The setup: Just the Uno and LCD connected so:

LiquidCrystal lcd(7, 8, 9, 10, 11, 12, 13);
  • Simple sketch to send and receive characters by monitor.
  • Powered By Laptop USB
  • LCD Back Light 8.5 mA. @ 5.04 v.
  • Experiencing some flicker on LCD and Pin 13 led.

  • Powered By 12v battery to DE-SWADJ - 10W 1 Amp Step down adjustable switching regulator @ 9 v.

  • Battery @ 12.7 v.
  • With no load, regulator is using 19.1 mA.

  • Connected battery to Uno at Vin pin and also tried using the plug w/ 9 v.

On battery only, w/ Uno and 2 Line X 16 Character LCD Display with Blue LED Backlight

  • 12.7 v. @ 65.2 mA. For 828 mW.
  • 9 v. Regulator @ 54.1 mA. For 487 mW.
  • 487 / 828 = 0.5882 59% efficiency

On Battery and Laptop USB

  • 12.7 v. @ ~ 40 mA.
  • 9 v. Reg. @ 32.6 mA For 293 mW.
  • So USB is providing ~ 487 - 293 = 194 mW.

I was under the impression that when Uno is on external power that it would not use laptop or USB power.

Question: Am I wrong? Or Is my Uno board partially damaged?

  • \$\begingroup\$ Anything that is powered by your computer's USB is going to draw less current in your circuit hence why you're probably seeing a flickering LED. This also happens when you try to charge your cell phone via your computer and so your phone warns you that because there's less current being drawn, it'll take a longer time to charge. Have you loaded any code onto the Arduino? Sometimes their IDE is capable of catching if something is wrong with the Arduino physically. \$\endgroup\$ – KingDuken Jan 13 '18 at 18:41
  • \$\begingroup\$ Perhaps this is better on SE Arduino? \$\endgroup\$ – MatsK Jan 13 '18 at 19:03

On an Arduino Uno the USB communication is accomplished by a separate controller chip which is directly connected to the USB 5v. If there is no external power (or if external power is low) the USB 5v will also power the rest of the circuitry. If external power is supplied and it is high enough a comparator and mosfet switch selects the external power for the system. In either case the USB 5v line will likely always draw some current when connected.

The LED flicker may or may not indicate a problem. First test the Uno without the LCD or external supply (for example try loading and running the simple LED blink code). If the Uno is operating correctly re-connect the external supply then the LCD to see if the problem is related to one of those.

One or more of the on-board LEDs may flicker during serial communication. The LED on D13 (that you have connected to the LCD) will obviously flicker when that line is toggled.

  • \$\begingroup\$ @Chris, agreed, edits made. \$\endgroup\$ – Nedd Jan 13 '18 at 21:11
  • \$\begingroup\$ Guess I'm not supposed to use comments to respond. Having trouble logging in. At any rate, \$\endgroup\$ – L. Martin Jan 13 '18 at 22:36

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