Capacitor Charge Race

Race different capacitors to see which charges faster—a hands-on demonstration of RC time constants.

Overview

Which capacitor charges fastest? Build three identical circuits with different capacitors and watch them race! The smallest capacitor wins—but why? This project makes the RC time constant formula tangible and memorable.

Components Needed

  • 1x Capacitor 100µF electrolytic
  • 1x Capacitor 470µF electrolytic
  • 1x Capacitor 1000µF electrolytic
  • 3x Resistor 10kΩ
  • 3x LED (3 different colors)
  • 3x Resistor 1kΩ
  • 3x NPN Transistor (2N2222)
  • 1x 9V Battery
  • 1x 9V Battery clip
  • 1x Push button (momentary)
  • 1x Breadboard (large)
  • manyx Jumper wires

Instructions

  1. Plan Your Layout

    You'll build three identical circuits side by side. Use different colored LEDs (e.g., red, yellow, green) to easily see which is which. The only difference between circuits is the capacitor value.

  2. Build the First Racer (100µF)

    Connect: Power → 10kΩ resistor → junction point. From junction: 100µF capacitor (positive to junction, negative to ground). From junction: 1kΩ resistor → transistor base. Transistor emitter to ground. LED from power through 1kΩ to collector.

  3. Build the Second Racer (470µF)

    Build identical circuit next to the first, but use the 470µF capacitor. Use a different color LED.

  4. Build the Third Racer (1000µF)

    Build identical circuit with 1000µF capacitor and third color LED.

  5. Add the Start Button

    Connect a push button between the battery positive and the power rail. This lets you start all three circuits simultaneously. Make sure all capacitors are discharged before starting (briefly short their leads with a resistor).

  6. Race!

    Press and hold the start button. Watch the LEDs—the 100µF (red?) lights first, then 470µF (yellow?), finally 1000µF (green?). Time them with a stopwatch and compare to your calculated values!

Challenges

  • Time each LED with a stopwatch and compare to calculated τ values
  • Try different resistor values (4.7kΩ, 22kΩ) and see how it affects race times
  • Add a "reset" button that discharges all capacitors through resistors
  • Swap positions of resistor and capacitor—does it change behavior?
  • Calculate and verify: how long until each LED reaches full brightness (5τ)?
  • Use a multimeter to measure actual capacitor voltage during charging