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# Making a wattmeter with 3 digit LED/LCD display

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#### Duke

##### New Member
Hello,

This is my first project in electronics so I will need a lot of advice

I want to output to the current wattage by an amplifier in my car to an LCD or LED display. I'm looking for a simple 3-digit display. I assume that I can tap into the car's voltage somewhere. How can I measure the current from the amplifier? Then, I can multiply these 2 results together to get my wattage.

Will I need a microprocessor to execute the multiplication? Also, I'd like to have a looped timer set to about 250 ms, so that it delivers the output on these intervals.

It's probably a simple project for you guys...but this is new to me and I hope I can do it!

Thanks a lot,
Brian[/b]

can anyone help me out?

Hey Duke,

I think people here are a little stumped on your question, so I will attempt to shed a little light on your problem.

To measure current, there are several methods used. You might want to look into www.allegromicro.com for their non-intrusive hall-effect based current sensors which will put out a proportional voltage. I believe these sensors can measure +/-100A.

Alternatively, you can use a simple sense resistor. This would go in-line with your power supply and you would measure the drop-voltage across it to calculate your current. (Ohms Law!!). Remember to make sure the resistor is the proper wattage (I^2XR or VI) or it will cook.

I am not sure how you would measure your actual wattage to your speakers.. You have to remember that you aren't getting 100% efficiency in your switch-mode power supply (your 12V to a higher +/- voltage) or your actual amplifier. If you know the approximate value of your efficiency here, then it would be possible to get a rough estimate.

I am not to sure about measuring directly from your speakers, maybe somebody else here can help you with that. I would think you would need to consider the output impedance and maximum power transfer to get this, but I am not positive.

Steve

Audio power meter

Ok Duke ... how does this sound...

Rather than measuring current to the amp (which will give a constant offset to your readings) why not work on the speaker line directly --

P = I * V
I = V / R

So P = V^2 / R

As R is the speaker and so can be considered to be constant this is just a fixed multiplier.

If you measure the voltage, square it and divide by R ...

A simple exponential circuit can be made with a couple of transistors, you just recalibrate a voltmeter on the output of this to read watts directly.

#### Now all you have to do is work out if there is a display for each of your (four?) speaker channels or are you going to attempt to sum the measurements :?:

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