| Electronics > Beginners |
| How to see effect of True RMS |
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| David Hess:
A square wave source is what I use to calibrate my average and RMS responding meters. The RMS meter will indicate the peak voltage of the square wave but a sine calibrated average responding meter will read 11% high. Going backwards, this is one way to calibrate a square wave source to be used to calibrate an oscilloscope. |
| spec:
mwb1100 Another way to measure true RMS of any voltage waveform is to use a chip, like the Linear Tech LTC1967. They are quite pricey though at around £8UK. https://www.analog.com/media/en/technical-documentation/data-sheets/1967f.pdf The older Analog Devices AD567 RMS to DC converter is around £15, but I have some in metal cans, not surface mount. If you want one PM to get my address and send a SAE, and I will send you one. It Needs +-15V supply lines though. https://www.mouser.co.uk/datasheet/2/609/AD536A-1501510.pdf UPDATE- just noticed that you are in the US, so not sure about the SAE but PM me anyway if you want. |
| spec:
--- Quote from: ArthurDent on November 23, 2018, 02:38:52 am ---The description by spec in post #2 is correct as to how it used to be done. Below is a diagram of how a low mass thermocouple is attached to a heater with a small glass bead inside a glass container with a vacuum inside and the heater is switched between D.C. and A.C.. The heating effect of the D.C. voltage is the same as the heating effect of the R.M.S. value of the A.C. voltage. I have an old thermal transfer voltmeter model 6A made by Holt and that had added circuitry, mainly voltage dividers, to allow calibration of D.C./A.C. voltages up to 1000 volts. The low voltage from the thermocouple was fed to an analog meter with adjustable sensitivity and the closer you got to a match, the higher the gain of the meter could be adjusted while still maintaining balance. The accuracy of the divider resistors and the meter accuracy wasn’t too important because as long as the circuit was stable enough to check comparison readings as you switched back and forth between the known accuracy D.C. and A.C. to be calibrated, that was all that was needed. Agreement of D.C. to A.C. better than .01% could be achieved. --- End quote --- Those were the days ;D |
| Zero999:
I'm being picky here, but an averaging meter will just display zero, when given an AC signal with no DC component and is what a meter does when set to read DC. There are different types of non-true RMS meters: An averaging non-true RMS AC meter will high pass filter the signal first and rectify it, before taking the average value, which will be less than the RMS value of a pure sine wave, so it will multiply it by a correction factor. A peak detector will high pass filter and rectify the signal, before taking the peak value, which will be more than the value of a pure sine wave, so it will be divided by √2. In other words, in a true sine wave meter, the measured waveform is assumed to be sinusoidal and peak or average value is scaled to reflect this. |
| spec:
--- Quote from: Hero999 on November 23, 2018, 09:03:50 am ---I'm being picky here, but an averaging meter will just display zero, when given an AC signal with no DC component and is what a meter does when set to read DC. There are different types of non-true RMS meters: An averaging non-true RMS AC meter will high pass filter the signal first and rectify it, before taking the average value, which will be less than the RMS value of a pure sine wave, so it will multiply it by a correction factor. A peak detector will high pass filter and rectify the signal, before taking the peak value, which will be more than the value of a pure sine wave, so it will be divided by √2. In other words, in a true sine wave meter, the measured waveform is assumed to be sinusoidal and peak or average value is scaled to reflect this. --- End quote --- My understanding is that a true RMS meter, or chip, will give the RMS value of any signal, including DC. So, for example, if you applied 1VRMS or 1VDC, the reading would be 1V RMS. If you applied a 1V p/p square wave (equal mark and space) the reading would be 500mV and so on. RMS gives the total area (integral ) under the waveform curve, both positive and negative (intregal). It only has magnitude, not direction. In fact it is often said that RMS is the heating effect, as implied in replies #2 and #9 above. |
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