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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.
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-8.2Off set-4.1v-so as we have 0 volts at the bottom of the wave(62% off time)
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.
Mark,This is what I did.
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.
I don't think OUR is blocked to read, is it? Let me know if it is.
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