It makes sense when power is being transduced into some other signal in a linear fashion, and you which to know the RMS of the resulting signal, not its average power. In these cases, RMS is interesting because for AC waveforms it is also the standard deviation, which is often of interest.My doubt is about applying the RMS specific calculation to power quantities, squaring instantaneous power values. I don't see a technical relevance for it, because power already represents squared magnitude (current, voltage, sound pressure, whatsoever) values.
I didn't find it complicated in battery powered radio applications yet. An oscilloscope with math capabilties is a versatile tool for the measurements. If you want to measure average current with a multimeter, it may be helpful to add filter capcitors to reduce the variance of samples.Yes it is complicated, the problem is not how to calculate the power consumed, the problem is which one is the correct one and correct way in calculating.
According to the problem description given by Mariwan, I would suggest the below simplified model. You may want to place an arbitrary current waveform instead of the switched load, or you can discuss a pure resistive load as special case.
But isn't it obvious that the integral expression reduces to Pavg = V * Iavg for constant V?
P.S.: The previous mentioned inequation should be changed into
|Pavg| ≦ Irms*Vrms
because Pavg is a signed quantity
Yes, I added the abs() operation to take account of the power sourcing case. It's not relevant for the present problem.I assume you mean that because it can be a vector quantity and √i = -1 so when Irms is squared it could be a signed vector quantity. But actually if a negative power would indicate a power source, rather than a power load.
P = Vpk * Ipk * %d.f.
P = Vavg * Iavg * %d.f. << Preferred method
P = Vrms * Irms * %d.f.
P ≠ Irms * Vpk & %d.f. { n.b. invalid. }
The first three methods should all give the same result for an estimate of RSM power.
Dear SunnySkyguy;
Thanks for clarifying a big part of the problem, thanks for your time you put to clarify this. But still I don't know what to do.
What I would do is, use a standard Integrate & Dump cheap DMM or better yet an HP DMM , not the TRUE RMS meter . Use a filtered DC source approached for both Voltage and Current and switch between selections with a 3 wire connection. Put the shunt on the ground side. Put a small 0.1uF and low ESR cap across both the current ports and voltage ports at the meter.
Then trust those readings as pure DC equivalent power readings.
To attempt to match those readings in the scope, select a shunt that drops less than 1% of desired measurement error. So if 3V and drops 30mV or 1%, make sure you use a NON WIRE-WOUND power resistor and make sure the area of the loop of your power loop is near zero. i.e. twist all wire pairs. This will help in reducing ingress. Then move the shunt to the ground side of the load and use a coaxial cable soldered to the shunt resistor and a coaxial cable soldered to the power connector. with RF cap across both if necessary. In this there is not too much RF for Zigbee, but it it were say > 10MHz this would be critical. and if > 300MHz use SMA or SMB connectors with controlled impedances throughout to avoid ingress and perhaps even additional filtering with ferrite & RF cap. Then measure both V & I as V1,V2 on scope relative to 0Vdc and multiply RMS readings for each.. THey should now agree with Average DMM meter mode. Mark the TRue RMS Meters with their spec'd Max Frequency so others do not repeat your bad experience with a quality label approved by Lab Mgr. That ought to do it and one last thing, dont worry. I've seen seasoned pro's at Aerospace and Disk drive companies make similar mistakes of not reading the manual. You won't forget this one. .
Dear SunnySkyguy;
Thanks for clarifying a big part of the problem, thanks for your time you put to clarify this. But still I don't know what to do.
What I would do is, use a standard Integrate & Dump cheap DMM or better yet an HP DMM , not the TRUE RMS meter . Use a filtered DC source approached for both Voltage and Current and switch between selections with a 3 wire connection. Put the shunt on the ground side. Put a small 0.1uF and low ESR cap across both the current ports and voltage ports at the meter.
Then trust those readings as pure DC equivalent power readings.
To attempt to match those readings in the scope, select a shunt that drops less than 1% of desired measurement error. So if 3V and drops 30mV or 1%, make sure you use a NON WIRE-WOUND power resistor and make sure the area of the loop of your power loop is near zero. i.e. twist all wire pairs. This will help in reducing ingress. Then move the shunt to the ground side of the load and use a coaxial cable soldered to the shunt resistor and a coaxial cable soldered to the power connector. with RF cap across both if necessary. In this there is not too much RF for Zigbee, but it it were say > 10MHz this would be critical. and if > 300MHz use SMA or SMB connectors with controlled impedances throughout to avoid ingress and perhaps even additional filtering with ferrite & RF cap. Then measure both V & I as V1,V2 on scope relative to 0Vdc and multiply RMS readings for each.. THey should now agree with Average DMM meter mode. Mark the TRue RMS Meters with their spec'd Max Frequency so others do not repeat your bad experience with a quality label approved by Lab Mgr. That ought to do it and one last thing, dont worry. I've seen seasoned pro's at Aerospace and Disk drive companies make similar mistakes of not reading the manual. You won't forget this one. .
Thanks again,
From the beginning I didn't depend on the multimeter so much. All my calculation are already based on the waveform captured over the resistor. Two thinks I think we mixed between them, the voltage over the resistor - shunt resistor is variable as it represent the current requirement by the XBee module, while the VCC is constant and is = 3.3 Volts.
I think, it will be impossible for me to arrange the requirement you put
I hope that we learned something by this question .. I do by myself and appreciate all comments
Just one question . Do I do a mistake when I calculate using the equations mentioned in my post before ? In my research I put both values as a reference as I am not sure which one would be the exact correct one?
Thanks again
But let me perfectly clear. RMS POWER is a statistically valid if Shannon, Nyquist requirements are used in good measurement techniques. If you wish to debate this. Anytime offline.
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