If that's all I've really got, I might as well have not even included the darlington array to sink the current. And supplying 10mA to each of the illuminated pushbuttons leaves me with only 21mA.Ģ1mA to drive an array of 64 LEDs. 20mA for the two green leds, and 5mA for the piezo eats up another 25mA leaving me with 41mA. Subtracting the atmega's current requirements from that leaves me with 66mA. 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.Įven 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. Also, it's based on the minumum heat sink.) I figure nobody'll be using the prop on a day which is hotter than 90 or so. (I did the calculation based on an ambient temperature of 105 degrees Farenheit. Pd = (125 degrees C - 40 degrees C) / 220 degrees C/W 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.Īnd according to the datasheet, the power dissipation can be calculated as follows: This, I've heard, is the datasheet for the Pro Mini's voltage regulator: It now looks like I may not have enough current to spare to run the array without making the LEDs really dim. (20mA per LED per row, one row lit at a time, current sunk by darlington array connected to GND.) I'd planned to allocate 160mA to this array. Lastly, I have a multiplexed 8x8 array of LEDs. I have a red power led, and a green mode LED on illuminated pushbuttons. Though it may be drawing much less than this. I've put a 1000ohm resistor on this to ensure that it only draws 5mA. They're wired in series, so the total current draw for the two would be 20mA. I have two green leds which I want to power with 20mA becuase they're kinda dim. The microcontroller itself, from what information I could find on the forums, seems to draw between 27-50mA. However, the numbers I've come up with are way lower than what I originally calculated. 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.īecause 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. I need to know how much current the 5v Pro Mini's regulator can supply.
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