I'm currently working on an Arduino UNO based 8x8x8 mono-color LED cube with TLC5940 ICs to support PWM dimming and at the moment I'm thinking about how to construct my case and how to power the project.
One of the requirements is that the cube should be powered only by a wall-wart power supply with at least 2A@5V (64 LEDs per layer at 20mA each = 1,28A + a little more for the rest of the electronics). (The voltage level for the supply is being discussed further below.)
While the cube is powered by the wall wart supply I nevertheless want to be able to plug in a USB cable to the cube to connect it to a PC to reprogram it. This should be possible while the cube is powered without plugging out power, plugging in USB, programming, plugging out USB, plugging in power to see the changes or to remove the Arduino from the case. This could then also be used to stream visualization data from the PC to the Arduino in real time.
Here the Arduino is powered by the PC via USB while the peripherals are powered via a bench power supply set to 5V. Since the bench supply is not isolated I connected the grounds of the Arduino (and therefore the USB) and the bench power supply to have a common reference. Then the Arduino communicates with the peripherals via normal signal lines. In this configuration the Arduino consumes less then 100mA and everything works fine (at least no smoke so far ;)).
A power supply should be connected to the cube powering the Arduino and the peripherals at the same time to enable complete functionality. If the cube is powered I want to be able to ADDITIONALLY plug in a PC USB connection to reprogram the Arduino or the stream serial data, but the cube has to work without the USB connection as well.
The USB plug in the wall of the case would be extended to the USB socket on the Arduino board, therefore to power the Arduino using the the power supply there are only two options left:
- VIN or or power barrel jack
- the 5V pin
From what I read on the internet I understood that it is really unadvisable to connect USB and external 5V via the 5V pin at the same time (due to two voltage supplies "fighting" each other as it was said). Therefore I would be left with option 1. This however requires the voltage supply to be 7-12V and this in turn need to be stepped down separately for the peripherals.
The questions that I currently have due to this situation are as follows:
Is my current development set up with the connected GNDs of the USB powered Arduino and the external power supply safe or I was just lucky so far that nothing was damaged?
What type of power supply is applicable for the final project? I would tend to a switched mode supply due to size and efficiency. May there be problems with high frequency ripples for the Arduino and other ICs stability or will this work fine?
Will my plan work to power the Arduino with 7-12V via the VIN pin (or the power barrel jack) to get the cube running while at the same time being able to communicate with the Arduino from a PC via USB without letting some smoke out or having other problems?
Is it advisable to get a 7V supply instead of 12V or to reduce 12V to 7V before feeding it into the Arduino in order not to overload the onboard voltage converter? (I read this tip in some forum, but I'm not sure if this is needed in every case since the input of the Arduino is rated 7-12V)
(possibly I want to have 12V available in the case at some point instead of only 7V to be able to use an illuminated pushbutton at the front of the case, those are most often powered by 12V)
What will be the best solution to step down the voltage from 12V to either 7V or 5V? Buck converters for efficiency (what about noise?) or rather linear regulators? Or maybe something else entirely? Should I buy those premade or build them myself? (in the end I'm planning to make a PCB for the whole project, so I guess I could fit the parts on there as well)
Answers my questions as well as additional inputs/ideas are very welcome!