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I've connected eight 8x8 leds matrices to an Arduino, to display a scrolling banner.

When I developed it on an Arduino Leonardo powered by USB, everything worked fine.

When I moved the solution to an Arduino Pro Mini, I tried to use the onboard voltage regulator (using the RAW pin) but it only worked half of the time (the other half of the time, I only had 2V on the VCC pin of the Arduino).

Even when it was working, the Arduino was very hot after a few seconds, so I assumed the leds required too much power for the onboard regulator, and I moved to a breadboard power supply (see picture).

From left to right: The Arduino Pro Mini, the power supply and the power brick - The led matrices are on the top

That's where things start being confuse :

  • If I keep the power supply switch on and plug/unplug the power-brick to the main, it works flawlessly and I got 4.8V on the Arduino's VCC pin
  • But whenever I try to use the power switch on the power supply, the leds turn on but the Arduino doesn't do anything and its VCC pin is about 2.4V

I would like to know what I'm doing wrong, and how to prevent this kind of failure to happen.

PS: The power brick is ratted for 12V 1.5A; There are 512 leds but they are driven by eight MAX7219 and AFAIU only 64 of them can be lit at the same time

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    \$\begingroup\$ Start reading datasheets, especially those parts where it states how much power things consume and how much power they can deliver, switch etc. \$\endgroup\$ – PlasmaHH Mar 20 '17 at 10:17
  • \$\begingroup\$ I tried to do that, but it seems I failed... The issue may be that for some reason when using the switch, the leds initialize with "full on" pattern... which I took care of avoid in my program, but I didn't expect it to be the default state (still, I don't understand why they don't do this when I plug directly the power supply...) \$\endgroup\$ – LeFauve Mar 20 '17 at 13:31
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    \$\begingroup\$ You likely get into display test mode and stuck there by leaving the SOA of any of the involved devices. What you are seeing is not the issue, but a symptom, the issue is you are leaving the SOA of at least the regulator. \$\endgroup\$ – PlasmaHH Mar 20 '17 at 13:42
  • \$\begingroup\$ Thanks for the feedback. The regulator is rated for 1A which seems to not be enough (also, I failed to take into account the 7W dissipation @SamGibson mentions in his reply...). I learned a few things tonight, thanks to you guys :o) \$\endgroup\$ – LeFauve Mar 20 '17 at 14:09
  • \$\begingroup\$ 1A might be enough, but the SOA is not just a single number that needs to match up, you may need to think of it more as a multi dimensional simplex. \$\endgroup\$ – PlasmaHH Mar 20 '17 at 14:11
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Considering the number of LEDs, the current draw must be quite high, like 1-2 amps.

I assume the LED matrix needs 5V supply. It would be unreasonable to power it from a linear regulator (such as the one on your YwRobot board) because it would dissipate lots of heat. I see a SOT223 regulator on this board; if the LEDs draw 1 Amp, from 12V, it will dissipate (12V-5V)*1A = 7 watts, which is way above what a SOT220 can do. Therefore, it will overheat and shut down, or burn.

You should power it directly with a supply of appropriate voltage and current capacity.

I see from the picture that you run the high current supply through the arduino, which is definitely not recommended!

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  • \$\begingroup\$ Thanks for the reply. I had calculated I needed less than 800 mA for all the leds. I'm not sure for the Arduino but since the bigger version can be powered by USB I assumed it would be less than 500 mA. Of course, I was probably wrong... So if I understand you, I should find a 5V*2A power supply? Also I'm not sure what you mean by "run the high current supply through the arduino". The VCC of the Arduino is connected to the 5V output of the SOT223 thing. How should I connect it (assuming I don't try to power as much leds). \$\endgroup\$ – LeFauve Mar 20 '17 at 13:39
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    \$\begingroup\$ Yeah, get a 5V 1-2A cellphone charger. And connect GND/VCC to LED display directly, without routing high current through arduino. \$\endgroup\$ – peufeu Mar 20 '17 at 13:48
  • \$\begingroup\$ I think I already do that (the black & white wires are GND/5v from the power supply to the Arduino GND/VCC and the black & red wires are the same GND/5V from the power supply going directly to the LED display). I apologize for the quality of the picture though, which makes it hard to see which cables go out of the power supply. \$\endgroup\$ – LeFauve Mar 20 '17 at 13:58
  • \$\begingroup\$ OK! I didn't see it on the pic. Then this part is OK. \$\endgroup\$ – peufeu Mar 20 '17 at 14:01

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