I have an Arduino Uno drawing power from a 9V wall adapter plugged into its power jack. I have a 12V LED strip connected to the VIN pin of the Arduino (positive side to the VIN, negative to GND, of course) and the strip won't light up. I'm checking the voltage on the VIN and it's about 9V, which should be enough, because I'm testing the same strip with a 9V battery and it works OK.

I'm guessing that the issue is with the current, i.e the amperage output of the VIN pin, which is about 1A. In the specs of the LED strip it says it has input current of 400mA. Is this the issue? What should I do?

  • \$\begingroup\$ VIN has nothing to do with Arduino. VIN is output of wall adapter right? \$\endgroup\$ – User323693 Jul 9 '15 at 10:12
  • \$\begingroup\$ It's a 12V LED strip so you should not expect it to work on anything below 12V. A fresh 9V battery can be up to 10 V, which might be just enough to light up the 12V LED strip. The 9V adapter will be much closer to 9V, 9.5 V would already be on the high side (assuming good quality adapter). Do you have a multimeter ? Then measure. If not: buy or borrow one and then measure. \$\endgroup\$ – Bimpelrekkie Jul 9 '15 at 10:21
  • \$\begingroup\$ @Umar:No, there is a pin labeled Vin on the Arduino UNO board, that's supposed to output the same voltage as the power jack receives(in this case 9V) and a current of 1A. See here arduino.cc/en/Main/arduinoBoardUno \$\endgroup\$ – damjandd Jul 9 '15 at 10:21
  • \$\begingroup\$ Yes and this Vin pin is wired STRAIGHT to the jack plug you're using to connect 9V. The 1A is just the maximum current that is allowed to go through this connection. Although this is on the Arduino board, it has nothing to do with the microcontroller. Heck, if you remove (desolder) the Atmega chip, this conection will still be working ! \$\endgroup\$ – Bimpelrekkie Jul 9 '15 at 10:29
  • \$\begingroup\$ what is the rating of the wall adapter you are using? \$\endgroup\$ – User323693 Jul 9 '15 at 10:42

Typical dumb led strips that are 12V will work from ~ 10V to 14V, based on the resistor that is used. A 9V battery may be enough for a while for a few segments at best.

The Arduino VIN is NOT connected directly to the power jack input. There is a reverse protection diode D1 between Power Jack and VIN. That will result in a 0.7 Volt drop from the power supply, enough not to turn on the led strip segment connected. (Not to mention any transistor or mosfet voltage loss)

Finally, a non regulated power supply can be putting out a higher or lower voltage if not matched to the current it's rated for. You should be measuring the VIN pin under load for a true measurement. A regulated supply would not have this problem.

As you have switched to a 12v supply you no longer have these issues, but keep in mind the Uno has a linear regulator, so drawing too much current on its 5V rail can cause it to over heat.

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