GDPR and DSGVO law

Storing Cookies (See : http://ec.europa.eu/ipg/basics/legal/cookies/index_en.htm ) help us to bring you our services at overunity.com . If you use this website and our services you declare yourself okay with using cookies .More Infos here:
https://overunity.com/5553/privacy-policy/
If you do not agree with storing cookies, please LEAVE this website now. From the 25th of May 2018, every existing user has to accept the GDPR agreement at first login. If a user is unwilling to accept the GDPR, he should email us and request to erase his account. Many thanks for your understanding.
Amazon Warehouse Deals ! Now even more Deep Discounts ! Check out these great prices on slightly used or just opened once only items.I always buy my gadgets via these great Warehouse deals ! Highly recommended ! Many thanks for supporting OverUnity.com this way.

User Menu

Powerbox

Smartbox

3D Solar

3D Solar Panels

DC2DC converter

Micro JouleThief

FireMatch

FireMatch

CCKnife

CCKnife

CCTool

CCTool

Magpi Magazine

Magpi Magazine Free Rasberry Pi Magazine

Battery Recondition

Battery Recondition

Arduino

Ultracaps

YT Subscribe

Gravity Machines

Tesla-Ebook

Magnet Secrets

Lindemann Video

Navigation

Products

Products

WaterMotor kit

Statistics

  • *Total Members: 82673
  • *Latest: edu

  • *Total Posts: 499703
  • *Total Topics: 14806
  • *Online Today: 44
  • *Most Online: 103
(December 19, 2006, 11:27:19 PM)
  • *Users: 8
  • *Guests: 22
  • *Total: 30

Author Topic: Using a resistor to messure power consumption of a circuit.  (Read 11097 times)

Offline tinman

  • Hero Member
  • *****
  • Posts: 5019
Using a resistor to messure power consumption of a circuit.
« on: September 12, 2014, 08:26:22 PM »
I was testing some very simple circuit's today,and found one that has my up most attention. As the frequency is around 127KHz,i don't trust my DMMs to give me an accurate amp average reading-although all 3 show the same current draw. I have drawn a basic circuit below,which we will use to answer my question.

Using the 3 ohm resistor in the circuit,can i use my scope across that 3 ohm resistor to work out the circuit's power draw,or will the rms voltage across that resistor only tell me what the resistor is dissipating?. (I tried a 1 ohm resistor in it's place,but i get a very noisy signal on my scope-the 3 ohm is much smoother on the scope.)

SG setting's.
Square wave at tuned frequency (around 127KHz)
Duty cycle-38%
VPP-8.2
Off set-4.1v-so as we have 0 volts at the bottom of the wave(62% off time)

Free Energy | searching for free energy and discussing free energy


Offline gyulasun

  • without_ads
  • Hero Member
  • *****
  • Posts: 3773
Re: Using a resistor to messure power consumption of a circuit.
« Reply #1 on: September 13, 2014, 12:42:28 AM »
Hi Brad,

The rms voltage across the resistor can indeed tell you the dissipation across the 3 Ohm:  P=V*V/R.  But you can learn about the current that flows in the circuit if you use Ohm's law for the rms voltage and the resistor values,  I=V/R

Here I assume the rms voltage value across the 3 Ohm is correct i.e. the scope considers duty cycle.

Finally the circuit power consumption could be calculated by taking the rms value of the 8.2Vpp square wave, 38% duty cycle and divide it by the rms current. 

I hope I am correct... :)

Gyula

Offline mscoffman

  • Hero Member
  • *****
  • Posts: 1377
Re: Using a resistor to messure power consumption of a circuit.
« Reply #2 on: September 13, 2014, 05:02:21 AM »

tinman,

You should do what our lab instructor in school said to do: always draw the signal generator as a circle with the AC signal
for the picture *then* draw the generator's internal impedance as a resistor in series with it. So really your diagram should
have two resistors. Those coaxial cables that connect to
the circuit from the generator are supposed to match the impedance of the generator so as to not reflect any power back.
The manufacturer of the generator does quite a good job making the generator emit a near perfect signal, but he can't
create the impossible - a zero ohm impedance output. The generators output impedance is usually marked near that coaxial spigot.
Probably 50ohms. That step will help you analyzing what is happening. The reason is what you want your current shunt resistor
value to be is a trade off. The other thing is to not forget the AC power information; A two channel scope will let you determine
the phase shift between the voltage signal across the circuit and the current signal through it.  You can check whether your circuit
has a PF power factor that is inductive, capacitive, or resistive at the given frequency. Any inductor will have a self capacitance and
some frequency will cause parallel resonance and will allow you to tune to parallel frequency resonance of the inductor + self capacitor.

By the way a sinewave output RF frequency generator is easiest to understand, a pulse generator is somewhat different because
digital circuits require +/- unbalanced signals of fixed voltages creating asymmetric wave signals.

Also gyulason while you power numbers will be close to accurate, you really need to integrate (summate) the powers of the individual
Fast Forrier decomposition (sinewaves) of the voltage waveform with the Fast Forrier decomposition (sinewave) of the current waveform
then use that to calculate the power of each decomposition then add them back together to get total power. One of TB's capacitor
overunity proofs did this wrong and showed overunity in a capacitor where there really was none.

This is why it is better to measure the power after rectification then to try to calculate it.


:S: MarkSCoffman

Free Energy | searching for free energy and discussing free energy

Re: Using a resistor to messure power consumption of a circuit.
« Reply #2 on: September 13, 2014, 05:02:21 AM »
Sponsored links:




Offline tinman

  • Hero Member
  • *****
  • Posts: 5019
Re: Using a resistor to messure power consumption of a circuit.
« Reply #3 on: September 13, 2014, 05:47:51 AM »
Looking at the scope shot below(across the SG),which voltage do i use to calculate my P/in-as i already have the I/in.
My DMM is showing a very accurate I/in,as i checked it against my scope using a known value resistor across my SG at the same frequency.

In regards to the scope shot,it dosnt seem right that we should be using the RMS voltage to calculate power. As the duty cycle is 38%,and the vpp is 8.2,how is it that the RMS is 4.72?-which is more than half the 8.2 vpp,and yet only a 38% duty cycle.
The mean voltage is the same as the average voltage,and my DMM also shows 2.8 volts across the SG-same as means and average voltage on the scope.

My I/in is 2.75mA-can you work out the P/in from the scope shot and current(2.75mA)

Offline TinselKoala

  • Hero Member
  • *****
  • Posts: 13763
Re: Using a resistor to messure power consumption of a circuit.
« Reply #4 on: September 13, 2014, 06:40:11 AM »
You have a situation where your input voltage is pulsed. So what you need to do to find the _average_ input power over many cycles, is to compute the VxI value during the pulse, then multiply that by the 0.38 duty cycle. Your voltage varies a little during the pulse, going from 8.2 down to perhaps 7.5 volts. So really you want an "average" here, take 7.8 volts for example. Your scope is computing the "average" and "mean" (here the same thing for this waveform) by doing something like this, taking the "average peak" and multiplying by the duty cycle. So you take the average voltage during the pulse, multiply by the average current during the pulse, and then multiply the result by the duty cycle, and this will be the average power. This won't be as strictly accurate as performing the instantaneous multiplication of the V and I traces and then integrating that resulting trace over time but it will be close.
I think.



Free Energy | searching for free energy and discussing free energy

Re: Using a resistor to messure power consumption of a circuit.
« Reply #4 on: September 13, 2014, 06:40:11 AM »
Sponsored links:




Offline tinman

  • Hero Member
  • *****
  • Posts: 5019
Re: Using a resistor to messure power consumption of a circuit.
« Reply #5 on: September 13, 2014, 08:06:35 AM »
You have a situation where your input voltage is pulsed. So what you need to do to find the _average_ input power over many cycles, is to compute the VxI value during the pulse, then multiply that by the 0.38 duty cycle. Your voltage varies a little during the pulse, going from 8.2 down to perhaps 7.5 volts. So really you want an "average" here, take 7.8 volts for example. Your scope is computing the "average" and "mean" (here the same thing for this waveform) by doing something like this, taking the "average peak" and multiplying by the duty cycle. So you take the average voltage during the pulse, multiply by the average current during the pulse, and then multiply the result by the duty cycle, and this will be the average power. This won't be as strictly accurate as performing the instantaneous multiplication of the V and I traces and then integrating that resulting trace over time but it will be close.
I think.
Thanks TK.
This is what i asumed,and lines up exactly with my DMM reading's.

Offline MarkE

  • Hero Member
  • *****
  • Posts: 6830
Re: Using a resistor to messure power consumption of a circuit.
« Reply #6 on: September 13, 2014, 10:21:55 AM »
I was testing some very simple circuit's today,and found one that has my up most attention. As the frequency is around 127KHz,i don't trust my DMMs to give me an accurate amp average reading-although all 3 show the same current draw. I have drawn a basic circuit below,which we will use to answer my question.

Using the 3 ohm resistor in the circuit,can i use my scope across that 3 ohm resistor to work out the circuit's power draw,or will the rms voltage across that resistor only tell me what the resistor is dissipating?. (I tried a 1 ohm resistor in it's place,but i get a very noisy signal on my scope-the 3 ohm is much smoother on the scope.)

SG setting's.
Square wave at tuned frequency (around 127KHz)
Duty cycle-38%
VPP-8.2
Off set-4.1v-so as we have 0 volts at the bottom of the wave(62% off time)
At 127kHz, you will need to take a little care, not too much to avoid measuring the inductance of your wiring instead of the real current.  My first suggestion is that you purchase some low inductance resistors.  If you want to be really accurate, buy four terminal Kelvin resistors.  If you can afford a small amount of error then Ohmite WNE series Aryton-Perry resistors cost a dollar or two.  The thing to remember about current sense resistors is that the frequency at which the inductance adds 41% extra amplitude is:  R/(2pi*L).  So, the higher value current sense that you can tolerate the less frequency dependent gain the inductance introduces.  Here is a link to Digikey's in stock parts.

http://www.digikey.com/product-search/en?pv7=2&FV=fff40001%2Cfff80482&k=wne&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=500

Free Energy | searching for free energy and discussing free energy

Re: Using a resistor to messure power consumption of a circuit.
« Reply #6 on: September 13, 2014, 10:21:55 AM »
Sponsored links:




Offline MarkE

  • Hero Member
  • *****
  • Posts: 6830
Re: Using a resistor to messure power consumption of a circuit.
« Reply #7 on: September 13, 2014, 10:29:12 AM »
Looking at the scope shot below(across the SG),which voltage do i use to calculate my P/in-as i already have the I/in.
My DMM is showing a very accurate I/in,as i checked it against my scope using a known value resistor across my SG at the same frequency.

In regards to the scope shot,it dosnt seem right that we should be using the RMS voltage to calculate power. As the duty cycle is 38%,and the vpp is 8.2,how is it that the RMS is 4.72?-which is more than half the 8.2 vpp,and yet only a 38% duty cycle.
The mean voltage is the same as the average voltage,and my DMM also shows 2.8 volts across the SG-same as means and average voltage on the scope.

My I/in is 2.75mA-can you work out the P/in from the scope shot and current(2.75mA)
Anyway:  rms voltage or current is the equivalent DC voltage or current that would cause the same amount of heat in a pure resistor.  If your circuit is not a pure resistor, then in order to obtain power, you measure both voltage and current and multiply them.  Then you can process that to find peak, minimum, and/or average power over some time interval.

The measurement that often goes wrong is the current.  If you get some of the resistors I recommended and you follow good hook-up practices with your scope, then you will be OK.  Poynt99 published some nice YT videos on measuring current, voltage and power in pulsating circuits.

Offline tinman

  • Hero Member
  • *****
  • Posts: 5019
Re: Using a resistor to messure power consumption of a circuit.
« Reply #8 on: September 13, 2014, 01:33:38 PM »
Anyway:  rms voltage or current is the equivalent DC voltage or current that would cause the same amount of heat in a pure resistor.  If your circuit is not a pure resistor, then in order to obtain power, you measure both voltage and current and multiply them.  Then you can process that to find peak, minimum, and/or average power over some time interval.

The measurement that often goes wrong is the current.  If you get some of the resistors I recommended and you follow good hook-up practices with your scope, then you will be OK.  Poynt99 published some nice YT videos on measuring current, voltage and power in pulsating circuits.
Hi Mark

Below is the scope shot across the 3.2 ohm resistor,and second shot is across the SG/circuit. Are you able to have a go at calculating the P/in from these two shot's.No matter what resistor i use,there will always be the wave form you see below,due to the way the inductor and LED's are hooked up.3.2 ohms is the exact value of the 1 watt resistor im using.

@TK-please have a guess aswell,so as i can see how close you and Mark are to what my DMM amp meter read's.
Thanks

Free Energy | searching for free energy and discussing free energy

Re: Using a resistor to messure power consumption of a circuit.
« Reply #8 on: September 13, 2014, 01:33:38 PM »
3D Solar Panels

Offline MarkE

  • Hero Member
  • *****
  • Posts: 6830
Re: Using a resistor to messure power consumption of a circuit.
« Reply #9 on: September 15, 2014, 07:47:11 AM »
It would be a lot easier if you could post CSV files.  Otherwise, I will have to extract the values which is possible, but somewhat time consuming.

Offline poynt99

  • TPU-Elite
  • Hero Member
  • *******
  • Posts: 3556
Re: Using a resistor to messure power consumption of a circuit.
« Reply #10 on: September 16, 2014, 04:22:45 AM »

Free Energy | searching for free energy and discussing free energy

Re: Using a resistor to messure power consumption of a circuit.
« Reply #10 on: September 16, 2014, 04:22:45 AM »
3D Solar Panels

Offline MarkE

  • Hero Member
  • *****
  • Posts: 6830

Offline poynt99

  • TPU-Elite
  • Hero Member
  • *******
  • Posts: 3556
Re: Using a resistor to messure power consumption of a circuit.
« Reply #12 on: September 16, 2014, 04:39:44 AM »
I don't think OUR is blocked to read, is it? Let me know if it is.

Quote
Here's what we can do manually.

Eyeballing the current wave form, an equivalent pulse wave would be about 4mV (1.25mA) peak. Now we can simply multiply it out:

1.25mAp x 6Vp x 0.38 = 2.85mW. Add our 78uW for the 50 Ohm, and we have 2.93mW for Pin.

WARNING!

Keep in mind that scopes exhibit terrible accuracy (and resolution or precision) down in these low millivolt levels. The offsets in the inputs is often 5mV (or more), so there could be 100% or more error here.

Offline MarkE

  • Hero Member
  • *****
  • Posts: 6830
Re: Using a resistor to messure power consumption of a circuit.
« Reply #13 on: September 16, 2014, 05:06:54 AM »
I don't think OUR is blocked to read, is it? Let me know if it is.

Quote
Warning!
The topic or board you are looking for appears to be either missing or off limits to you.
Please login below or register an account with OverUnity Research.

Offline gyulasun

  • without_ads
  • Hero Member
  • *****
  • Posts: 3773
Re: Using a resistor to messure power consumption of a circuit.
« Reply #14 on: September 16, 2014, 11:59:23 AM »
Mark,

This is what I did.

Hi poynt99,

I also have problem to read your post at your above link.  After login I receive this message :

" An error has occured!  The topic or board you are looking for appears to be either missing or off limits to you."

Why is that I wonder?

Gyula

 

OneLink