Variable attenuator contest

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o->|---+ 80V peak (400W) or 28.8V peak (50W)
       |
      [ ] R1 
       |
      [ ]<-+ RV1 0-10K
       |   |
       +---+
       |
       +--------------o 2.56VDC
       |
      [ ] R2
       |
       V

I have been trying to calc R1 and R2 so I can set RV1 for max attenuation (biggest R) and attenuate 80V to 2.56V. When RV1 is 0 ohms, I'd like 28.8V in to be attenuated to 2.56V. I can puts a legend on the skirt of the knob showing where to set it for 50 (full ccw) 100 200 400 (full cw) watts. I originally was using a fixed attenuator and was going to have the user set a threshold level with a 4 pos dipswitch, but that needed 4 inputs. If I use a tiny85, I can read the one a/d chan and run a pair of red/grn leds. Any other better block diagrams for this topology welcome. Like taking the out from the pot wiper, etc.

 

Imagecraft compiler user

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R1 = 4700

R2 = 470

 

Will put you close to what you want.

What did I win?

Jim

 

 

 

edit: spelling

 

 

 

 

Last Edited: Mon. Nov 16, 2015 - 10:28 PM
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Not sure what you're getting at.

 

If, with RV1 at 0R, a 28.8V input will yield a 2.56V output, won't an 80V peak input yield a 7.1V output?  That's a bit high for an AVR, isn't it?  I suppose just clamp it with a zener?

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

Last Edited: Mon. Nov 16, 2015 - 10:59 PM
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This is easily accomplished using the standard voltage divider rule ---> Vo = Vi ( R2/(R1+R2) ) <--- and a bit of Algebra.

 

Of course, while you specified Vo MAX, you never specified Vo MIN.   I mean, when the value of RV1 is maximum, Vo will be at some minimum because R2 is still as much a part of the voltage divider as R1 and RV1.

 

But then too, that isn't the particular voltage divider configuration I would be using, especially if you expect V MIN to be at or near GND potential.

 

 

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

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Cool! I filled up a page solving the voltage divider equation several times and came up with 3300 and 330, so N=2 makes me think thats about right. The gizmo sniffs the speaker out and 2.56 lights up red, 3db down is yellow, another 3db is green, below that is green slow quick blink. User has to dial in 400, 200, 100 or 50 watts.

 

Imagecraft compiler user

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bobgardner wrote:

Cool! I filled up a page solving the voltage divider equation several times and came up with 3300 and 330, so N=2 makes me think thats about right. The gizmo sniffs the speaker out and 2.56 lights up red, 3db down is yellow, another 3db is green, below that is green slow quick blink. User has to dial in 400, 200, 100 or 50 watts.

 

 

Does that mean I won the contest???

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

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First prize is a week with Bob. Second prize is two weeks... yeah I know you have all heard that old joke before. Only offered because Bob didn't describe the prize.

Ross McKenzie ValuSoft Melbourne Australia

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What's the saying? "If its too loud, you're too old".

I thought the impedance of a speaker varied widely, so just measuring the volts is going to have a bit of error when determining the watts methinks.

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Doesn't matter what the impedance is. The speaker emits smoke when the R in the voice coil gets glowing red like a stovetop. Its all I^2R. Temperature follows rms. Rms tracks about 3dB above the avg. Don't believe it? Read in a music file sample by sample. Calc the avg and rms over a couple seconds of samples, graph em.

 

Imagecraft compiler user

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microcarl wrote:

bobgardner wrote:

Doesn't matter what the impedance is. The speaker emits smoke when the R in the voice coil gets glowing red like a stovetop. Its all I^2R. Temperature follows rms. Rms tracks about 3dB above the avg. Don't believe it? Read in a music file sample by sample. Calc the avg and rms over a couple seconds of samples, graph em.

 

 

It's been a really long time but, you see, a coil based speaker isn't designed for a DC voltage but rather, to have a nominal range of impedance over some specified frequency range.  And so, your concept of it having a pure I^2R is erroneous!!!

 

I believe the correct equation might be I^2Z.

 

If memory serves: Z = SQR(R^2 + XL^2)

Where:

       R = the internal resistance of the coil in the speaker

       XL = 2 x Pi x F x L

And:

       Pi = 3.14156

       F = the frequency of operation

       L = the inductance of the speaker coil

 

You can easily find the approximate XL of the speaker at a given test frequency by placing a rheostat in series with the speaker coil and adjusting the rheostat until R/L node is 1/2 the amplitude of  the test signal.  Once the 1/2 amplitude of the test signal is found at the R/L node, measure the the value of the rheostat.  The value of the rheostat that you measure will be the approximate XL of the speaker coil at that specific test frequency.

Just remember... when finding XL and Z over some specified frequency range, be sure to use the same test frequency for a given data point when calculating XL and Z for that particular data point.

 

And with one simple algebraic manipulation, you can now determine the approximate inductance of the speaker coil

 

If:  XL = 2 x Pi x F x L

 

Then the approximate L of the speaker coil is:

 

              XL
       L = ----------
           2 x Pi x F

 

From this point, I believe you have enough information to solve your problem.

 

But... I still think you need to take a refresher and then head back to the drawing board!!!

 

NOTE: the above thesis does not take into account parasitic capacitance of the speaker coil, among other things.  This presentation is only a simple approximation and should be treated accordingly.

 

 

 

And so, Bob, you didn't answer my question!  Do I get the prize???

 

EDIT TO ADD:

 

And if I did win the prize, and if it is one or two weeks hanging out with you, I defer it to Ross McKenzie, with my condolences!!!

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

Last Edited: Wed. Nov 18, 2015 - 03:53 AM
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You didn't give the R1 and R2 values I was looking for. No prize for you! Impedance doesn't matter. Heat comes from IR. I'm right, you're wrong.

 

Imagecraft compiler user

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Cogent as ever, Bob.
 

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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Thanks. Terse too.

 

Imagecraft compiler user

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bobgardner wrote:

You didn't give the R1 and R2 values I was looking for. No prize for you! Impedance doesn't matter. Heat comes from IR. I'm right, you're wrong.

 

 

What!  Do you expect me to do all of your work for you? surprise

 

Surely, Bob, you are able to derive the DELTA-V at the Rheostat, using a modified version of the voltage divider rule! kiss

 

Is this a school assignment? frown

If a school assignment, you need to do some foot-work, your self! enlightened

 

Is this a work assignment? frown

If this is a work related assignment and you are getting paid, I expect at least 30% of your take! yes

 

 

Come on, Bob, if you can't figure out how to find the delta V of the Rheostat slider, you'd better be hitting the books!!! crying

 

Oh!  And one other thing...  How about you demonstrate your effort by posting the work you've already done on this project!!! laughlaughlaugh

 

Now, where have you heard these things before???

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

Last Edited: Wed. Nov 18, 2015 - 09:58 PM