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Using Arduino => Project Guidance => Topic started by: ScottAS on Feb 13, 2013, 09:51 pm

Title: Maximum Current Draw from Arduino Mega 2560
Post by: ScottAS on Feb 13, 2013, 09:51 pm
Hi,

I often hear that 500ma is the maximum that can be drawn (due to the fuse on the USB VCC line), but looking at the at the datasheet for the Atmega16U2, 200ma is the absolute maximum rating for it's VCC output (which is what connects to the Arduinos +5V net).

So is 200ma the absolute maximum, if powering by USB, or am I missing something?

Thanks!
Title: Re: Maximum Current Draw from Arduino Mega 2560
Post by: CrossRoads on Feb 13, 2013, 10:14 pm
USB can only source 500mA, limitation of the PC you connect to.
PTC fuse enforces that.
Barrel jack has series diode with 1A current limit.
I just posted what Atmel said the '2560 can source/sink thru its VCC/GND & IO pins - 800mA.
Title: Re: Maximum Current Draw from Arduino Mega 2560
Post by: Grumpy_Mike on Feb 13, 2013, 10:30 pm
Quote
So is 200ma the absolute maximum, if powering by USB, or am I missing something?

What you are missing is that there are many maximum currents depending where you draw them.
From the processor itself this is taken from the sum of the current pins are sourcing and sinking. However do not mix up this with the current you can draw from the 5V line on the arduino board and it is subject to the limit you can get from the supply.
Title: Re: Maximum Current Draw from Arduino Mega 2560
Post by: ScottAS on Feb 13, 2013, 10:35 pm
CrossRoads: I assume you are getting the 800ma number from 4 VCC pins * 200mA/pin?

However, on the Arduino Mega 2560, everything is powered by the single VCC supply pin on the Atmega16U2 (when powered via USB), this pin also has the maximum rating of 200mA, but there is only one of them. This leads me to conclude that the maximum current draw for the Arduino must only be 200 mA. Or am I doing something wrong here?

Grumpy_Mike: I am mostly referring to the 5v line, I'm not drawing much from the I/O pins. If the 5V came directly from the supply, I'd agree, but it doesn't for the USB case... It comes out the 200mA limited pin on the 16U2.

Title: Re: Maximum Current Draw from Arduino Mega 2560
Post by: CrossRoads on Feb 14, 2013, 12:52 am
"This leads me to conclude that the maximum current draw for the Arduino must only be 200 mA. "
Why? You can draw 500mA from the USB connector. It provides current in parallel to:
the 16U2
the 4 VCC pins of the 2560
the 5V to 3V regulator
the op amp that does auto power switching
the 5V on the ICSP header

So if current comes from the USB port, the Arduino is limited to 500mA.
If current comes from the barrel  jack, the Arduino is limited to 1A.
If current is supplied to the board via the 5V power header, you bypass the reverse protection diode and the Arduino is limited to what the pin will support before it melts.
Title: Re: Maximum Current Draw from Arduino Mega 2560
Post by: retrolefty on Feb 14, 2013, 01:46 am

CrossRoads: I assume you are getting the 800ma number from 4 VCC pins * 200mA/pin?

However, on the Arduino Mega 2560, everything is powered by the single VCC supply pin on the Atmega16U2 (when powered via USB), this pin also has the maximum rating of 200mA, but there is only one of them. This leads me to conclude that the maximum current draw for the Arduino must only be 200 mA. Or am I doing something wrong here?

Grumpy_Mike: I am mostly referring to the 5v line, I'm not drawing much from the I/O pins. If the 5V came directly from the supply, I'd agree, but it doesn't for the USB case... It comes out the 200mA limited pin on the 16U2.

No it doesn't, The 16U2 chip get it's Vcc from the same 5 volt bus that the 328P does. USB 5 Volt current is not limited in anyway by the 16U2 chip, just the 500ma thermofuse.

Lefty
Title: Re: Maximum Current Draw from Arduino Mega 2560
Post by: Grumpy_Mike on Feb 14, 2013, 08:38 am
Quote
It comes out the 200mA limited pin on the 16U2.

ScottAS what makes you think that?
Title: Re: Maximum Current Draw from Arduino Mega 2560
Post by: ScottAS on Feb 14, 2013, 04:08 pm
Sorry guys, you are correct. For some reason I had it in my mind that power came in through UVCC on the 16U2, was regulated there and came out of it's VCC port to supply the 5V line. Seems silly, but this is the only thing that made sense to me, because the 16U2 is the only thing I saw USBVCC connected to. Now I see that it's connected to the MOSFET down below as  well...
Title: Re: Maximum Current Draw from Arduino Mega 2560
Post by: retrolefty on Feb 14, 2013, 04:19 pm

Sorry guys, you are correct. For some reason I had it in my mind that power came in through UVCC on the 16U2, was regulated there and came out of it's VCC port to supply the 5V line. Seems silly, but this is the only thing that made sense to me, because the 16U2 is the only thing I saw USBVCC connected to. Now I see that it's connected to the MOSFET down below as  well...

Still not correct. You must be looking at a different schematic then I am? The 16U2 chip has it's Vcc on chip pin 4 and it clearly wires to the board's +5V bus, not to the USBVCC bus. The USBVCC voltage from the USB connector goes only to one place, the T1 FET switch.

Lefty
Title: Re: Maximum Current Draw from Arduino Mega 2560
Post by: ScottAS on Feb 14, 2013, 04:52 pm

Sorry guys, you are correct. For some reason I had it in my mind that power came in through UVCC on the 16U2, was regulated there and came out of it's VCC port to supply the 5V line. Seems silly, but this is the only thing that made sense to me, because the 16U2 is the only thing I saw USBVCC connected to. Now I see that it's connected to the MOSFET down below as  well...

Still not correct. You must be looking at a different schematic then I am? The 16U2 chip has it's Vcc on chip pin 4 and it clearly wires to the board's +5V bus, not to the USBVCC bus. The USBVCC voltage from the USB connector goes only to one place, the T1 FET switch.

The schematic you linked shows USBVCC connecting directly to UVCC on pin 31 of the Atmega16U2. Yes, its VCC is connected to the 5V bus.

EDIT: I'm still confused about this though. If only USBVCC was connected to UVCC, and VCC was disconnected from the 5V bus, would 5V still come out of the VCC pin?
EDIT2: Ok, I think I see on the datasheet that 3.3V will come out of VCC if it is not pulled up by another source. Still a bit muddy to me though.
Title: Re: Maximum Current Draw from Arduino Mega 2560
Post by: CrossRoads on Feb 15, 2013, 03:27 am
UVCC powers the on-chip 3.3V regulator. VCC and Avcc connect to +5V and power the rest of the chip.
See page 3 of the datasheet
http://www.atmel.com/Images/7799S.pdf

More docs here
http://www.atmel.com/devices/ATMEGA16U2.aspx?tab=documents