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61  Using Arduino / General Electronics / Re: How many watts will this amplifier need to dissipate? on: May 05, 2014, 11:19:26 am
I linked to that same board in my posts above, wondering if it was okay for their board to be so small when my calculations indicated it would get pretty hot when running on a 12v supply.  Their description on that page you linked though says "it's completely cool running", which seems to contradict the power calculations I did, if one is truly running it at close to 20W per channel peak.
62  Using Arduino / General Electronics / Re: How many watts will this amplifier need to dissipate? on: May 05, 2014, 10:40:45 am
I'm aware that you can turn down the volume to reduce power consumption and the heat generated, but if they're claiming the amplifier is a 20W per channel amp, then I expect there is a way to get 20W of output power continuously.

Anyway, in looking at that continuous power rating again, I noticed something:
Continuous Power Dissipation (TA = +70°C)
44-Pin Thin QFN (derate 27mW/°C above +70°C, single-layer board) ...................................................2162mW

This bit:
(TA = +70°C)

I think it's a safe assumption that the ambient temperature isn't going to rise to 150F in my intended usage.

Also, there's this:
θJA, Single-Layer Board................................................37°C/W

So, if I assume 5W of power disspation, which I don't actually know is a correct assumption if I am driving two 4 ohm loads, then I can assume a temperature rise of 185C over ambient for which I think 30C (86F) is a reasonable assumption.

So that would give me a total temperature of  215C, which is too high.

Of course this also assumes I am running the amplifier at maximum power with something approaching a square wave at full volume.  But it's best to consider the worst case.

If I used a multi-layer board, which I am considering, then the temperature rise would be 27C/W, so at 5W now my total temperature rise above ambient is 135C, ,making the total temperature at 30C ambient... 165C.  This is slightly above the allowed junction temperature of 150°C.

So, assuming that 5W power dissipation is for two channels at once, and again, I am unsure if it is, then I think for the typical case of playing music in stereo, the power dissipation will probably be more like 2.5W, and my temperature rise will be 68C, making my maximum temperature 98C.  And 2.5W of power dissipation would require around 5 in^2 of four layer PCB.  Which seems, roughly, to be the the size of the Adafruit PCB.  Though it appears to be only two layers.




 
63  Using Arduino / General Electronics / How many watts will this amplifier need to dissipate? on: May 05, 2014, 09:54:32 am
http://datasheets.maximintegrated.com/en/ds/MAX9744.pdf

I'm trying to figure out how much PCB I'd need to dissipate heat from this amplifier when running it at 12V into a 4 ohm speaker.

The datasheet says on page 2:
Continuous Power Dissipation (TA = +70°C)
44-Pin Thin QFN (derate 27mW/°C above +70°C, single-layer board) ...................................................2162mW

So the chip can dissipate 2.1W continuously?  But only with thermal vias and a PCB of adequate size, I assume?

Page 5 of this document says a good rule of thumb is 2.37 in^2 per watt of power dissipated for a 40C rise:
http://www.ti.com/lit/an/snva419c/snva419c.pdf

But that does me no good as far as calculating PCB size if I don't know how much power the chip is actually dissipating.  I also have a PCB size calculator, but it needs JC not JA and this datasheet doesn't seem to supply that, so that's no good to me.

Then there's another snag   On page 8 of the amplifier's datasheet there's a graph for 12V into 4ohms, where it shows efficiency (%) & power dissipation (mW) vs output power, which doesn't make any sense.  If the amplifier is 85% efficient at  16W output, how is it the power dissipation is only 5mW?   If I assume 15% of 16W is wasted, that comes to 2.4W that I'd expect the amplifier has to dissipate.  Is the power dissipation axis labeled wrong?  Because while 5mW seems absurdly low compared to what I calculate it should be, it wouldn't surprise me if it was double what I calculated.

On a second look, I'm pretty sure they mean Watts, because the graph next to it for an 8 ohm load lists the power dissipation in Watts, and at 10W of output power the dissipation is 1.5W.  Which kinda meshes with the dissipation being 3.75W @ 10W output with the 4 ohm load.

But is that power dissipation for both speakers together?  Or just one?  If it was both together, than my calculation of 16W @ 15% efficiency dissipating 2.4W would make sense, since if you double that it's almost 5W, which is what that first graph says is the power dissipation at 16W into a 4 ohm load with a 12V supply.

But regardless, that brings us back to the first statistic I found in the datasheet:
Continuous Power Dissipation (TA = +70°C)
44-Pin Thin QFN (derate 27mW/°C above +70°C, single-layer board) ...................................................2162mW

If the amplifier can only handle ar ~2W of continuous power dissipation, what of that 5W of power dissipation?  Whether that's one or two speakers doesn't really matter, it's well above that 2W limit either way.  And a 4 layer PCB only gets you to 3W. 

Yet here Adafruit is, selling the same chip on a tiny board, with up to a 12V input and stereo output:
http://www.adafruit.com/products/1752

What gives?  Will that Adafruit amp melt if run continuously at 12V into a 4 ohm stereo load?  I highly doubt it or they'd warn about it.  And I've used smaller 10W boards.  So something must be up with my math here.

64  Using Arduino / Audio / Two Gravitech MP3 modules w/ Arduino Nano's for sale (Sold!) on: May 01, 2014, 11:44:51 am
Sold!
 
65  Products / Arduino Due / Arduino Due for sale. Open box. Never used. $35 + shipping on: May 01, 2014, 11:34:45 am
Like the title says, brand new, purchased it a month ago from Adafruit, opened the box but never powered it up and decided to go in a different direction with the project.  $6 priority shipping in the US, $12 international first class.  Send email address and I'll send you a paypal invoice and edit this post to indicate when it's sold.
66  Using Arduino / LEDs and Multiplexing / Re: How does this high power LED driver work? How much heat will it generate? on: April 27, 2014, 12:16:52 pm
Hm, the 488 isn't really an ideal solution at $1.68 a chip in quantity.

Is that really the only solution?  A chip like that, or use four pins?  That's disappointing.
67  Using Arduino / LEDs and Multiplexing / Re: How does this high power LED driver work? How much heat will it generate? on: April 27, 2014, 10:54:30 am
Bill:
I'm aware of the issues with using a linear regulator; that's why I asked what it was, it wasn't clear from the datasheet.

I'm also aware of buck converters.  There's one on Sparkfun that uses a similar design to the one you suggest, with one chip and one inductor per LED, but they're $20 each and I was wondering if there was a way to make them more cheaply.

By the way, in my search, I came across this chip:
http://www.ti.com/lit/ds/symlink/tlc5970.pdf

That chip can only supply 150mA per channel, but you only need the one chip to control three LEDs.  You also need only a single inductor.   It does seem to require four microcontroller pins though, unless you can drive a differential input with one pin tied to ground?  I'm not sure. 

At $0.25 per inductor and $1 per chip for those chips you suggested that's $3.75 for parts, plus the cost of placing three times as many components.  With the 5970, the cost of the chip is $3.23, and the inductor for it is $0.20, so your total BOM is a little over $3.43.  Not a huge difference, but remember, you're placing 3x fewer components and assembly costs go up with increased component count.  Also, if you're looking to make your board as small as possible, 3x fewer components will go a long way towards achieving that, and as the 5970 operates at 1.5mhz the inductor it needs is much smaller.

The only downsides are that differential interface and the lower current handling capacity.  If someone knows if you can drive differential pins like that with a single IO pin somehow that would be nice to know.
68  Using Arduino / LEDs and Multiplexing / How does this high power LED driver work? How much heat will it generate? on: April 27, 2014, 03:47:30 am
http://www.ti.com/lit/ds/symlink/tl4242.pdf

This is an interesting looking chip.  I would assume it works like a linear regulator, simply turning excess voltage into heat, except the diagram seems to indicate the only thing between the input and the output is a transistor, which presumably is toggled at some frequency to limit the current, as one would do in a buck regulator, except... there's no inductors on the output.   There is however a thermal pad on the bottom which indicates it may get hot.

I was looking at this for driving something like a 3W RGB led.  1W per channel, around 350mA each.

So how would I go about calculating how much power this is going to dissipate?  Excess voltage * current?  Something else?  What about the voltage drops they mention, how should I account for those in my calculations?
69  Using Arduino / General Electronics / Re: Build your own solenoid / linear voice coil actuator? on: April 27, 2014, 03:25:03 am
BTW: If it is not to much of a secret, what is your application for the Stewart platform?

It was on page 1 of the thread:
http://forum.arduino.cc/index.php?topic=83606.msg626707#msg626707

I never did get to building it.  I'd still like to eventually though.
70  Using Arduino / General Electronics / Re: Buck converter inductor current calculation - Am I doing it right? on: April 24, 2014, 03:39:13 am
Oh, and that other equation is IL = Iout + (/\IL / 2), so if /\IL is 0.625 then at 3A output IL =  3.3A, or I could have a 2.6A output with an inductor that saturates at 3A.

Hard to decide if I should go for the smaller 3A inductor:
http://www.vishay.com/docs/34295/sc15ah01.pdf

Or this larger inductor rated for 5A:
http://www.bourns.com/data/global/pdfs/SRP4020.pdf

I'm thinking I should just go for the smaller one.  Costs half as much, and it actually states that it works at up to 5mhz which the Bourns does not.  I assume it does, but it doesn't say, so...
71  Using Arduino / General Electronics / Re: Buck converter inductor current calculation - Am I doing it right? on: April 24, 2014, 03:31:33 am
Oops, I just realized there was another equation on the previous page in the datasheet, and I'd failed to notice that one of the IL's has a little triangle in front of it and that specifies that it represents the ripple current. 

Now off to the Eevblog to find out what ripple current represents.  I'm sure I watched Dave give a tutorial on that recently.
72  Using Arduino / General Electronics / Re: Buck converter inductor current calculation - Am I doing it right? on: April 24, 2014, 02:17:55 am
Hm, I just found this document:
http://www.power-mag.com/pdf/feature_pdf/1319729361_Murata611_Layout_1.pdf

It says on the first page that the ripple current is load independent.  Perhaps this means I should add 0.625 to my 3A load, and either decide that 2.3A is sufficient for my needs, or select a larger inductor capable of handing more current?
73  Using Arduino / General Electronics / Buck converter inductor current calculation - Am I doing it right? on: April 24, 2014, 02:12:42 am
I'm looking at using this switching regulator on my board:
http://www.ti.com/lit/ds/symlink/tps62130.pdf

My output voltage will be 5V, maximum output current 3A, input voltage 5-16V.  The regulator will run at 2.5mhz.

On page 19 of the datasheet it lists an equation for calculating maximum inductor current. 

I am considering using this inductor because it fits the reccomended 2.2uH inductance and has a maximum and saturation current of 3A.

The problem is, when I do the calculations on page 19, I arrive at a number that seems wrong.

I tried what I think is a worst case calculation:

IL = Vout * (1-Vout/Vin) / (Lmin * Fsw)
IL = 5* (1-5/16) / (0.0000022 * 2500000)
IL = 0.625

This seems way off.  I mean, I know that if I'm drawing 3A from the regulator 3A must be passing through the inductor.  So why is the value I get from this calculation so low?  Am I off by an order of magnitude here? 

Google says a microhenrie is 10^-6 henries, which is 0.000001.  Multiply that by 2.2, I get 0.0000022.  So the math seems sound.  What am I doing wrong?
74  Using Arduino / General Electronics / Re: Limiting current to motor? on: April 23, 2014, 09:29:09 pm
Why not use an appropriate sized thermistor? Kind of like an electronic clutch for current.
http://www.ohioelectricmotors.com/what-are-thermistors-and-how-do-they-limit-inrush-current-702

I'm not sure how to select one, and I'm not sure if it would limit the current quickly enough in a stall condition to protect someone's hand if it gets in the way of the moving parts.  It is only a tiny DC motor, but with a 30:1 gear reduction, so there's probably not much danger, but the whole point is to stop the thing immediately if something gets in the way. 

I did search Digikey though.  If any of them are suitable, it would be one of these through hole ones which are in stock:
http://www.digikey.com/product-search/en?pv69=521&pv69=80&FV=fff4001e%2Cfff803f7&k=thermistor&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25

I'm gonna check out the datasheets of a few and see if I can suss out how quickly they'd react, but I doubt it will be fast enough.
75  Using Arduino / General Electronics / Re: Wire reverse polarity indicator led to handle voltage above breakdown? on: April 23, 2014, 09:49:39 am
Well I stuck a resistor and an LED in a breadboard and measured it at various points and sure enough, 2.5V across the LED, 6V across the resistor, and 8.5V across either end from the battery.  I guess this will work then.  Thanks!

I have to wonder though why none of my searches pulled up any explanations of how to do this.  I actually found one circuit using an opamp to do it, and other places suggested diodes in series with the LED.  
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