I found out the other day that the Mega's voltage regulator only out's about 800mA... Is there a reason I shouldn't switch this voltage reg with something like the LM323T, which out's 3A (by switch I mean in the schematic while designing my own knock-off)? Would this fry the atmega or just add some extra "usable" current to the 5v line?
From what I know there is no reason it would fry the arduino because the chip will consume only the amount of current it needs.
And this change will give you more juice at the 5V line.
The on board regulator supplies all the current that the board would ever need for it's own use. If you just want more +5vdc current for externally connected components and devices, then it's better to supply that with an external power source and a beefer external +5vdc regulator.
Lefty
retro, I can't tell if your saying that it's safe to do what I said or not. I am short on space, so I don't want to have another regulator on the board when I can just use a more powerful one for the entire 5v line, including to power the atmega itself.
Well having a more powerful +5vdc regulator on-board regulator is not unsafe on it's own, but I just don't think there is any real useful advantage for upgrading the on-board regulator. A more powerful regulator is going to require a larger heat sink to reach it's max rated current anyway, so on board room will be a factor. Also there is a one amp polarity protection diode wired between the board's external power connector and the on-board regulator so that has to be upgraded also if you wish to keep that protection.
It's better if you explain what problem you are trying to solve specifly. What external devices or components are you wiring to the Arduino?
Also you realize that any increase in +5vdc current you gain with modifications won't be avalible when you are powering the board with USB connection only?
Lefty
I've removed the USB connection, so that's a non-issue. I have also removed the power-source-selector, so the only power can come from the dc power jack.
Basically, the mega will have to power:
A gyro
An accelerometer
A compass
ladyada's xbee breakout (well, same circuit anyway)
A small motor driver
3 Servo's (plan to run these from their own power source, as each will run at about 750mA)
4+ Ultrasound sensors
GPS module (ladyada's GPS shield circuit)
Perhaps more...
That's quite the amount of sensors, eh? I want to run everything on that list except for the servos from the 5v line going through the atmega.
There are a few reasons not to switch it but first you need to work out how much power you are dissipating the regulator.
-
If the MEGA has an SMD regulator then all of the heatsinking comes from the area of copper under the tab. If you start dissipating more power for the same heatsink area the regulator will run hotter.
-
Having a lower current limit is useful because it limits current in the event of an external fault.
-
The higher current regulators typically have a higher dropout voltage which
increases the power dissipation and the minimum input voltage.
(* jcl *)
www: http://www.wiblocks.com
twitter: http://twitter.com/wiblocks
blog: http://luciani.org
I want to run everything on that list except for the servos from the 5v line going through the atmega.
Give it a shot if you wish. I still don't think it's real pratical. What are all those devices going to mount on, some kind of mother board, breadboard,etc ?
If so there would probably be room for a TO-3 size regulator to power everthing mounted to it, that way only Mega signal lines and a ground would have to wire between them.
Lefty
that way only Mega signal lines and a ground would have to wire between them.
What do you mean?
It's basically a "motherboard", but not too large in size...
I was planning to have two of these:
which come both in to-3 and to-22 forms... One for the servos, the other for the rest.
As an addon question, what is the category/type of the "polarity protection diode" between the power jack and the regulator? I will need to change that accordingly too.
Not to bombard with questions, but much of this is a new area to me, so one more thing: What is the diode exactly for? I get the concept, but what exactly would cause "backflow" of current on the board?
but what exactly would cause "backflow" of current on the board?
Connecting a battery or wall wart transformer to the external power connector with the polarity of the voltage backwards. That would normally toast the board, but the series diode acts like a check valve and only allow current to flow in one direction and blocks in the other direction. It is a simple 1 amp diode most likely a 1N4001 or similar.
Lefty
Ok, now I get it 
So I should replace it with a 3A, since that's the max current output of the voltage reg, right?
EDIT: Wait, so how could you get a negative voltage with a DC power source?
So I should replace it with a 3A, since that's the max current output of the voltage reg, right?
That logic is sound, however it's usually normally good engineering practice to select componets that will have to run at their maximum rating. However above 3 amps, diodes can get kind of large, so you have to make sure it can fit.
Lefty
EDIT: Wait, so how could you get a negative voltage with a DC power source?
Come on, think a little
A given DC power source has two output wires, one is positive and one is negitive, can you hook them up in either direction? Will your car battery work in your auto if you hook the Red and black battery cables backwards to the battery?
Grasshopper needs to slow down and think, I think ;D
Lefty
REALLY? Hooking the wires up wrong? I ruled that out. I mean, I understand it can happen and all, but I didn't think that would be the reason. I was expecting a complicated ordeal of formulas regarding rate of discharge of the battery vs. power consumption or something.
I think I'll save myself the space (and time looking for the component) and just get rid of it... I'll be damned if I end up connecting a JST plug the wrong way.
dont be surprised when you do then.
and i too think your speed is a bit unthoughtful, shortcuts are ok when you understand why, but gun ho! fk that im not dumb at best can fry alot of work or money
at worse it could catch fire, stuff happens
crack open the oldest clock radio you can find at the thrift store and you will probally see some fat nasty diodes + a "ton o junk" for a couple bucks
I just got one last week for 1.25$, had 4 chunky diodes as a rectifier, a TI clock chip and a 4 digit 2 inch 7 seven segment display
- screws, switches, 120v to 5 v low amp transformer w ac plug and some radio junk
The speed of my "unthoughtful" questions is a result of an approaching deadline as well. So please excuse the question bombardment.
There is no reason not to replace the regulator in a new design with a different regulator capable of supplying more current.
However, there is more to making a 3A power supply than simply using a a regulator chip rated for 3A. The chip you mention is still a linear regulator, and is not any more efficient than the normal regulator chip. That means that if you want 3A out of it, it will end up dissipating (Vin - 5) * 3 Watts of power. For the "normal" input of about 9V, this is 12W of heat. The fact that the 323 is rated for 3A (and 30W) means that it IS capable of dissipating that much heat, but you WILL need a substantial heat sink, and it will get warm. (such regulators tend to have built-in over-temp protection, so if you build it wrong, it'll just turn itself off. Maybe.)
IMNSHO, if you want more than the 500mA or so that the normal arduino regulator provides, you should start looking at switching regulators for their greater efficiency. You can design your board for an external 5V regulated switching power supply like http://www.bgmicro.com/index.asp?PageAction=VIEWPROD&ProdID=12673, and you can avoid all sorts of issues...