### Author Topic: Measuring standby power of offline LED lamp  (Read 307 times)

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#### treez

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##### Measuring standby power of offline LED lamp
« on: March 27, 2018, 10:34:12 pm »
Hello,
The attached is the Mains input current to our product  (a 40W LED lamp) when in shutdown.
Those  spikes happen near the zero cross of the mains voltage.
Its the voltage across a 30R sense resistor in the neutral line.
It was supplied with 240VAC mains.
We  need to find the  average active power of this.
Do you know an instrument that can do this?

We have  Hameg 8115-2  power meter but its datasheet does not allure as to whether it can accurately measure such currents  with such high crest factor, low level, and low power factor.
Unfortunately we don’t have a sampling storage scope  to be able to measure power via the means of summing up a period’s worth of sampled v.i. products and then dividing by the number of readings taken.

Hameg 8115-2 Datasheet:
http://www.farnell.com/datasheets/493586.pdf
« Last Edit: March 28, 2018, 12:19:19 am by treez »

#### jolshefsky

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##### Re: Measuring standby power of offline LED lamp
« Reply #1 on: March 27, 2018, 11:17:04 pm »
I'm pretty sure you can "be" the RMS meter. First ensure your sampling frequency is high enough to grab all the spikes accurately (Nyquist's rules). Then, assuming the waveform is reliably periodic (one cycle is basically identical to any other), get sample the voltages at each point. Put it in a spreadsheet and square the voltage at each point, take the mean (the mean-square), then the square root (root mean-square).

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#### daveshah

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##### Re: Measuring standby power of offline LED lamp
« Reply #2 on: March 28, 2018, 01:24:48 am »
If you want power (as opposed to apparent power), the RMS is not the correct approach.  In the spreadsheet you should have both voltage and current columns, multiply at each point to obtain instantaneous power, then average that to obtain average active power.

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#### Ice-Tea

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##### Re: Measuring standby power of offline LED lamp
« Reply #3 on: March 28, 2018, 02:14:38 am »
The answer seems to be: a more expensive power meter?

Yokogawa has some nice gear:

https://tmi.yokogawa.com/solutions/products/digital-power-analyzers/digital-power-analyzers/
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eBay shop with all the gear you need!
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#### Fungus

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##### Re: Measuring standby power of offline LED lamp
« Reply #4 on: March 28, 2018, 03:30:46 am »
Its the voltage across a 30R sense resistor in the neutral line.

Add a capacitor across the sense resistor. This will filter the spikes into a much smoother, easier to measure signal.

(or add an inductor in series with the resistor - same thing)

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