Pro-Mini with external regulator?

Forum 2005-2010 (read only) Hardware Interfacing
1

Hey guys,

About a week ago, I discovered I had serious issue with one of my PCB's... I had grossly overestimated how much current the Arduino Pro Mini's voltage regulator could supply.

I'd believed the onboard voltage regulator could supply up to 450mA. But if my latest calculations are correct, it can actually only supply around 100mA.

That means my circuit, which was designed in such a way that it required the regulator to supply 335mA needs way more power than the onboard regulator can supply.

Now, thankfully, I made another mistake in my original calculations, and I specified that the ATmega itself would draw 120mA. That brings the actual total for the original circuit down to 265mA.

I was also running some illuminated touch switches in my original design off the 5v pin, with each drawing 20mA. If I reduce each of those to 10mA, now I've got:

  • 20mA from +5v pin for illuminated touch switches.
  • 30-50mA to power the microcontroller.
  • 175mA from the Arduino's pins to run an 8x8 led matrix, piezo speaker, and a couple leds wired in series.

That brings the total down to 240mA, 200-220mA of which will actually need to flow through the Arduino's circuitry.

Getting a new PCB made isn't an option because I'm out of time, so I've had to figure out a way to modify the ones I had made to solve this dilemma.

Here's my circuit:

The solution I've come up with is this:

  1. Cut the trace from PwerSw to the Raw pin on the Arduino.
  2. Run a jumper from the +9v PTH on the right side of the board to the Vcc PTH above the ULN2803A.
  3. Wire a 7805 5v regulator to my 9v battery pack, and wire its ground and output to one of the 9v connectors.
  4. Add a 0.1uf decoupling capacitor across the other 9v connector, and a 47uf capacitor on C1 if need be.
  5. I think I can probably get away without a capacitor on the input of the 7805 since I'm running off batteries, but if I need to stick one on there that'll be easy enough.

Now my question to you is this... will this work? Can the Pro Mini handle 200mA, seeing as it was only designed with a regulator that could put out 100mA?

Also, if I'm not mistaken the Arduino's Vcc and Gnd pins can only handle 200mA each. I assume that includes what the processor needs as well? In which case I need to subtract 50mA for that, leaving me with, at most, 150mA to source from the Arduino's pins?

I guess if that's the case, it'll have to do. At least then I might have 100mA with which to power my array.

2

I think I can probably get away without a capacitor on the input of the 7805 since I'm running off batteries

No you can't you need it to drop the input impedance which is even higher on a battery.

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.

3

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.)

http://www.fairchildsemi.com/ds/LM/LM317.pdf

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. :slight_smile:

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

4

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:
http://www.micrel.com/_PDF/mic5205.pdf

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.

5

Why do I need to drop the impedance?

To prevent it from oscillating and improve the transient response. While different manufacturers offerings do require different values for long term stability you should use them. You can even get the situation where they don't oscillate when you have an oscilloscope connected to them to see if they oscillate and then start when it is removed. I know I have seen it.

From the data sheet of the regulator http://www.micrel.com/_PDF/mic5205.pdf:-

Input Capacitor
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.

The tracks are not about to burn out because you have a bit more power running through them.

6

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

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.

Or are my calculations way off?

Haven't seen you calculations.

7

I posted the calculations in this earlier thread:
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1276895551/0#4

I read your tutorials on power and I think I did the calculations right, but I'm not certain.

8

From the data sheet of the regulator http://www.micrel.com/_PDF/mic5205.pdf:-
Quote:
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.

9

I think I did the calculations right

yes they look right to me. :slight_smile:

10

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. :slight_smile:

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.

[edit]

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

External regulator it is then.