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
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
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
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
Forgot to attach the image....
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 shouldn’t go much over 80 mV, that’s about 700 mW of power dissipation, I can live with that.
I could go for an analogue solution, but where’s the fun in that?, I'm most likely going to be using an MCU for other reasons anyway.
Are you seriously insulting the Floyd?
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 shouldn’t go much over 80 mV, that’s about 700 mW of power dissipation, I can live with that.
I could go for an analogue solution, but where’s the fun in that?
, I'm most likely going to be using an MCU for other reasons anyway.Are you seriously insulting the Floyd?
Attachments
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.
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.
somewhere on my website I have a routine to convert to logarithms -- but you can also find it on Tracy Allen's website:
http://www.emesystems.com/BS2math3.htm
while it is written in Basic for the stamp, you can quite easily make the conversions to other programming languages.
a logarithmic converter like an AD8307 or AD606 will give you far greater range, btw -- it's much easier to use a log converter and an 8 or 10 bit ADC to read its output than trying to squeek the range out of a 16 bit ADC. The AD537 which I cited in my prior post can also be used as a log converter.
here's an RF wattmeter based upon the AD8307 (92dB):
http://hem.passagen.se/communication/wattm.html
some of these chips show up on EBay from time to time -- if you do use a log converter make sure you do so in a static free environment.
http://www.emesystems.com/BS2math3.htm
while it is written in Basic for the stamp, you can quite easily make the conversions to other programming languages.
a logarithmic converter like an AD8307 or AD606 will give you far greater range, btw -- it's much easier to use a log converter and an 8 or 10 bit ADC to read its output than trying to squeek the range out of a 16 bit ADC. The AD537 which I cited in my prior post can also be used as a log converter.
here's an RF wattmeter based upon the AD8307 (92dB):
http://hem.passagen.se/communication/wattm.html
some of these chips show up on EBay from time to time -- if you do use a log converter make sure you do so in a static free environment.
Hey..ispik inglish prease...hey, speak english please
I have troubles with the language....not a native in English language...and never studied...had some boering Englishman in my family that i had to learn some, because the one was so terrible that no one tried to talk with him....i was elected by the family to be closer the F---- guy.....my God, that one, i suppose, was banished from England.... what a mood!
Them.... a group.....a musical group?... is that the meaning you are trying to say to me?
He ....this is a personal pronom.....related a one single person....
The Pink Floyd i am talking about, as a classical music, is an old music recordings that i used to here "The other face of the moon"...and others music.....
I am confused....me good guys....me OK....me Indian...very, very old culture.... thousand years culture....more than milenium.....many mileniuns guys..... we did not destroy our ground, our plants, we did not polute our seas...we did not kill our people....we are a very stupid people...isnt'it?
Carlos
I have troubles with the language....not a native in English language...and never studied...had some boering Englishman in my family that i had to learn some, because the one was so terrible that no one tried to talk with him....i was elected by the family to be closer the F---- guy.....my God, that one, i suppose, was banished from England.... what a mood!
Them.... a group.....a musical group?... is that the meaning you are trying to say to me?
He ....this is a personal pronom.....related a one single person....
The Pink Floyd i am talking about, as a classical music, is an old music recordings that i used to here "The other face of the moon"...and others music.....
I am confused....me good guys....me OK....me Indian...very, very old culture.... thousand years culture....more than milenium.....many mileniuns guys..... we did not destroy our ground, our plants, we did not polute our seas...we did not kill our people....we are a very stupid people...isnt'it?
Carlos
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