Two quick-and-dirty Arduino Power Current Taps for the Arduino Survival Guide: Workbench Edition class I taught for SqWr a while ago. These give you the current drawn by the entire board + LEDs + whatever, so you can calculate the power dissipation in that poor on-board regulator.

The USB version:

USB Current Tap

The general idea is to cut a USB extension cable (Type A plug on one end, Type A receptacle on the other) in half, splice the two data wires, splice the ground / common wire, but connect the +5 V wires to a dual banana plug that goes into a current meter. The Big Box o’ USB Junk produced a cutoff cable end with a Type A plug and a PC jumper that was supposed to connect an internal USB header to the back panel; the red blob of silicone tape conceals the jumper’s socket strip and a five-pin male header with all the wires soldered to it.

You could use a Type B plug that would go directly into an Arduino UNO (or similar), but I figured this way everybody can bring whatever cable they need for their particular Arduino, not all of which have bulky Type B receptacles these days.

The External Power version:

External Power Current Tap

The coaxial power plug goes into the Arduino and whatever you used for power goes into the socket. The Big Box o’ Coaxial Power Stuff actually had a more-or-less properly sized coaxial jack with two wires on it and silicone tape wrapped around it; I regarded that as a Good Omen and pressed it into service as-is.

These will also replace the horribly rickety collection of alligator clip leads I usually use for such measurements…