Help With A Watt Meter Senior High School Project

[andrewgoad]

I am a senior in high school and have been in electronics classes for three years. This year we have to complete a senior project. I have been really interested in Sound recently as I have just purchased a sound system for my car. For my project I had the idea that I could make a wattmeter to measure the watts running to each of my speakers and subwoofers. I have fund circuits that would work for my project that use programable processors as the main part of the circuit, but they seemed pretty complicated for my ability level and way too expensive. What I was thinking would work is if I could make a voltmeter with no display, a current meter alongside of the voltmeter with no display, and a chip that would connect the two circuits as inputs and multiply them together to give me my ouput in an LCD display of Watts. I could use switches to control which area in my sound system I want to monitor. Also the voltage and current sensors must be large enough to cover up to 1200 Watts. Does this idea seem logical, and can somebody help me with a schematic? Also what type of senosrs do I need for the Watts that I am monitoring? Any help would be greatly appreciated!!!

 

[chemelec]

Since your Speakers have a Specific Impedance (Resistance) (In Theory Typically 2, 4, 8 or 16 Ohms), You don't need to measure the current.
Just Make a peak detector citcuit with an Analogue voltmeter on the output.
Knowing the Resistance (Impedance)
The wattage is Volts Squared/Resistance

Example 10 volts at 8 ohms would be:

10 times 10
-------------- = 12.5 watts
8

Than RE-Calibrate the scale of the Voltmeter to read directly in watts.

This is Not a Precise wattmeter, But for Simplicitity, typically Close Enough.

Gary

 

[Nigel Goodwin]

Since your Speakers have a Specific Impedance (Resistance) (In Theory Typically 2, 4, 8 or 16 Ohms), You don't need to measure the current.
Just Make a peak detector citcuit with an Analogue voltmeter on the output.
Knowing the Resistance (Impedance)
The wattage is Volts Squared/Resistance

Example 10 volts at 8 ohms would be:

10 times 10
-------------- = 12.5 watts
8

Than RE-Calibrate the scale of the Voltmeter to read directly in watts.

This is Not a Precise wattmeter, But for Simplicitity, typically Close Enough.

But to give any meaningful results, you need an RMS voltmeter (or only input sinewaves) - using a peak reading meter will only give imaginary results, like the existing spec on the amps.

 

[Oznog]

Well, there's several ways to run this:
1. Measure voltage and assume the speaker impedance to be the book value. This is flawed since the impedance changes a lot per freq, but of course the meter is often just for show anyways.
2. Measure voltage and current and multiply them. Note taking current is a bit complicated, there is a question of where you'd insert it and its impedance will affect the system. You could use an induction sensor, only wraps around the wire, but you'd need to take into account the freq response of the sensor.

Now here's the thing. With any reactive load, some of the current is not real. Thus a 1kHz 10v sine wave at 1 amp is not actually 10 watts of real power. To get real power, you'd need to take phase angle into account. This might mean a high speed ADC above the Nyquist rate for both voltage and current, and some efficient microcontroller math. If it was an induction sensor, you also need to take into account the phase lag for the frequency.

 

[jrz126]

I've been thinking about doing this for my system. It seems quite complicated to measure the power on the speaker side, so why not measure it on the voltage input to the amp?
Seems like it would only involve a current sensor, then just multiply the current by the voltage.

Of course the power measured will be the power through the speakers and the power lost in the amp, but it's much easier than what was mentioned above.

 

[chemelec]

No Matter What you do you will not get a TRUE Wattage Reading. There is Inductive Reactance to contend with as well as Square wave Distortion in the sound.

But As I also said Above, In "THEORY", those are the Resistances.
I Never said it was Exact.

But Based on Calibrating it with a Reasonable Sine Wave Signal at 1000 Hz, and at a Known Wattage, For the most part it should be accurate enough.

But if you are using a Conventional Analogue Meter, Your reading isn't all that accurate anyway.

Now if you want a More accurate reading, Use a Microphone at an appropriate distance from the speraker and Measure the Acoustic Power radiated from the speakers. That is the Power that you Really Hear.

 

[hotwaterwizard]

Here is a Simple Circuit

 

[hotwaterwizard]

The only way to get a true reading is to run a Test CD with a signwave at a different frequencies.
Do this throughout the whole spectrum 20hz to 20khz.
Then average out the readings by adding them together then deviding them by the number of readings you recorded.
The readings will be Volts and Amps at a given Frequency.
Volts times Amps are the Watts.
That will give you your Peak Wattage to get RMS you just multiply the Watts by .707 and that will be your Continuous Power output over the entire spectrum.

 

[David Bridgen]

That will give you your Peak Wattage to get RMS you just multiply the Watts by .707

See https://www.davidbridgen.com/watts.htm
.
.

 

[Teletech]

All of this is really getting complicated. How about just building a really kicka__ amp and letting them here your project. There is nothing like listening to your system knowing that you are the one that built the amp plus they are fun to build.