction, which is referred to frequency, also well defined in terms of the Caesium clock) there is no definition of the AC volt, certainly not in terms of fundamental constants.

So the way it is done is to use a very very precise and well insulated (usually in vacuum) thermocouple (or series connection of many thermocouples), and the heating effect of the AC is compared to the heating effect of DC. When they are the same value, the rms of the AC is equal the DC, which you know to high accuracy.

http://www.npl.co.uk/measurement-ser...rrent-transfer

The third panel down shows the old Fluke 540B thermal transfer standard, of which I have two. Basic accuracy of that is 0.01%from 5Hz to 50kHz, although it goes to 1MHz with reduced accuracy of 0.1%.

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Old 11th Oct 2018, 4:35 pm   #32
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Default Re: Full wave rectified RMS voltage

We've got into thermal transfer standards. There's a fairly common HP thermal RMS voltmeter, pretty much the same as the Solartron one the 3400A and a wider bandwidth 3403A

These amplified units will, of course clip large peaks so the crest factor of the signal is limited (literally!)

THere are non-amplified ones for RF/ microwave use like the 432A power meter and the 478A family of bolometric thermal heads. These add a neat trick by using thermistors as the load resistor for the RF. THe thermistor is part of a bridge which senses its temperature and adjusts the DC drive to the bridge to get the thermistor exactly on the target value. Add RF and the extra heating means the bridge re-balances itself with a lower amount of DC power into the thermistor. The reduction in DC power equals the RF power.

DC substitution and sensing all in the same component.

This unit was taken further as the NIST type IV RF transfer standard.

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Old 11th Oct 2018, 5:25 pm   #33
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Default Re: Full wave rectified RMS voltage

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Only the .707 figure is relative to a pure sign wave
Or a rectangular wave with the right mark space ratio, many other waveforms will do that too.
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Old 11th Oct 2018, 6:04 pm   #34
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Default Re: Full wave rectified RMS voltage

Yes, but we were discussing mains derived voltages, so that doesn't apply here. You can get mains derived near square wave (the corners are rounded) using a special transformer, as I said before, but that is still a 50-50% ratio. However, the RMS value will be much closer to 50% than 71%.

In other applications, then yes, you are right but I can't think what use it might be to anybody. Also, the precise mark space ratio to which the .707 figure will apply will be a different constant for every different waveform so I can't see it being very useful knowing what that that precise ratio is. A true RMS meter, of course, neither knows nor cares!
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Old 12th Oct 2018, 2:51 pm   #35
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Default Re: Full wave rectified RMS voltage

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So how would you measure the RMS voltage of the output that is un-smoothed?
Take the mean average reading (i.e. what you get using the DC range on your meter), and the 'true RMS' reading (which is AC only), square them both, add them together, then take the square root. So in your example:

sqrt[ 12.43V^2 + 6.49^2 ] = 14V
This is close enough to the expected 13.7V. Obviously it is limited by your meter's ability to take a mean average reading of a difficult waveform.

EDIT: I didn't see the second page of posts!
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Old 13th Oct 2018, 1:00 pm   #36
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Default Re: Full wave rectified RMS voltage

This thread shows the importance of really understanding the behaviour of your test gear. I don’t own a DVM with claims to show true RMS, but I would expect a True RMS meter to read the same, give or take a diode drop, either side of a full wave rectifier, assuming no smoothing cap. After all, they have the same energy or heating effect.

I can see applications where you would need to know the true RMS value of a DC and AC signal superimposed (heater chains with half wave rectification, LED lighting, etc). When would you need the RMS value of just the AC bit of the rectified voltage, or of the 2v ripple on a 200V supply?

A crude measure of the true RMS voltage or current (if your meter insists on AC coupling) could be made with the old trick of an appropriate light bulb and a light meter. For example, put a small bulb in series with your heater chain, note its light level, then connect the same bulb to a DC supply and note the current needed for the same brightness. The light meter can be any photosensitive device and a multimeter, you’re always measuring at only one light level, so linearity and calibration of light levels are not important.

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Old 13th Oct 2018, 3:15 pm   #37
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Default Re: Full wave rectified RMS voltage

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Originally Posted by G8HQP Dave View Post
Maybe the AC coupling ought to be optional, as for oscilloscopes.
Some meters can achieve the equivalent. My old Fluke 45 bench meter can do AC Vrms (and AC Irms) and can also combine AC+DC if required. However, I never use this feature and I think that on this meter you have to avoid the 'fast' display mode when combining AC and DC. Otherwise significant errors can creep in.

I think that it measures the AC Vrms and then the average voltage and then it calculates the AC +DC based on these two measurements and sends it to the display. So it isn't a direct measurement as such. It is a built in feature that does the sums internally based on the above two measurements. To get it in this mode you have to press the ACV and DCV buttons at the same time.
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