Virtual Workbench: LED Circuit
Try batteries, resistors and LEDs on screen before you build, and see exactly why every LED needs a resistor.
The LED workbench
Choose a battery, a resistor and an LED, then flip the switch. The workbench works out the current with Ohm’s law and shows what would happen to a real LED.
About this model: it’s simplified. It treats each LED colour as needing a fixed voltage and assumes perfect batteries. Real LEDs vary (check the datasheet), batteries sag as they’re used, and parts have tolerances, so real measurements will differ a little. It’s a guide, not a substitute for careful building.
Challenges
- With 3 × AA and a red LED, which resistors are “just right”? Which is too much, and which too little?
- Switch to the 9 V battery. Which resistor keeps a red LED safe now?
- Try a blue LED on 2 × AA. Why won’t it light? (Hint: Lesson 5.)
- Pick “None” for the resistor. Now you know why every LED project has one.
Answers
1. On 4.5 V with red: 220 Ω (11.4 mA) and 470 Ω (5.3 mA) are just right; 1 kΩ (2.5 mA) is dim but safe; 100 Ω (25 mA) is too much; 10 kΩ (0.25 mA) is very dim. 2. On 9 V, 470 Ω (about 14.9 mA) and 1 kΩ (7 mA) are both safe; 220 Ω gives about 31.8 mA, which would damage the LED, and 100 Ω would also overheat the resistor. 3. A blue LED needs about 3.0 V, and 2 × AA supplies about the same, leaving almost nothing to push current through. It glows faintly at best. 4. Without a resistor, nothing limits the current.
Learn more
- Lesson 5: LEDs and resistors: the math behind the workbench
- Build it for real: My First LED Circuit
- Back to Electronics School
Electronics School is part of the Discovery Zone. Every activity here runs on small batteries or a USB cable: never wall outlets. The one plug-in tool is the optional soldering iron, set up by an adult.