Solution
Hints
- Voltage is the push (volts); current is the flow it produces (amps). Ohm's Law connects them -- V = I x R.
- A lit LED eats a fairly fixed chunk of the supply voltage no matter what, about 2 volts for a red LED.
- A multimeter measuring current sits in series (inline), not in parallel like a voltage measurement.
Steps
- 5V - 2V (the LED's share) = 3V left over for the resistor.
- I = V / R = 3V / 220 ohms = about 0.0136 amps, or 13.6 milliamps.
- Break the loop, insert the multimeter in series with jumper wires, set it to DC milliamps, and power the circuit.
- A reading near 13.6 mA confirms the prediction.
Circuit
A red LED in series with a 220 ohm resistor on a 5V supply drops about 2V across the LED, leaving 3V across the resistor. Ohm's Law (I = V / R) then predicts about 13.6 mA -- a safe, typical LED current, well under this chapter's 20 mA safety line. Measuring it for real means breaking the breadboard loop and routing current through the multimeter's DC milliamp range, in series rather than in parallel like a voltage reading. A result close to but not exactly 13.6 mA is expected, since a real LED's forward voltage varies slightly by part and brightness.
See also: Ohm's Law Current Calculator diagram (Chapter 16) -- this scenario matches the diagram's own 5V/220-ohm default settings.