arduino power adaptor

Hi everyone,

When I power my Arduino NG with an DC adapter many times the script does not start. The power led is ON but the program does not start running. I have to try many times until (usually) it works. That does not happens when powered through USB.

I’ve try the following adaptors and they all have the same problem:

  • 9V 1200mA 15W
  • 12V 1200mA 15W
  • 12V 2A ?W

Any idea of what can cause the problem? To many mA??

Thanks a lot in advance.



You can't have too many mA, that is a maximum rating, how much it actually is is governed by the circuit you connect it to. The problem is likely a lack of smoothing and/or decoupling, both of which can be cured by adding capacitors to the circuit. I'd suggest a 100-1000uF electrolytic and a 100nf ceramic across gnd and vin of the Arduino. Watch the polarity on the electrolytic, its exact value depends on how much current your arduino is pulling and how much ripple you can get away with. It won't hurt anywhere in the range I suggested.

Please don't post threads multiple times.

Are the adapters you are trying have DC voltage output and not AC voltage output? Your list just says volts. It should be clearly labled on the adapters if they are DC or AC output.


I don't think he'd get them to work at all if they were AC lefty.... ;)


Thanks everyone for the quick answers. I am sorry for having posted twice.

Obviously I used an adaptor with DC output ;-)

What you say about the capacitors looks complicated to me. Maybe buying a "better" power source that renders a "softer" output would work (?)

Thanks again.


I don't think he'd get them to work at all if they were AC lefty....

the protection diode can also function as a half wave rectifier, thus not cooking but probably not working right either

This sounds more like the serial port hanging issue from back in the NG days.

Hello everyone,

I tried the 10K resistor brindging RX and GND, as explained in the post pointed out by Digger

and the problem remains...

I forgot to say that everything goes fine with a battery (9V). So the problem arises only with a DC adaptor (??)


When you connect with a 9v battery - where do you connect it to? the power jack or Vin?

Are your power adapters regulated?

I have found that if you underpower the Arduino, the power LED lights but the ATMega won't run. Do you have a voltmeter? Measure between the 5V pin and ground, it should be very close to 5.00 volts.

Hello again,

Daveg, I connect the battery to the power jack.

Things, I will check the voltage of the +5V pin. Where can I found a “regulated” power adaptor. Do I ask just for that in a store?

Thanks a lot!