# Questions tagged [rms]

Root Mean Square. Mathematical method to calculate effective value from an AC signal.

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75 views

### How do I calculate the root mean square voltage of a square wave (with drawing)?

I need to find the root mean square voltage based on this drawing, knowing that the period (T) is equal to 6 seconds, and the voltage (V) is equal to 14 Volt. I know that the rms can be calculated ...
70 views

### RMS calculation in cheap microcontroller

I normally see that RMS calculation is done using some kind of DSP or more high end microcontroller. I'm using this microcontroller and I would like to know if it could do RMS. I need to read 60 Hz ...
38 views

### Understanding voltage sampling rate of induction motors

Why it is required to have a sampling rate of at least 4kHz when sampling the voltage waveform of a three phase induction motor? I find most research papers use 4kHz but they don't state why. I want ...
274 views

### Strange measurements for Vrms in LTspice

For my experimental methods class (basically an intro to electronics) in university we had to simulate a circuit, which was nothing out of the ordinary, it just produces different sine waves of 2 ...
74 views

### Why is the output DC voltage of full wave rectifier not equal to the RMS of the original AC input?

https://www.electronics-tutorials.ws/accircuits/average-voltage.html https://www.electronics-tutorials.ws/accircuits/rms-voltage.html I understand that the RMS of an AC voltage is the magnitude of ...
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### Formula seems to be wrong into a PFC application note

Here is the formula that I found into the following application note: https://www.infineon.com/dgdl/an-1166.pdf?fileId=5546d462533600a40153559aabdf1128 The formula seems to be fault? Am I wrong? ...
79 views

### Why isn't RMS called SMR, when it's (linguistically) a square mean root? [closed]

This question falls into the funfact category, but I'm anticipating this question from my next course students and I'd like to have the answer, if there is any. In the spirit of English language, RMS ...
55 views

### Calculating RMS of Simple Waveform

A problem in my textbook gives this task: A solution is given as: I tried to work this problem using a different method. I simply calculated the areas of the triangles as if the height of the ...
152 views

### How do you differentiate between rms and peak voltage?

In the circuit above I've been asked to calculate the average power absorbed by the 10 Ω resistor. I managed to calculate a value for $V_0=40\sqrt2\,\angle{-25}$. But I'm not sure if this is peak ...
56 views

### Why are inductive loads an issue for the power grid?

I understand from Power = V_rms * I_rms * cos(phi) Being phi the phase difference between current and voltage. The power consumed by the a circuit is reduced in the case of having inductive or ...
45 views

### AC-AC Converter 3 phase, Line to line voltage calculation

I’m working one some assignment like this circuit (AC-AC converter 3 phase, 89 degree firing period). I've done this on PSIM software with 110 Volts 3 phase voltage source. I've been told to analyze ...
72 views

### What is the difference between RMS and DC?

The "RMS" value of an alternating voltage is equivalent to a DC voltage that would generate the same amount of heat in a resistor as the AC voltage would. And RMS = 0.707 * Peak value [N.B. Sine ...
246 views

### How to calculate RMS current from the rated current? Are they the same in a stepper motor?

I want to use the TMC2130 driver with the stepper motor I show below. This driver has these main features amongst others: Integrated power MOSFETs handle motor currents up to 1.2A RMS (QFN package) / ...
115 views

### Capacitor Vrms vs Vpeak Rating in Transformerless Supply

Why is it that when calculating for voltage across the capacitor in transformerless power supply we need to use peak voltage not RMS voltage value. To be clear I am referring to the rating of C1 in ...
117 views

### What's the use of the average voltage in rectifiers?

I've noticed that there is much talking about $V_{dc}$ in rectifiers (specifically, the half-wave rectifier) and less focus is given to $V_{rms}$. Why is that? I thought the RMS value was the much ...
100 views

### What is the RMS value of this waveform

I would like to know what is the RMS value of this waveform. I would like to see the mathematics. I ve done the mathematics but it seems that the result has a lot of terms ... If some approximations ...
50 views

### Load RMS voltage of a bridge rectifier

I was given a homework assignment shown below and I have a few quick questions that I appear to be stuck on. Any help would be appreciated. I can get Part i) and the answer is 168.64 degrees. Part ...
66 views

### What is the required VA rating of the transformer in this LTSpice power supply?

I have this circuit that I'm going to build, including the transformer. It is a simple power supply, with RC filters and one choke, I know all the voltages and currents. Because I will design, wind ...
276 views

### How to calculate RMS power of an offsetted sine wave

I know when it is centered at 0, Vpeak/sqrt(2) gives the RMS. When I do FFT on LTspice, it gives a peak at -17dB which is equal to 0.141254 On the FFT page, program says that it shows RMS values. ...
41 views

### Factor of 1/2 in RMS Voltage

Given the following circuit, I would like to calculate the Thevenin equivalent of the voltage in RMS. I know that R = 400 [Ω]; e(t) = 40 cos(10^4 t) [V]. However, I can't understand why there is a ...
34 views

### change in ac reading relative to the frequency

I'm working on a school project, just measuring the Vrms of two different waveforms at different frequencies. The function generators output is connected directly into an oscilloscope therefore the ...
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### How to calculate rms current from impedance and voltage?

If a circuit has these values Vac = 20Sin 130πt, L = 32.6 mH. Do I have to get Xl first and V/Xl gives the rms current value?
119 views

### Why is RMS used for P=VI (where there are no squares)?

I understand why RMS values are used for $$P = {\bar{v^2} \over R}$$ because $\bar{v^2}$ would be the "MS" so to speak, and then by square rooting we get the "RMS" ($\sqrt{\bar{v^2}}$), which ...
238 views

### RMS value of fundamental harmonic vs RMS of third harmonic

If we have a periodic signal (square wave) with amplitude of 100V. Then what would the RMS value of the fundamental harmonic be compared to the RMS value of the third harmonic? After some suggestion ...
143 views

### Why are different rms noise sources added in this way?

I'm wondering why noise sources are added such a way their rms values are added in square root of sum of squares fashion. For instance if the rms noise values from three sources are X,Y and Z, then ...
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### Question about calculating the rms noise from sampled DC measurement

If I want to estimate the rms noise of a DC signal, like a DC offset reading of an analog transducer sampled at a certain frequency. So imagine I used the anti-aliasing filter correctly and and fed ...
152 views

### Is A4988 and DRV8825 Current Limit Setting RMS or Peak?

I'm trying to aggregate information on config settings and current limit calculations for all the popular "StepStick" style motor drivers. However, I've noticed that while the older type (A4988 & ...
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### bidirectional (biphasic) RMS value

I have searched online and can find RMS calculations for many waveform shapes. What I cannot find however, is a calculation for a typical neuro stimulation wave as shown in the attached image. What ...
70 views

### How to find the RMS value of a sine wave with DC offset and retain the negative sign

I'm trying to build a Quadrature setup where an input signal is mixed with a reference signal that is in-phase and another one out-of-phase by 90deg. The idea is to extract the real and imaginary ...
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### Lead-acid battery charging fixed current stage specification

This post doesn't question the charging process or its theory. I seek to understand how to read the battery manufacturer's requirement of fixed current stage. They prescribe a current to be held at ...
199 views

### Clamp multimeter RMS current measurement, AC versus DC confusion

This question doesn't relate to a specific meter, I am using AMPROBE AMP-220 manufactured by Beha-Amprobe. I believe am an experienced engineer but only in the digital world, not really in power and ...
116 views

### RMS value of a line to neutral voltage on a three phase inverter

Basically, I feel like this is an incredibly simple thing, but my brain is stuck on it for hours now, and I can't find the answer anywhere. How do I find the RMS value of the signal shown, assuming ...
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### Low DC power consumption: RMS or mean?

I've been measuring low DC-powered devices consumption, by measuring the current at the needed frequency (able to detect the fastest consumption changes). I can then have the mean, max, min. To get ...
41 views

### Oscilloscope V_rms vs V_avg for effective voltage across an LED?

I'm using a PWM signal to vary the effective voltage seen by the high side of an LED. When probing this signal on an oscilloscope, I can read the RMS voltage or the average voltage, which vary ...
205 views

### RMS power and phase angle control

We have a piece of industrial equipment using a phase angle controller (PAC), and are having some issues calculating the RMS power used by the load. Personally I am of the opinion that you measure ...
57 views

### Tripping a circuit breaker when using a half wave rectifier

How does a half wave rectifier or phase control dimmer affect the triggering of a circuit breaker? For example, if a 20A resistive load was on a 15A breaker with a half wave rectifier. Now average RMS ...
65 views

### Confused on when to use RMS current or voltage in calculations (regarding rectifiers)

For question a, why is the power absorbed by the DC source : P=Vo*Io However, for the same exact question, the power absorbed in the resistor is P = (Irms)^2 * R Hence when do we use RMS current/...
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### Is the input voltage on power adapters RMS voltage or peak AC voltage?

For example, would a laptop power adaptor with a voltage input of 100-240V ~ be safe to use in a country with 230V RMS wall voltage? Thanks in advance!
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### What is the typical RMS noise of a lithium battery?

Take a typical CR2032 battery, if it is not discharging, what is the typical variation in voltage over a short period of time (eg a few seconds or minutes)?
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### Random Noise Voltage

I have a signal I am interested in obtaining power from. The best way to describe the signal is that it is random. In the graph, the voltage fluctuates from negative to positive with an RMS of 0.5 ...
344 views

### PCM5102A: analog output level not what I expected?

I cannot achieve a 2.1 Vrms mentioned in their datasheet (pages 4 and 9). I can only output 1.04Vrms from the 1kHz source. Do I measure it wrong or this is what I should expect? Or maybe I get such ...
524 views

### Peak Current vs RMS Current when calculating Reactance

I am having a hard time getting a grip on AC circuit analysis. I have been taking my peak voltage/current and converting it to RMS current and RMS voltage so I can use ohms law and things like that to ...
664 views

### Calculate Angle for Triac Phase Control

Triac-based phase control of the mains power into resistive load. According to this post, https://www.physicsforums.com/threads/how-to-calculate-rms-voltage-from-triac-phase-angle.572668/#post-...
191 views

### How to Find R.M.S value [closed]

how can I find the R.M.S value of this question?
3k views

### BLDC Motor Constants: Why is my Kt and Ke different?

I'm new to working with motors. Reading up on BLDC theory, it is repeatedly said that in SI units, the Ke and Kt will be numerically equal. I downloaded the motor spec excel sheet of the T1101 motor ...
1k views

### Multimeter true RMS [closed]

I'm just getting started with electronics (arduinos and friends) and I would like to get a multimeter. As far as I can tell arduinos and any other logic chips and circuits I'm gonna be using are using ...
441 views

### AC Circuits: Determine the RMS current in the Inductor in the circuit diagram attached below. (Voltage is 40V, RMS)

Here is the image of the circuit diagram. I wanted to know the method to evaluate the RMS current in the inductor branch. I would prefer to use complex analysis!
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### LTC1966 Sallen-Key post-filtering application question

Just a quick question about an application circuit in the LTC1966 datasheet. Figure 13 on page 18 shows a third order (when considering output resistance of the LTC1966) Sallen-Key filter, with a ...