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Topic: The USB Port - Full of magic, or no? (Read 173 times) previous topic - next topic

keith204

Arduino Nano input voltage is specified as 7-9V.  However, 5V USB works well. 

My project uses 2 Arduinos communicating via I2C to split processing timing.  Currently, I plug the prototype into 2 USB ports for power.  I believe the power requirements are low enough to use 1 USB port.  So, I'm trying to figure out how to simplify. 

I have some micro USB ports on hand, so I was considering using that to split the output to the two Arduinos through the VIN pins.  But then I remembered the input voltage requirement, which doesn't seem to make sense since USB is 5V.

So, I guess there are 2 questions:
1) Why can the USB port get away with 5V, where the VIN needs 7-9?
2) If you have general advice for powering my project, I would be happy to hear it. 

I don't really feel like spending a lot of money or hassle to eliminate one wire, since I can fairly easily just plug in two USB cables.  But, if simplifying is relatively easy/cheap, I'd like to.


Arduino Nano One
  • Ulblox Neo 6m GPS
  • 1 green LED @10mA
  • 2x tm1637 4-digit displays
  • inexpensive rotary encoder (assumption that current consumption is minimal)


Arduino Nano Two
  • 8x WS2812B - never more than 1 color at once, so assume 20mA x8 = 160mA round to 200mA



Graynomad

The USB voltage feeds into the board downstream of the VIN regulator, at the 5v stage.

I think you can power the 2nd board by running a wire between the 5V connection on the socket strips, although I haven't looked at the schems for ages to see if the Nano's regulator can hand being back fed.
Rob Gray aka the GRAYnomad www.robgray.com

Wawa

#2
Aug 29, 2016, 01:59 am Last Edit: Aug 29, 2016, 05:08 am by Wawa
The USB voltage feeds into the board downstream of the VIN regulator, at the 5v stage.

I think you can power the 2nd board by running a wire between the 5V connection on the socket strips,
True.
You can link the 5volt pins (and grounds) of the two Nanos, and supply one via the USB socket, but...
The Nano has a <500mA rated USB backflow diode. More current drawn by both Nanos will fry that diode.
Leo..

PieterP

In case you are wondering how we know all this: it can be found in the schematic on the Arduino product page.

INTP

If you have a trusted regulated 5V source, you would plug that into the 5V pin.

The reason VIN needs more than 5V is because all regulators drop some voltage, and it explicitly states that it can't guarantee a steady 5V source if it isn't fed 7V+

MorganS

In this case the "trusted 5v source" can be the 5v pin on the other Arduino.
GoForSmoke: "What GShield? You never mentioned a shield."

keith204

In this case the "trusted 5v source" can be the 5v pin on the other Arduino.
Ooooh that makes sense...

Am I OK with current?  The phigixxx pinout says 200mA max current for entire package.  That actually seems a bit low, but I am in no position to challenge it.

Usign approximations, if B consumes 50mA when running, +160mA when all the LEDs of a single color are on (will be very rare), that's 210mA already without even considering the power usage of A and A's components.

Am I interpreting the 200mA note correctly?  It sure seems low.




PieterP

Am I interpreting the 200mA note correctly?  It sure seems low.
It means that the total current drawn from or sunk to the Nano's microcontroller cannot exceed 200mA, so this is only the current going through the I/O pins (and reset).
The current that can be drawn from the 5V pin is limited to 500mA by the on-board voltage regulator, or by the polyswitch (resettable fuse) when using USB power.

Keep in mind that the Arduino itself also takes power from this 5V rail (+/- 30mA), and all the power drawn from the I/O pins comes from the 5V as well.

Also, 200mA is quite a lot of current actually, @5V, that means 1W of power.

keith204

Ok, thank you.

Would it be safe to assume that the shared power rail needs to be disconnected/switched when connecting either of these to the computer via USB for uploading sketches, or is it safe to leave connected to both arduinos?

PieterP

If you connect them to the same computer, you can leave them connected, since the 5V of the computer is the same, and even if it were 2 different rails or even computers, no current could flow from 1 USB port into the other, because of the diodes in series with the 5V USB on the Arduino (D1 on the schematic). But this doesn't mean you can draw 2x500mA though, because then you have 2 diodes in parallel (see schematic):
Quote
The issue with putting diodes in parallel is that as they heat up their resistance decreases. As a result, that diode ends up taking on more current then the other diode, resulting it in heating up even more. As you can probably see this cycle will cause thermal run away causing the diode to eventually burn if you give it enough current.
(Stack Exchange)

keith204


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