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1111  Forum 2005-2010 (read only) / Interfacing / Re: How much currrent can Pro Mini's regulator supply? on: June 30, 2010, 12:12:50 pm
Heh. :-)
1112  Forum 2005-2010 (read only) / Interfacing / Re: How much currrent can Pro Mini's regulator supply? on: June 30, 2010, 06:32:33 am
I'm no expert, but I'm falling into the Grumpy Mike camp here.  Contrary to popular belief... LED's are not magical devices with unknown properties only Merlin the magician could fathom...

Of course not.  Nobody suggested they do.

It's your visual system which has unknown properties which only merlin the magician can fathom:

In this illusion, the colored regions appear rather different, roughly orange and brown. In fact they are the same color, and in identical immediate surrounds, but the brain changes its assumption about colour due to the global interpretation of the surrounding image. Also, the white tiles that are shadowed are the same color as the grey tiles outside of the shadow.

Your eyes perform lots of tricks to make the world look the way it does.  I've researched this in great detail because last year I was working on a graphics application where I was trying to extract shape and color information from photographs in order to create bumpmaps and color maps for a texture making application I wrote.  Since no algorithm exists which can do this well, I was trying to invent one and I was studying how the brain interpets visual stimuli to see if I could copy some of its tricks.

I don't know much about how this led trick is supposed to work, but off the top of my head, I can think of several possible explanations for it.  One would be that doing this prevents your pupil from dialating as much, and so it gathers more light.  Another is it could be tied to the response times of your rods or cones.  Or it could be how your brain interprets the input.

An led doesn't have to actually be brighter to appear brighter.
1113  Forum 2005-2010 (read only) / Interfacing / Re: How much currrent can Pro Mini's regulator supply? on: June 30, 2010, 06:14:41 am
Well, 5% brightness is 1mA if normally you're putting 20mA through the leds.

And with my 8x8 array, I was lighting one whole row at a time, supplying each led with 20mA.

Divide that by 8, since each row was only on 1/8th of the time and effectively I was supplying each led with 2.5mA, and running them at a 12.5% duty cycle.

So, I'm not far off from where they were running their leds at.

Also just for kicks I looked up the green leds they used:

Look like they might be 5000mcd.  So they could get away with that 5% duty cycle and still have a fairly bright led.  Me, I'm stuck with 32mcd leds because they don't make bargraphs with green leds which are any brighter, and they don't seem to make rectangular green leds which are any brighter either. :/  

(Least none I could find on Mouser, Digikey, or Newark.)

Anyway. can't hurt to try it and see if I get any results. :-)  It's not like I have many other options.
1114  Forum 2005-2010 (read only) / Interfacing / Re: How much currrent can Pro Mini's regulator supply? on: June 30, 2010, 05:13:44 am
Reading the article some more...

When a short-cycle pulse voltage with a frequency of approximately 60Hz is applied to an LED at a duty ratio of about 5%

So what exactly are they saying to do here?

Are they saying if I have an led I might normally drive at 5mA then I drive it at 100mA, and turning it on and off 60 times a second, or once every 16 milliseconds, and when I have it on, I should leave it on for 5% of that period, or .833 milliseconds?  Ie, 833 microseconds?

In oither words, are they saying I should turn the led on for 833 microseconds while putting 100mA through it (or whatever it's designed to handle with very short duty cycles) and then off for 16 milliseconds, roughly?

I guess it won't hurt to give it a shot.  I'm updating my display fast, but I think I was simply updating at 480hz, and lighting a whole row at a time.  Not trying to force as much current through an led as I could in as short a period as possible.
1115  Forum 2005-2010 (read only) / Interfacing / Re: How much currrent can Pro Mini's regulator supply? on: June 30, 2010, 04:55:53 am
Hm... you didn't happen to do your test with a red led did you?  The article says that:

Based on an evaluation test using subjects, the group reported that a blue LED looks 1.5-1.9 times brighter while green and red LEDs look 2.0-2.2 and 1.0-1.3 times brighter, respectively.

So if you were using a red led, at most it would only appear maybe 30% brighter.  I'm using green leds, so I would have gotten the most difference.
1116  Forum 2005-2010 (read only) / Interfacing / Re: How much currrent can Pro Mini's regulator supply? on: June 30, 2010, 04:49:12 am
Not according to some researchers in Japan:

Also, I have tried it, and have seen results with my 8x8 matrix when driving the leds with much less than 10mA.

It may come down to how many mA and how bright the leds are in the first place.  I'm using 10 segment bargraphs with LEDs that are fairly dim to begin with.
1117  Forum 2005-2010 (read only) / Interfacing / Re: How much currrent can Pro Mini's regulator supply? on: June 19, 2010, 05:55:03 pm
Problem is, the circuit boards have already been made.  :'(

I'll find some way to make it work.  Shave some mA off here and there.  Maybe improve the multiplexing.  Give up pulse width modulation, and light one LED at a time instead of a whole row.  Feed it 40mA for a few microseconds...  You can trick the eye with multiplexing.  An led flicking quickly, but brightly looks brighter than an LED lit dimly, but constantly.  Was already using that trick, but with less current and leaving the leds on longer.
1118  Forum 2005-2010 (read only) / Interfacing / Re: How much currrent can Pro Mini's regulator supply? on: June 19, 2010, 07:57:14 am
Done some more calculations.  If I assume there's no diode in front of the regulator (and I don't know that there is one) and I take ground current into account (whatever that is... the datasheet references it in its equations) but I assume a best case scenario of running at room temperature, I get a maximum of 108mA continuous from the regulator before it goes into thermal shutdown.

And once again, the ATMega328 itself draws between 30-50mA, so that leaves me with only 58-78mA to play with.  

That's less than half what I oriignally planned to put through the LED array alone.
1119  Forum 2005-2010 (read only) / Interfacing / Re: How much currrent can Pro Mini's regulator supply? on: June 19, 2010, 05:32:16 am
Trying to find ways to solve this in case my calculations are correct.  So far I've only come up with one potential solution.

It turns out using brighter LEDs may not be an option because there don't seem to be any available.  The bargraphs I'm using are rated at 19mcd, and that's as bright as they come.  Same goes for discreet LEDs.  There don't seem to be many green rectangular leds which are brighter than about 10mcd.

The only other option I've come up with is to use a different voltage regulator.  One which can provide the Pro Mini with more current.  Or try running it off an unregulated 4.5v.  Though I don't know how long 3 AA or AAA batteries would be able to prvide sufficient voltage to keep it powered.  I think I calculated recently it could continue to run down to 3.7 volts at 16mhz.

Anyway, even if I use a diffrent voltage reuglator, I'm not sure how much current the Arduino will be able to supply.  

I guess I'll pick up a 7805 when I head out to Radio Shack today though.
1120  Forum 2005-2010 (read only) / Interfacing / How much currrent can Pro Mini's regulator supply? on: June 18, 2010, 04:12:31 pm
Hey guys,

I need to know how much current the 5v Pro Mini's regulator can supply.  I think I may have made a miscalculation when I was originally determining how much current I had available to power one of my circuits.

Because I'm just about to assemble the circuit, I decided to redo my current calculations to make sure I was running everything within suitable parameters.  However, the numbers I've come up with are way lower than what I originally calculated.

Here's my current requirements:

1) The microcontroller itself, from what information I could find on the forums, seems to draw between 27-50mA.

2) I have two green leds which I want to power with 20mA becuase they're kinda dim.  They're wired in series, so the total current draw for the two would be 20mA.

3) I have a piezo speaker.  I've put a 1000ohm resistor on this to ensure that it only draws 5mA.  Though it may be drawing much less than this.

4) I have a red power led, and a green mode LED on illuminated pushbuttons.  Each of these will draw 10mA.

5) Lastly, I have a multiplexed 8x8 array of LEDs.  I'd planned to allocate 160mA to this array.  (20mA per LED per row, one row lit at a time, current sunk by darlington array connected to GND.)  

The problem is that last one.  It now looks like I may not have enough current to spare to run the array without making the LEDs really dim.

This, I've heard, is the datasheet for the Pro Mini's voltage regulator:

With a 9v input, a 5v output, and a 0.7 voltage drop across a diode I believe may be in front of it, the regulator needs to drop 3.3v.

And according to the datasheet, the power dissipation can be calculated as follows:
Pd = (125 degrees C - 40 degrees C) / 220 degrees C/W
Which gives: Pd = 386mW

(I did the calculation based on an ambient temperature of 105 degrees Farenheit.  I figure nobody'll be using the prop on a day which is hotter than 90 or so.  Also, it's based on the minumum heat sink.)

So, if we take 386mW, and we divide it by the 3.3 volt drop across the regulator, we arrive at 116mA as the maximum current the regulator can supply if the ambient temperature is 40 degrees celsius.

Even if I assume the ambient temperature is less than that, say, 20 degrees celsius or 70 degrees farenheit, the picture doesn't get a whole lot better.

So... 116mA.  That's not much.  Subtracting the atmega's current requirements from that leaves me with 66mA.  20mA for the two green leds, and 5mA for the piezo eats up another 25mA leaving me with 41mA.  And supplying 10mA to each of the illuminated pushbuttons leaves me with only 21mA.

21mA to drive an array of 64 LEDs.  If that's all I've really got, I might as well have not even included the darlington array to sink the current.  And I've got a big problem because the bargraphs I planned to use for the array only have 16mcd LEDs.  

So what do you think?  Are my calculations correct?  When I originally designed this thing I thought the Pro Mini reuglator could handle 335mA, but now I wonder if there wasn't a mixup somewhere along the way, and what was actually meant was 335mW.

If I can't get more current out of this thing, my only option may be to buy 64 rectangular LEDs which are way brighter than the ones in these arrays.
1121  Forum 2005-2010 (read only) / Interfacing / Re: Pro-Mini with external regulator? on: June 30, 2010, 04:34:47 am
Because it says maximum output.
It says DC input 5V up to 12V.
That does not mean you can have 150mA output when you feed it with 12V input. For any input voltage the current will be limited by the power dissipation. The page doesn't say that all 150mA is available for other uses.

I reread what you said, and apparently I didn't understand the first time.

So what you're saying is the voltage you input to the raw pin affects the output capability.  So if I input 9v, I can use fewer mA before the regulator overheats.

Okay.  I can agree with that.  That's how I arrived at my numbers.  By calcuating how much 9v would overheat the regulator.  I guess I just forgot what I did since I did it a week ago. :-)

But in that case, I guess I did get my numbers right.  So the regulator can't supply more than 100mA... when powering it with 9v.  And I do need an external regulator.


And I see you just confirmed that.  Oh well.  I got my hopes up. :-)

External regulator it is then.
1122  Forum 2005-2010 (read only) / Interfacing / Re: Pro-Mini with external regulator? on: June 30, 2010, 04:24:32 am
From the data sheet of the regulator
A 1[ch956]F capacitor should be placed from IN to GND if there is more than 10 inches of wire between the input and the ac filter capacitor or if a battery is used as the input.

Yes, but that's the regulator built into the Pro Mini, not the LM317 which I also posted a datasheet for which seems to imply they're optional:

Ci is required when regulator is located an appreciable distance from power supply filter.
Co is not needed for stability, however, it does improve transient response.

But if you think one is necessary, I'll include one... if it turns out I need the external regulator at all.  Please verify the onboard regulators ability to supply current to the output pins if you would.  

If I don't need to use an external regulator to get 150mA from the pins, that would be great.
1123  Forum 2005-2010 (read only) / Interfacing / Re: Pro-Mini with external regulator? on: June 30, 2010, 04:16:40 am
I posted the calculations in this earlier thread:

I read your tutorials on power and I think I did the calculations right, but I'm not certain.
1124  Forum 2005-2010 (read only) / Interfacing / Re: Pro-Mini with external regulator? on: June 30, 2010, 03:56:30 am
Another question I have is:

Why does the Sparkfun page say the Pro Mini can output 150mA when according to my calculations it couldn't do anywhere near that?

As I already mentioned, using the datasheet for the voltage regulator it has onboard:

I calculated it could handle a little over 100mA.  That's before it has to power the ATMega with 30-50mA.  

So shouldn't their page list the output in the range of 50-70mA?  Or are my calculations way off?  If it can output 150mA then there's no need for an external regulator after all.
1125  Forum 2005-2010 (read only) / Interfacing / Re: Pro-Mini with external regulator? on: June 30, 2010, 03:19:02 am
No you can't you need it to drop the input impedance which is even higher on a battery.

Why do I need to drop the impedance?  What function does that serve?  If it's to stablilize it, I thought that wasn't strictly necessary in all circumstances?  The datasheet for the LM317 I have seems to imply they're optional.  (Though I have no datasheet for the 7805 because it came from Radio Shack.)

I figured I'd try it without them and if it ran stable then I should be fine without them.  If not, I'd know fairly quickly and could solder them on.  But is it your opinion that there's absolutely no (or very little) chance the 7805 will run stable without them?

Can the Pro Mini handle 200mA, seeing as it was only designed with a regulator that could put out 100mA?

That seems to imply you don't know anything about ohms law.
The pro mini will not take more current than it needs if fed the correct voltage. The absolute limits for the chip still apply, that is don't source 9or sink) more than 200mA in total. Also see the data sheet for cumulative limits on individual ports.

No, you misunderstand.  What I want to know is can the traces on the board and the other components on it handle 200mA?  I assume they can, but I don't like to assume. :-)

(Also, I'm just trying to cover all the bases.  I don't know what I don't know about how circuits function.)
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