Questions tagged [rms]
Root Mean Square. Mathematical method to calculate effective value from an AC signal.
140
questions
-1
votes
3answers
93 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 ...
-1
votes
0answers
39 views
0
votes
1answer
48 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 ...
0
votes
2answers
49 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 ...
0
votes
4answers
107 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. ...
0
votes
1answer
38 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 ...
1
vote
1answer
31 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 ...
-1
votes
2answers
67 views
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?
0
votes
2answers
117 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 ...
1
vote
1answer
201 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 ...
3
votes
3answers
105 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 ...
0
votes
1answer
43 views
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 ...
1
vote
1answer
86 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 & ...
-1
votes
1answer
60 views
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 ...
0
votes
0answers
59 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 ...
2
votes
2answers
63 views
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 ...
2
votes
3answers
153 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 ...
0
votes
1answer
67 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 ...
2
votes
2answers
41 views
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 ...
1
vote
1answer
36 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 ...
1
vote
3answers
121 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 ...
0
votes
1answer
52 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 ...
0
votes
2answers
60 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/...
1
vote
2answers
42 views
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!
5
votes
1answer
316 views
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)?
0
votes
1answer
86 views
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 ...
1
vote
1answer
246 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 ...
0
votes
1answer
316 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 ...
0
votes
3answers
515 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-...
-2
votes
2answers
161 views
4
votes
2answers
2k 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 ...
6
votes
2answers
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 ...
-2
votes
2answers
328 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!
0
votes
1answer
111 views
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 ...
0
votes
1answer
354 views
What matters, the rms or peak value of the Analog signal to choose appropriate ADC for digitization?
i have an 23 bit ADC with voltage reference of 1.4 V.
If my input Analog signal is AC with frequency 50 HZ and its rms ranges from 0.2 Vrms to 1.3 Vrms. Then its peak voltages will vary from 0.28 V to ...
0
votes
1answer
359 views
How can I reduce the RMS value for a bridge rectifier?
I need to reduce the rectified DC voltage. Is it possible to reduce the RMS value after the bridge rectifier i.e before the capacitor? Practical value (at the capacitor) is 34 DC voltage. I need to ...
0
votes
1answer
233 views
Why use RMS instead of the average voltage of half the cycle for power
In a AC sine wave, the RMS voltage value is \$\frac{V_{p}}{\sqrt{2}}\$ where \$V_{p}\$ is the peak voltage whereas the average voltage value over half the cycle is \$V_{p}\cdot \frac{2}{\pi}\$
If we ...
0
votes
1answer
106 views
What current should be used to calculated the drop across the diode? RMS current or average current?
I was designing a power supply, following is the output bridge diode current waveform from LTSpice simulation .
My intention is to calculate the power drop across the diode. What value of current (...
0
votes
3answers
577 views
Convert Power to a Peak Voltage, when to multiply by sqrt(2) - Not so simple?
I have a dBW value and dBW I understand to normally be given as representing a RMS Power (however this is not specificically ever stated and I am told its just assumed?)
from dBW to Watts = \$10^{\...
-2
votes
2answers
577 views
Measure RMS voltage from AC Grid
Circuit design
I need design a circuit for read rms AC grid, its function is obtain the RMS value and generate a PWM signal, this PWM signal feeds my AC/DC LED driver (PWM/1-10V input pin), the ...
0
votes
0answers
108 views
Can I measure the voltage noise with True RMS function
I have a measurement setup with which I want to measure high voltages. Now I want to evaluate the noise this setup produces (preferably in V/sqrt(Hz)). I short circuited the input and I am now ...
0
votes
2answers
54 views
Finding phase angle of AC
After solving for a system of complex equations, I got the following currents:
$$I1= -.80669-.35675j$$
$$I2= -1.8546+1.30730j$$
Firstly, I found the current magnitude:
$$|I_1|=.88205A$$
$$|I_2|=2....
3
votes
3answers
120 views
Finding the average and RMS value of this signal
How can I find the RMS and the average voltage of the following diagram?:
My work:
The RMS:
$$\sqrt{\left(\sqrt{\frac{1}{3\times10^{-3}}\int_0^{3\times10^{-3}}\left(\frac{5000}{3}\cdot t\right)^2\...
0
votes
2answers
45 views
Which one is correct for this power consumption estimation?
If a MOSFET gate driver is supplied by a 12V DC power supply and if it is drawing a current I(t) from the power supply, what can we say about the power consumption in terms of voltage and current?
I ...
0
votes
4answers
192 views
AC Effect Value
"The effective value of an alternating current is that current which will produce the same heating effect as an equivalent direct current. The effective value is called root mean square (r.m.s.) value
...
0
votes
1answer
110 views
Mistake in reading a rms voltage
I got a really strange result. I've build a full-bridge rectifier with a capacitor filter. And I used the following values \$\text{C}=500\cdot10^{-6}\$ and as a load \$\text{R}=1000\$. The maximum ...
0
votes
1answer
450 views
Do I have a true rms meter
I bought an Excel XL 830L multimeter with the understanding that it was a RMS voltmeter.
I also have UEI ElectroMate DM 200.
When I checked the output of my APC unit with both meters, the readings ...
0
votes
1answer
530 views
Is the Diode RMS Reverse Voltage spec important in a DC circuit?
Consider a 12Vdc automotive power supply design where +12V power connects first to a Vc=49.9V bidirectional TVS diode (e.g., TPSMB36CA) and subsequently connects to an SB370 Schottky diode. The SB370 ...
1
vote
1answer
127 views
Question about the RMS value of the real input power
I've the following circuit:
simulate this circuit – Schematic created using CircuitLab
I know that the relation between the input voltage and the input current is given by:
$$\begin{cases}
\...
1
vote
0answers
104 views
Calculating dissipation of a CCR powered via a full bridge rectifier
I'm putting together a string of UV/purple LEDs for photoetching purposes.
The idea is the following:
simulate this circuit – Schematic created using CircuitLab
The 80 LEDs are in series, ...
