Can I supply a large current load to the 5V pin and a small load to 3.3V pin

So when using the Arduino UNO and powering it externally via the barrel jack with a 9V 1.5 A source, can I apply a large current load to the 5V pin (such as 800 mA) without seeing any spikes in current on the 3.3V pin? Essentially, I would be powering a wireless transceiver such as the Longruner NRF24L01 off of the 3.3V pin, and then I would apply a larger load of resistors to the 5V pin (for a university test) that would likely draw around 800 - 1000 mA. Before conducting this test, I wanted to be sure that I wouldn't damage the transceiver connected to the 3.3V pin, as I am aware that the 3.3V pin feeds off of the 5V rail. A large current surge on the 5V pin wouldn't also affect the 3.3V pin right?

Well, the regulator on the board is rated for 1A max, and can't dissipate nearly enough heat to supply 1A from 9v input (ie, that's asking it to dissipate two watts), potentially either damaging the regulator or causing it to detect that it's overheating and shut off.

Taking more current from 5V does not "push" more current into the 3V circuits.

with a 9V 1.5 A source

The 9V is fixed (and hopefully regulated). The 1.5A rating is the maximum you can get. The actual current depends on the load ([u]Ohm's Law[/u]). With nothing connected, the voltage is still there but there is no current.

A car battery can supply hundreds of amps into the starter but if you connect a little LED (with the usual series resistor) you'll get a few milliamps into the LED just like with any other 12V power source. if you start the car and draw a bunch of current from the battery, nothing happens to the LED (except it might dim if the voltage drops).

DrAzzy:
Well, the regulator on the board is rated for 1A max, and can't dissipate nearly enough heat to supply 1A from 9v input (ie, that's asking it to dissipate two watts), potentially either damaging the regulator or causing it to detect that it's overheating and shut off.

If powering this for long periods of time sure, but would I be able to do this for a short period of time? Let's say 3-5 seconds.

but would I be able to do this for a short period of time? Let's say 3-5 seconds.

No.

With the regulator in mind then, what is the absolute maximum current I can draw from the 5V pin when being powered from an external power supply?

Why not wire up a buck converter (available cheaply on ebay, powered from Vin) and use that to get the 5v for this power hungry external thing?

This testing is actually more for academic purposes than anything else. Before being integrated into a larger system, I have to prove what the Arduino UNO can actually handle as far as a current load, while taking in this external voltage supply of 9V 1500mA maximum.

I have to prove what the Arduino UNO can actually handle as far as a current load

Then asking in the forum is a waste of time. Just do the experiment.

You will find that the Arduino cannot supply "much" current, without overheating the on board regulator.

If you connect 9volt to the DC socket of an Uno, then about 8.3volt is left on the input of the 5volt regulator.
Because of the ~0.7volt drop of the reverse protection diode between DC socket and V-in.

If you assume a max continuous long-term dissipation of about 1watt in a normal <=25C temp environment before shutting down (educated guess), then total current from the 5volt pin PLUS the 3.3volt pin is about 1watt / (8.3volt - 5volt) = 0.3A = 300mA.
Take 50mA from that for the Arduino itself, and you have about 250mA left.

Don’t know what your radio is using, but an Uno can supply 150mA max from the 3.3volt pin.
Current from the 3.3volt pin flows through the 5volt regulator, so “250mA - the 3.3volt draw” is available from the 5volt pin.
Short bursts of higher current draw might be possible, but you’re in uncharted territory.
Leo…

For calculations read Power & Heat

and

http://www.thebox.myzen.co.uk/Tutorial/Power_Examples.html

I hope I get a good mark for your assignment. Will you give me credit in the write up?