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61  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.
62  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.
63  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?
64  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.
65  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...
66  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.
67  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?
68  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?
69  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.
70  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.  
71  Using Arduino / General Electronics / Re: Wire reverse polarity indicator led to handle voltage above breakdown? on: April 23, 2014, 09:32:10 am
So like this?


That's it?  I would have imagined if I measured across the LED that was conducting at best I'd get Vin - Fv, and that's what the reverse voltage across the other LED would be.
72  Using Arduino / General Electronics / Wire reverse polarity indicator led to handle voltage above breakdown? on: April 23, 2014, 07:37:35 am
I will be implementing the reverse voltage protection circuit shown in this video on a board I'm designing:
https://www.youtube.com/watch?v=IrB-FPcv1Dc

I have a red/green led which I will be using for a status indicator, and to reduce unique component count, I figured why not use it for my power indicator LED as well?  Then I got to thinking that perhaps I could wire it to not only indicate power, but also indicate reversed polarity so it's easy to tell if the problem is a dead battery, or a reversed battery.

The problem is, my circuit can be powered by anything from 4.5V to 16V, and if I simply placed one led in reverse across the power input, I assume even if I have a resistor on the led sufficient to limit the current to 20mA at the maximum voltage I expect, that I may still damage the LED thanks to overvoltage.

Is that true, and if so, how can I implement this cheaply?  I'd rather not add another unique component to the board, like a diode.  I was hoping to get this for free.

I considered a voltage divider, but if I divide by 2 while 5v becomes 2.5v which will still be enough to light the led, 16v becomes 8v which is still too high for the reverse breakdown which is 5V.
73  Using Arduino / General Electronics / Re: Limiting current to motor? on: April 15, 2014, 06:59:39 pm
Yeah, I've got the PWM part nailed down.  I will either be using a dedicated controller IC, or mosfets with flyback diodes.

Or are you saying I can just use a lower duty cycle to reduce the torque?  That will reduce the speed of my motor under load though.  What I really want is a way to prevent the motor from exceeding the current it draws under load, so that if it's stalled there will be insufficient current, and therefore torque.
74  Using Arduino / General Electronics / Limiting current to motor? on: April 15, 2014, 05:57:07 pm
I have a small DC gear motor and I'd like the limit the torque it outputs, without impacting the speed of the motor under a normal load.  But I also want to keep costs and component count low.

So I was wondering, what effect a simple resistor attached to the motor would have, and why it would be preferable to use a more sophisticated current limiting circuit over that.   Ie, how would it affect the speed and torque? 

I'm guessing with a motor things are going to be a little more complex than just choosing a particular resistor to limit the voltage and current to a specific level.

Oh, and I'm going to be controlling this motor with PWM.
75  Using Arduino / General Electronics / Reset into programming mode circuit for an Xmega. on: March 31, 2014, 05:21:17 am
Hey guys,

I think I finally figured out that reset/programming circuit I was working on.

This circuit is designed to allow an XMEGA to be reset into programming mode with a single button press.

Normally, if you have the board powered from the USB port, you'd have to press and hold reset, then press and hold a program button, then release reset, and finally release the program button.  Or you'd need a switch or jumper on the board to enable programming mode. 

With this setup, you just push the button, the board resets, and assuming you don't release the button faster than the reset occurs, the program line will be pulled low when that happens, and the board will enter programming mode.

In addition, because the programming pin by default is on one of the SPI lines, and I didn't want to lose an SPI port, I needed to find a way to ensure that SPI on the programming pin wouldn't reset the board, that it would still be possible to reset the board if the programming line were being pulled high or low, and that pressing the button while SPI was active would not short the pin to ground. 

This circuit does all of the above:

Java simulation: http://goo.gl/Jp7jex

Note that the forward voltage drop on the diodes (well, the one on the reset pin at least) needs to be below 400mv or the capacitor discharge will cause the voltage to spike above the 3.8v max on the MEGA's reset pin.  It isn't tolerant of high voltage like the Atmega and 32U4.

Also, the 25K capacitor by the reset pin is the internal 25K pullup on that pin, so the final part count is 2 resistors, 2 diodes, and a capacitor.

I have not tested this circuit myself yet, so buyer beware. 

It should be possible to add an additional reset button that performs a normal reset with the addition of another diode, but I never end up using those things, I just reprogram.  It's easy enough to unplug the board to reset it anyway. 

Anyway, if anyone has any thoughts on the circuit let me know.  The only thing I'm not certain about is that the reset line is pulled low for long enough to trigger the reset, but I think the timing is the same as the Uno here, so it should be okay.  The Mega requires a maximum of 1000ns low I think to trigger it which is a really small amount of time.

[edit]

Oh, and in the above circuit, the resest pin is the dot on the right side of the schematic, and the programming pin is the white dot at the top left.  The switch highlighted in blue is the pushbutton that triggers the reset and programming mode.  Above it is a switch to test what effect spi communication via the programming pin would have.  It's the miso line, and without that diode there, going low would trigger the board to reset.  There's also a switch next to it which disconnects that other switch from the circuit so the programming pin will be as it would be in its normal input state.

Also, the scope on the left is the reset pin, and the scope on the right is the program pin.
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