Using a resistor to messure power consumption of a circuit.

[tinman]

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.

[MarkE]

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

[MarkE]

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.

[tinman]

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

[MarkE]

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.