Hey, I have a quick question. I have an AC to DC adapter that outputs 16 volts and 4 amps. This page says that it should be at least 500 mA, but there wasn't a cap. How high is too high regarding the amps?
Thanks,
marco
The question doesn't make any sense if you understand electronics. The current is determined by the load , not the source. The "cap" as you put it for your adaptor is 4 A according to you . Assuming the output is actually 12V and not more , it should be ok to use because the arduino is not going to draw more than 55 mA and cannot supply more than 800 mA through it's onboard regulator. There is no advantage to using an adaptor rated for 4 A but also no reason you can't.
The current depends on the load... If you connect an LED and appropriate current-limiting resistor to a car battery you'll get about 20 milliamps. If you connect a car starter you might get 300 Amps.
If you connect your 4 Amp power supply to the same LED circuit you'll get the same ~20mA and if you connect a car starter to your power supply you'll draw too much current and the voltage will drop (and the power supply will either shut down or burn out).
[u]Ohm's Law[/u] describes the relationship between voltage, resistance (or impedance), and current. In summary - More voltage = more current, and more resistance means less current. ("Resistance" is the resistance to current flow.)
With a dynamic/active circuit like the Arduino, we don't know it's "resistance", but if you measure the current under some specific conditions the we can calculate the effective resistance under those particular conditions.
So it's safe to use?
Thank you for the extra information by the way, I definitely need to read more on how this works as I clearly have no idea how this works.
16volt is too much for most Arduinos, although the Uno could theoretically handle 16.7volt on the DC jack
(47uF/16volt input cap and 0.7volt reverse protection diode drop).
The onboard regulator has to knock it all back to 5volt, could get very hot, and shut down (if you're lucky).
You can't power anything else from the 5volt pin at this high input voltage.
Maximum current rating of the supply is irrelevant. The Arduino only uses what it needs.
Minimum current rating has to be higher than what the Arduino plus sensors LEDs etc. use.
A regulated 7.5volt/1A supply with a DC plug is best, but 9volt supplies are more common.
Another way to power the Arduino is with a 5volt phone charger, connected to the USB socket.
Leo..
Hmm... I see. Well I do have a portable phone charger so that could be of use. I think I'll play it safe and steer away from the adapter.
The adapter is actually for rechargable batteries(aa and aaa), but they stopped selling them because they'd ruin the batteries. But it only took 15 mins to charge.
chummer1010:
The adapter is actually for rechargable batteries(aa and aaa), but they stopped selling them because they'd ruin the batteries. But it only took 15 mins to charge.
Riiiiiight! 
You saw that coming , right ?
raschemmel:
You saw that coming , right ?
I mean this is a very old Energizer product, I got this maybe 5 years ago. Yes, I did see it coming a other chargers did take hours.
It looks like I'll have to buy something online. My portable phone charger only outputs 5V / 1000mA
Actually, that comment was directed at Paul,
but Getting back to your OP, the 16 V exceeds the maximum input for an external dc supply plugged into the barreljack.
Wawa:
A regulated 7.5volt/1A supply with a DC plug is best, but 9volt supplies are more common.
Well, if it is going to be regulated - such as most phone chargers with a USB outlet, or in fact any other USB-type supply, you supply the 5 V directly to the 5 V terminal on the Arduino. That is the most sensible way and of course, the most efficient.
What do you mean the 5V terminal?
Generally, the one with "5V" written next to it.
Paul__B:
Generally, the one with "5V" written next to it.
Isn't the 5V pin an output? Don't you use the Vin pin for that?
Isn't the 5V pin an output? Don't you use the Vin pin for that?
Yes , it's an output. So what ?
It's connected to everything on the board that needs 5V. That's all that matters.
raschemmel:
Yes , it's an output. So what ?
It's connected to everything on the board that needs 5V. That's all that matters.
If I connect my 9V battery to 5V and ground, how do I send electricity to a breadboard?
Reread what Paul said:
Well, if it is going to be regulated - such as most phone chargers with a USB outlet, or in fact any other USB-type supply, you supply the 5 V directly to the 5 V terminal on the Arduino. That is the most sensible way and of course, the most efficient.
Where did he say anything about connecting 9V to the 5V terminal ?
If you absolutely must use a 9V adaptor then of course it needs to be connected to the external dc barreljack (Vin) and NOT the 5V pin. Paul's comment was meant to mean if you are going to use an external wall wart (ac/dc adaptor) you should choose one that outputs 5V, not 9V. How expeensive are they anyway ? Any voltage abobe 5V has to be disipated as heat and is wasted energy. That's why he suggested a 5V adaptor. If you are going to use a 9v adaptor is must be plugged into the barreljack or connected to Vin.
chummer1010:
If I connect my 9V battery to 5V and ground, how do I send electricity to a breadboard?
If you connect your 9V battery to 5V and ground. you should seriously give up on electronics and take up knitting instead. That would be much safer.
No, wait! Knitting needles are kind of sharp, aren't they? 
If I connect my 9V battery to 5V and ground, how do I send electricity to a breadboard?
This question is just plain scary...(this is wrong on so many levels...)