DIY Power meter - Page 2 - Home Theater Forum and Systems - HomeTheaterShack.com

Old 10-13-09, 09:10 PM
Elite Shackster

Greg

Join Date: Nov 2007
Location: NY
Posts: 2,601
Re: DIY Power meter

OK. I'm back, and I've done some thinking. It' sgetting late, but maybe I can get you 90% of the way there and you can finish or experiment... Sorry fr the verbosity in the interest of thoroughness:
We start by sort of reverse engineering the circuit we have. We start with the power levels the LEDs are supposed to fire at:
P
150
100
50
25
10

We assume we're talking about 8ohm speakers here, so P=V^2/Z, V=sqrt(P*Z), and we see the voltages that resent those powers:
P V(8ohms)
150 34.64
100 28.28
50 20
25 14.14
10 8.94

Now based on the original values, I see that for 9V input, you will get 2 V across the lowest LED, so I've used that moving forward. So we know that at 9Vin we want 2 V across the lowest LED:
P V(8ohms) 8.94
150 34.64
100 28.28
50 20
25 14.14
10 8.94 2

Now that means at 14.14Vin, we should get 3.16V at the same spot:
P V(8ohms) 8.94 14.14
150 34.64
100 28.28
50 20
25 14.14
10 8.94 2 3.16

Now we know that at 14.14Vin we need the same 2V across the 25W LED, so at that point we want the anode of that LED at 5.16 V:

P V(8ohms) 8.94 14.14
150 34.64
100 28.28
50 20
25 14.14 5.16
10 8.94 2 3.16

Carrying this through, we can populate most of this chart:
P V(8ohms) 8.94 14.14 20 28.28 34.64
150 34.64 20.56
100 28.28 15.15 18.56
50 20 9.3 13.15
25 14.14 5.16 7.3
10 8.94 2 3.16

Now we can use these voltages to show us the simple ratios we want each voltage divider equation to yield (for instance, for the equation relating to the lowest diode, Vin/Vout=9/2...
P V(8ohms) 8.94 14.14 20 28.28 34.64
150 34.64 20.56 0.59
100 28.28 15.15 18.56 0.54
50 20 9.3 13.15 0.47
25 14.14 5.16 7.3 0.37
10 8.94 2 3.16 0.22

So, if we take the entire string and call it Rtot, that's the sum of all resistors (R1 is small enough to be negligible here): Rtot=R2+R3+R4+R5+R6+R7. Rtot becomes the denominator in all our divider equations:
P V(8ohms) 8.94 14.14 20 28.28 34.64 Rtot=R2+R3+R4+R5+R6+R7
150 34.64 20.56 0.59 0.59=(R3+R4+R5+R6+R7)/rtot
100 28.28 15.15 18.56 0.54 0.54=(R4+R5+R6+R7)/Rtot
50 20 9.3 13.15 0.47 0.47=(R5+R6+R7)/rtot
25 14.14 5.16 7.3 0.37 0.37=(R6+R7)/Rtot
10 8.94 2 3.16 0.22 0.22=R7/Rtot

Now we have 5 equations, but we have 6 variables. Need one more to make this solvable. Truth be told, we may not want to, as we need to choose standard resistor sizes, but since we're reverse engineering, it's easy... R2=5600. Now we have a system of 5 simultaneous equations with 5 variables. That should be easily solvable. Hopefully when you do it, it will yield values close to the original design. I think you can probably scale that appropriately to your desired situation.

That being said, I started to do it, and I see that your lowest power levels yield voltages that won't bias the diodes, so you're going to have an issue doing exactly what you want with this topology.

If the math proves what I've done works, then I can show you how to get around this problem (I think) and at least have a few LEDs that fire at a minimum output voltage, so you can have the long scale you want...
Attached Files VU Meter.xls (9.0 KB)

Last edited by glaufman; 10-13-09 at 09:18 PM. Reason: Text didn't format, attached excel file
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Old 10-14-09, 06:41 AM
Elite Shackster

Greg

Join Date: Nov 2007
Location: NY
Posts: 2,601
Re: DIY Power meter

OK. Before someone else points it out, some much needed sleep has helped my thinking. The general idea of building up the equations should be sound, but... once a given LED fires, it no longer is part of the voltage divider per se, it should then regulate the voltage across its section of the divider... hopefully I'll be guessing correctly at 1 V, but I suppose this might depend on the LED. That means even though the lowest LED at 10W should see 2V, it won't. It'll see 1V. So I say it'll fire before it's at 2V, specifically at 1V, or 5W. I'll have more time later to figure out the rest of the levels (minimum input voltages at which each LED will fire in the original design). Sleep has also shown me that were this done around 4ohm speakers, 10W would represent 1 V at the lowest LED, so ...
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Old 10-14-09, 08:17 AM Thread Starter
Shackster

Vido

Join Date: Jul 2009
Location: Pula, Croatia
Posts: 12
Re: DIY Power meter

Wow, thank you very much, you wrote so much I'm gonna need few hour to translate and understand this all but I think I know what you meant and I will try to follow your advices as soon I finish all school bussines for this week.
Thanks for giving me idea to make calculator in Excel cause I totally forgot about that nice program

If you have something more to write, please do, any help is welcome.
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Old 10-14-09, 10:17 AM
Elite Shackster

Greg

Join Date: Nov 2007
Location: NY
Posts: 2,601
Re: DIY Power meter

I've refined the equations in the spreadsheet to more accurately reflect how the divider should work (LEDs regulating around 1V). I also added a section that takes the RC into account (sort of) to calculate the power each LED would fire at. It's imperfect for a number of reasons, but I suspect it's close enough for your purposes. For these purposes, I assumeed we'd be working with a pure sine wave at some frequency. That allows me to easily calculate the RMS voltage see at the diode from the peak voltage the divider string sees after the diode. Then, at that frequency, the C has a given Z and R1C can be treated as a voltage divider, giving the voltage seen by the whole circuit. Now that's used to calculate the power, assuming the speaker's impedance is 8ohms at that frequency. Here's where it gets quite arbitrary: What frequency? Since I'm reverse engineering, I played with values until the numbers got close: at 75Hz, the lowest LEDs are a little low, and the highest ones are a little high, but hopefully close enough to illustrate the procedure. I suspect the remaining innacuracy is due to the voltage divider equations not taking into account the current flow through the LED when it's firing, but that may have to wait for the next go-around... Also bare in mind that this circuit will naturally respond less to higher frequencies than lower...
Aw, well, I'm kinda enthused now so I just might take a crack at the next version pretty soon... depends how soon my boss comes to talk to me today...
Attached Files VU Meter.xls (14.5 KB)
glaufman is offline
Old 10-14-09, 10:54 AM
Elite Shackster

Greg

Join Date: Nov 2007
Location: NY
Posts: 2,601
Re: DIY Power meter

Well, I tried adding in the LED currents... it seems I may have reached the point where the more I try to refine it the stranger the numbers get... got to let the mind rest for a while.. let me know if you try experimenting and what you come up with.
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