I've got the following circuit:
It works. The LED glows nice and bright.
However my math doesn't match the readings I'm getting from my digital multimeter. Using Ohm's Law, I take the voltage rating of the batteries and desired amperage of the LED to figure out how much resistance I need.
This is my math:
V = I*R = 4.8V (4x AA 1.2V/2400mA in series) I = V/R = 0.02A (1x 3mm Green LED 3.0-3.2V/20mA) R = V/I = 240Ω (1x 220Ω & 1x 20Ω in series)
These are my multimeter readings:
Voltage across the batteries: 5.17V Voltage across the LED: 2.93V Voltage across the resistors: 2.24V Resistance across the resistors: 238Ω Amperage on the circuit: 9.56mA
Now obviously I am getting more volts from the batteries than I anticipated but even adjusting for that I still don't get ~9.5mA of current in my math.
V = I*R = 5.2V I = V/R = 0.022A R = V/I = 238Ω
What am I missing?