precise power meter
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 4th October 2004, 06:16 AM #1 pfloyd_is_god   diyAudio Member     Join Date: Sep 2004 Location: texas precise power meter I’m planning on building an accurate power meter for my guitar amp I’m building. I’m sure one can’t rely on p=v^2/r for a speaker, as the impedance is going to change with frequency. So what I’m planning on doing is taking a current signal and a voltage signal from the speaker, multiplying them, and integrating this for an rms output. However, as a speaker is an inductive load, the current and voltage signals will probably be out of phase, so I’ll integrate the voltage and current signals first, and then multiply them, is this mathematically accurate? Secondly, what should I be looking for in terms of ADC specs? How much resolution do you need to accurately measure power output? I’m sure you don’t need 16 bits, and the same for sampling rate, I have my doubts that the high end of the spectrum adds much to power. Attached is the basic analogue circuitry I’m aiming for. Will the current sensing resistor, R1 degrade the performance of the system much, other than reducing output power? One could also put it before the speaker, after the amp, and use another differential amplifier to take the voltage drop. any help is apreciated, cheers __________________ listen to pink floyd
 4th October 2004, 07:41 AM #2 rpapps   diyAudio Member   Join Date: Jul 2004 Location: nowhere of interest Power Meter Hi Why not sense the current inductively using a current transformer? Pass the speaker lead through a ferrite toroid a few times and wins as many turns of a small diameter wire as you can fit on the toroid as a secondary. Now secondary voltage is proportional to primary current. Pass a known current through primary and measure secondary voltage and you'll have your scaling factor. You might like to look at analog multipliers as well. Would save a lot of digital hardware. Just a thought. Rob
 4th October 2004, 11:10 AM #3 destroyer X   diyAudio Member     Join Date: Feb 2004 Location: Recife - Brasil Northeast Please, correct your text, Pink floid (floyd) is not good. That music is one excellence.... magnificent, astonishingly, incredible... a classic of our days. Not so simple as good...no , not, non, não, niet, nichst, nein! regards, Carlos __________________ Restriction removed ; ttps://www.youtube.com/watch?v=3ioyOjAzDt0; https://www.youtube.com/watch?v=uFcnddpe3MU
pfloyd_is_god
diyAudio Member

Join Date: Sep 2004
Location: texas
Forgot to attach the image....

Quote:
 Why not sense the current inductively using a current transformer?
From what I gather, current sensing transformers are designed for applications which require limited accuracy but high current capacity, whereas a current sensing resistor will be more precise, but will be less practical when dealing with high current, p=I^2*r. The voltage drop across a 10 mOhm resistor for my amp shouldnâ€™t go much over 80 mV, thatâ€™s about 700 mW of power dissipation, I can live with that.

I could go for an analogue solution, but whereâ€™s the fun in that?, I'm most likely going to be using an MCU for other reasons anyway.

Quote:
Are you seriously insulting the Floyd?
Attached Images
 power_meter.jpg (29.8 KB, 349 views)
__________________
listen to pink floyd

 5th October 2004, 04:17 AM #5 runebivrin   diyAudio Member     Join Date: Jul 2003 Location: Huddinge, Sweden I think Carlos read "good", rather than "God" in your user name, and he's saying they're better than just "good". Reun __________________ Do wizards use spell checkers?
 5th October 2004, 10:29 AM #6 djk R.I.P     Join Date: Feb 2001 Location: USA Is there a point to all this? Why don't you make something useful, like a circuit that measures voice coil temperature rise? Speakers go into power compression and eventally burn up due to the coil getting hot. Put a 0.05 ohm sense resistor in the ground side and amplify the voltage. Divide down the voltage from the amplifier hot side until equal. Drive a set of differential comparators. The output of the comparators could be an LED ladder, say 5 green, 3 yellow, and 2 red. If this is for a 'class', you could make it a lot more complicated by using a µP.
 5th October 2004, 10:43 AM #7 Upupa Epops   diyAudio Member   Join Date: Aug 2003 Location: Prague,Czech Republic Whatabout phase shift in voltage sensing ?
 5th October 2004, 11:17 AM #8 destroyer X   diyAudio Member     Join Date: Feb 2004 Location: Recife - Brasil Northeast Rnebrivin is rigth...he understand my "special indian english!" And you texan...hehe... have ten steps, turn and shot! having ink pistols please! I love Pink Floyd...he will be the Classic Music in near future. carefull with brazilians.... we will turn in the first step!!!! regards, Carlos __________________ Restriction removed ; ttps://www.youtube.com/watch?v=3ioyOjAzDt0; https://www.youtube.com/watch?v=uFcnddpe3MU
pfloyd_is_god
diyAudio Member

Join Date: Sep 2004
Location: texas
like I said, I'm going to use an micro processor, and Iâ€™m going for a little more precision than a 3 stage bar graph.

Quote:
 What about phase shift in voltage sensing ?
Hence the integrate voltage and current before multiply, Iâ€™m fairly confident this will work. So does anyone have any ideas on how much dynamic range and bandwidth I need?
__________________
listen to pink floyd

 23rd October 2004, 05:08 PM #10 jackinnj   diyAudio Member     Join Date: Apr 2002 Location: Llanddewi Brefi, NJ if you want to acurately measure power consider using and AD737 or AD536 True RMS Converter -- they do the multiplying, square - rooting etc. for you. The AD737 is 200mV max input and requires an input attenuator, the AD536 doesn't.

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