New to this - DC current spec question

Hello everyone!

This is the first time I'm using an arduino. I've got the UNO r3 Board. All I've got to power it is a 9V power supply that will supply a max of 100 mA to the board. The specs for the UNO r3 say the DC current for the I/O pins is 40 mA. Would I be supplying it with too much current if I plugged it in?

First you need a better understanding of current.

1) The Specifications you cited are maximum capability not requirements. a) The adapter you have can only supply up to 100ma. If you connected nothing to it the current would be 0. If you connected a 10k resistor the current would be I = E/R = 9/10000 = 0.00009 amps or 0.9 milliamps.

b) the outputs of the arduino can only supply up to 40 ma. The same analogy applies, the higher the resistance of the load the less current will be required.

The best I can tell, the Arduino Uno board with nothing connected to the pins requires about 40 to 50 ma. If you use your power adapter you will only be able to draw 50 ma from the output pins. This is all the output pins added together.

When you study electronics the first thing you learn is [u]Ohm's Law[/u] which describes the relationship between voltage, resistance and current. (BTW - "I" represents current, and I don't remember why.)

Resistance is "the resistance to current flow". More voltage = more current, less resistance = more current.

We usually don't know the resistance (or impedance) of the Arduino, or a motor, etc., and it can change depending on how many LEDs are on, or the load on the motor, etc. But, we often have a maximum current spec for the device. Ohm's Law is a law of nature, and it's always true but it's not always easy to apply directly.

And there is the old water-flow analogy - Voltage is similar to water pressure, electrical current flow is similar to water flow, and resistance is the resistance to water flow. i.e. A fat pipe has low resistance and lots of water flows. A skinny pipe has high resistance and less water flows.

But, there are a couple of big differences - If you cut a water pipe you get zero resistance and water flows-out everywhere. If you cut a wire you get infinite resistance and no electrical current flows. And, zero resistance in a "water circuit" doesn't cause any problems. Zero (total) resistance in an electrical circuit is a "short circuit" and stuff gets fried!

This is over-simplified, but voltage tends to be constant. If you live in the U.S. there is 120VAC at the wall outlet all of the time, whether anything is plugged-in or not. If you plug-in and turn-on a light, current flows. If you plug-in a hair drier or toaster, more current flows. If you plug-in two toasters, too much current will probably flow, the circuit breaker will blow, and the voltage drops to zero until you reset the breaker.

"Constant voltage" really means that the voltage is (largely) independent of the load... If you write a "1" from an Arduino output pin, you get approximately 5V and that doesn't change if you connect an LED with the appropriate current-limiting resistor.

If you try to get too much current from your 9V power supply, the voltage will drop and unpredictable things can happen... Maybe an internal fuse will blow, or something will burn-up inside. Or maybe there will be no permanent damage and everything will return to normal you remove the load.


Perhaps the answer is simpler,

If you have a power supply of current capacity greater or equal to the consumption of the device, you can use, as long as the voltage is compatible with the voltage range that the device can receive.

The Arduino board has other components, such as the serial USB adapter, which will also consume some power.

There is another matter also.

If you are powering it with 9 V via the “barrel jack” or “Vin” terminal, the internal regulator on the UNO is unable to provide more than about 100 mA to the 5 V power rail in the UNO itself before it overheats and shuts down, and the processor will be using some of that current itself, so you should not expect to power more than a few LEDs or other components and low-current sensors from the 5 V terminal.

If you need to use other modules which require any significant power, you will need a proper regulated 5 V power supply of adequate rating to connect via the USB jack or 5 V terminal.