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Old 13th December 2007, 09:21 PM   #1
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Default Gross power indicator

I have an application where I need to devise a method to add Three LED indicators to a speaker to determine the Gross amount of power being fed to the speaker. This is an industrial application where they use large PA speakers to literally shake devices.

The problem is that the amps are in one remote location and the control room is in another location. Access to the speakers is limited so the indicators must run purely from the speaker lines. the speakers are viewable by camera remote control.

I need to figure a way to light a Green LED's at 1 watt, and then some division of power there after. something like 200 or 250 watts triggers a yellow LED and then a Red LED at 500 watts which is the program rating of the speakers used. something like that.

But, whatever circuit is devised cant affect the audio signal being fed to the speaker.

I was thinking of a resistor and a Zener that would feed the LED and current limiting resistor. Just duplicate that 3 times with zeners/resistors calculated for the various power levels.


BUT, when the diodes conduct. wont that effect that audio signal? or would the effects be negligible???


Zc
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Old 13th December 2007, 09:32 PM   #2
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Do some research on LM3915 power meters. They are basically an AC voltmeter that you can sort of calibrate to watts by assuming a constant resistive load.

Have used them for years, they have no effect on the music or a sine wave.

HTH, Marcus
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Old 13th December 2007, 09:40 PM   #3
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Thanks, but the LM chips require power. In this application there is no power available and batteries are impractical. The circuit needs to run entirely from the speaker line.

I thought about using a diode bridge across the speaker line and then take the DC from the bridge and use that to feed the three LED's with current limiting resistors just calculated for the thee voltage levels. But would the bridge effect that speaker signal???

They dont seem to effect the output of a transformer...but what about at higher frequencies??

Zc
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Old 13th December 2007, 10:02 PM   #4
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If you add another conductor you could do phantom power.
Or, if you don't mind big capacitors on your speaker wires you could run DC down the wires.
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Old 13th December 2007, 10:59 PM   #5
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If you run the LM3915 in dot mode the current consumption is around 15 milliamps. A SLA battery of 12 volts 7 Ah would run at least a month straight.

Marcus
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Old 14th December 2007, 01:03 AM   #6
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Default Hi Zero Cool !

Why don't you use limiter, it is cheaper than breaking your brains with Gross power indicator?

Regards zeoN_Rider
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Old 14th December 2007, 01:32 AM   #7
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I cant go into much detail about what they are doing. they wont even tell me all of it. But. No other AC or DC power lines are allowed in the room during testing. and the cables that do extend into the room now were installed as part of the structure with extreme measures taken for shielding etc. Lets just say this place is making things you or I cant buy (legally)nor could we ever afford. and they basically have one use and thats not a good one....

Anyway.

I once had a pair of PA system monitor speakers made by community sound that had Green "Signal present" LED's in them. If i still had the dang things i would just take them apart and see what they did.


But if they can put a LED in a speaker without having to resort to battery's or external power. then this should be an easy thing to come up with.

The client has given me a directive. find a way to make a level indicator with at least 3 steps that works without an external power source. speaker line only.


SO, the LM signal chips are great. But for this application they are out of the question. so lets talk about other methods.


Now if community put a signal present LED in there speaker. they had to have done it somehow.

Now if i used a diode bridge on the speaker line and rectified the AC coming in to DC, then the next problem becomes how to trigger the LED at 2.8V for the first step and how to limit the voltage to it so that it doesn't die from gross over voltage???
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Old 14th December 2007, 01:34 AM   #8
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Default Re: Hi Zero Cool !

Quote:
Originally posted by zeonrider
Why don't you use limiter, it is cheaper than breaking your brains with Gross power indicator?

Regards zeoN_Rider
This was my first thought and they said no. they wont tell me why. they want a visual indication that there is minimal signal present, average signal and max signal.
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Old 14th December 2007, 01:57 AM   #9
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Default Use an analog meter

Get a 0 to 5V meter.

Run the amp output through a resistor.

R~= Vrms max the amp puts out * 0.9 / amps needed for full scale meter deflection.

Wattage = 4 X Vrms max^2 / Resistor value

Resistor goes to a 4 diode fullwave bridge (ultra fast diodes are first choice, I've used 1n4148s).

If buying a diode bridge with 4 diodes already in side it. The meter goes from + to - and the resistor goes to one AC input and the amp return goes to the other ac input.

Put a cap across the meter if desired.

10 years ago, Radio Shack would have had all the parts to build this.
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Old 14th December 2007, 04:15 PM   #10
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The people at Community were nice enough to supply me with a schematic of the speakers i used to own that had the signal present LED.

They are just using a Diode string and a resistor. if my math is correct and please correct it if it is wrong. each diode provides a .7v forward drop. so three of them in a string should be 2.1V. with the LED across those three diodes. the LED should start to conduct at 2.1V correct??

But there is the forth Diode that also has a .7v drop. I am assuming it i wired as such to also protect the LED from high reverse voltages. so this really should be a forward voltage drop of 2.8V before the LED conducts correct???

This would make sense as that is 1 watt at 8 ohms.

Now if the diodes are just providing a voltage drop. couldnt i use a reversed Zener rated at whatever voltage i want in place of the lower three diodes??

63vrms at 8 ohms = 500 watts, the max program rating of the speaker.

couldn't i use a Zener as close to 56v as i can find. as the lower diodes with everything else the same? i bet i would have to change the value of the resistor wouldn't i?


Zc


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