Meter accuracy

Relating all this good stuff back to meter accuracy, does any forum member know how digital meters work?


i.e do they somehow average out the voltage, perhaps by rectifying it,  and measure that or do they sample the voltage many times per cycle, measure each sample and average the results?


Clearly a peaky waveform will show a higher voltage than the true RMS is they rectify it whereas a sampling mechanism could be quite accurate.

Thank you Mike that does help to be able to look at those scope traces did you do all those experiments yourself? I'm a bit short of test gear now although I do have a scope and certainly recognise the rectifier waveforms I remember ages ago reading an article about rectifier noise and how to cure it and have done it on a couple of linear PSUs that are used in the shack. Anyhow looking at the red trace on the bottom picture it's clear that the current it shows does go negative slightly just after the mains current goes positive and the other way on the next zero cross  so I think I got it now maybe I need to do some more reading on it wanted to thank you for your efforts in explaining this

Most simple DVMs have a one chip DC analog to digital convertor of the dual ramp kind, typically running at about 10Hz and when switched to an AC range engage a rectifier circuit up front. There are models that drive a display direct or to interface to a micro-controller.

The correction for peak to RMS is handled by fiddling the values in the resistive divider chain, or by scaling the op-amp gain as part of the 'precision rectifier '

(precision my .. ahem sorry.)

Here is a reprint of an application note from the chip makers showing how to use their devices to build a DVM, and an examples of rectifier circuit.


That example is a re-badge (knock off) of a chip originally made by ICL in about 1980 and meant that  digital metering suddenly stopped being a thing that came in a carry case like an avo, needing arms like a gorilla to carry and a mortgage to buy, and suddenly became one hand held and cost less than a weeks wages. Originally with LED and then soon after with LCD display .

Since the patents expired they are now even cheaper.

Hey Lisa I posted a reply not long before the forum went off for maintenance now it's gone?Can you find it again please. X

Ok il rewrite my reply as I can't sleep and it will save Lisa a job here goes Mapj thankyou for a very detailed reply I recognise the waveform in picture 1 the mains waveform here is very similar although I don't think the crests are as crushed tho not far off. In picture 3 I can clearly see that the 150 cycle waveform goes to the opposite polarity a few milliseconds before the supply voltage crosses over at that point you could almost say it forms a mini sine wave all its own loosely speaking. Now I'm guessing that these effects are caused by the inductance and capacitance of not only the diodes but other parts of the wiring too. I read somewhere that even house wiring can have appreciable inductance to relatively fast moving events the comment was made in relation to the causes of  audible clicks on PA systems from things like the heaters in tea urns or pie warmers which I know is a different thing altogether I just added it for completeness.

Hi Kelly, both insomniac huh ?

You are over thinking this rather by worrying about the L and C  - all you need to get the voltage and current waveforms in figure 1, is the  circuit of figure 1 - so there are diodes, a finite supply resistance, and a smoothing cap and a load. No inductors in the model at all, and none are necessary to get this waveform. Adding in the real L and C of the diodes is a small extra impedance, and only really has a noticeable effect on far higher frequency terms than 150Hz. if you need to see the scales a larger version of the pictures, clocking it should open a full screen version.


The other 2 circuits show how those sort of current and voltage waveforms can be approximately represented as a sine wave plus some 3F.

And the frequencies are there - at very high frequency RF it is not unknown to use a diode to generate a high frequency, by using it to distort an RFat sub-harmonic frequency that being lower is easier to generate. Tripler circuits are very common, as a doublers that look like a full wave rectifier


Actually if you need it you get a model with a  better fit to the 'real' rectified waveforms, you an do so  by adding further higher order harmonics to the mix.



As to your waveforms not being so bad as my simulations, I have perhaps used a bit of artistic licence, and chosen values that make the effect very clear indeed.


If you or anyone else has a copy of LT spice installed and wishes it,   I can upload the source files so you can play. At your own risk.


It is surprising how much L and C you need on the input to pull out the conduction angle to give a respectable power factor.  Which is why in rectifer type cases, L_C filtering is not the weapon of choice for PFC, rather a switching circuit that tracks up and down the sine-wave to a degree.

If you want a more accurate reading you need to buy a "true RMS" meter of high accuracy. This measures the exact voltage at many points of the waveform and then calculates the proper RMS value. There is still some inaccuracy in the voltage measurement inherent in the digitisation process, but it can be very small, perhaps 5 parts per million for a 6 1/2 digit meter. This all comes at a considerable price! All other kinds of meters are usually called average reading, RMS calibrated, which are sensitive to any waveform distortion from a pure sine wave, in other words harmonics. The true RMS meter can cope with a "crest value", typically 10 times when it calculates the resulting voltage, so large waveform distortions are accurately measured. Some Fluke multimeters are true RMS, which are labelled on the front.

Were the meters calibrated? With a meter such as this, intended to be built into a control panel, it would often be the person commissioning the panel who would calibrate the meter, ensuring an accurate reading is obtained at the nominal value it is being used at. 


Regards,


Alan.