Arduino Forum

Forum 2005-2010 (read only) => General => Uno Punto Zero => Topic started by: nettigo.pl on Nov 16, 2010, 08:56 pm

Title: 3.3 DC current for Uno and Mega 2560
Post by: nettigo.pl on Nov 16, 2010, 08:56 pm
Hi!
Is there a reason that specs on Hardware page say that maximum current for 3.3 DC pin on Uno and Mega 2560 is 50 mA not 150 mA? Using LP2985 as 3.3V regulator should provide 150 mA...

Since this is my first post ;) I can not add links to hardware pages :) but on both is:
DC Current for 3.3V Pin      50 mA

arduino.cc/en/Main/ArduinoBoardMega2560
arduino.cc/en/Main/ArduinoBoardUno
Title: Re: 3.3 DC current for Uno and Mega 2560
Post by: mellis on Nov 17, 2010, 05:50 am
You have to worry about power dissipation as well as current.  Here's what Gianluca has to say: "With 150mA of load at 12V the regulator will go in thermal shut down (or burn) because it will dissipate 1.3W instead of the 0.45W it could not exceed without a proper heat sink

At  12V it could easily provide 50mA (75mA @ 9V)

it is the same for the Mega2560"
Title: Re: 3.3 DC current for Uno and Mega 2560
Post by: nettigo.pl on Nov 17, 2010, 09:00 am
Thank You for a clarification. That makes sense.
Title: Re: 3.3 DC current for Uno and Mega 2560
Post by: westfw on Nov 23, 2010, 01:17 am
Um.  The 3.3V regulator comes AFTER the 5V regulator, so it would never dissipate more than about 0.3W regardless of input voltage to the board.  (now, the 5V regulator could get in trouble, but it's still spec'ed at a much higher current.)
Title: Re: 3.3 DC current for Uno and Mega 2560
Post by: nathan7 on Dec 06, 2010, 01:18 pm
The dissipation at 150mA assuming it comes after the 5V regulator is 0.15*(5-3.3)=0.255W.
Seeing that the max dissipation at 25C is (125-25)/225[ch8776]0.444W (calculation from datasheet), you can safely pull 150mA.
Title: Re: 3.3 DC current for Uno and Mega 2560
Post by: Lilirama1 on Dec 20, 2010, 05:04 pm
Thx for posting that I had the same question, I just thought it was stupid and had been asked a million times,So Thank you.





        MakerMann
Title: Re: 3.3 DC current for Uno and Mega 2560
Post by: smeezekitty on Jan 07, 2011, 09:38 am
The data was carried over from when arduino used the FTDI chip to provide the 3v.
Title: Re: 3.3 DC current for Uno and Mega 2560
Post by: retrolefty on Jan 07, 2011, 05:05 pm
This can be a dilemma for people designing and building commerical shields for the standard arduino pin layout. If a shield is designed and built to require more then 50ma on the 3.3v pin and is plugged into any arduino board that uses a FTDI chip rather then the newer Uno or mega2560, then there is real possibility of damage to the FTDI chip on the board. So vendors will have to be awful careful how they document their shields requirements or there may be a lot of not happy users.

The new extra 3.3v current capacity for the uno is nice, but there must be cautions understood and obeyed.

Lefty