Capacitor and Light Bulbs

This demonstration uses 50V DC, which can cause painful shocks. The large capacitor stores significant energy and can deliver a sharp discharge even after the power supply is turned off. Always discharge the capacitor through a resistor before handling terminals. Never touch capacitor leads while powered or immediately after disconnection. Use insulated tools and keep hands away from exposed connections during operation.

Electrolytic capacitors must be connected with correct polarity. Reversing polarity can cause the capacitor to overheat, rupture, or explode. Always verify the negative terminal marking (usually a stripe with minus signs) before connecting to the circuit. Double-check connections before applying power.

After completing the demonstration, the capacitor may retain some charge even after apparent full discharge through the bulbs. Always discharge the capacitor through a resistor (or short the terminals with an insulated tool) before handling or storing. This is especially important if the demo is interrupted or if students will approach the apparatus.

Purpose

Students observe capacitor charging and discharging through the brightness of incandescent bulbs. One bulb fades as the capacitor charges (decreasing current), then both bulbs glow equally during discharge, making exponential RC transient behavior directly visible.

Two incandescent bulbs on clear plastic bases with white ceramic sockets connected by wires to a large blue Nichicon 50V capacitor; the left bulb near the capacitor is dim while the right bulb is brightly lit

Same setup; the left bulb near the capacitor is completely dark while the right bulb remains brightly lit

Both bulbs show faint, nearly identical glowing filaments as the capacitor discharges through the circuit

Figure 1: Charging in progress: the bulb in series with the capacitor (left) has dimmed as charging current decreases, while the bulb connected directly to the power supply (right) remains bright.

Figure 2: Fully charged: the capacitor-connected bulb (left) is dark — no current flows through it. The directly connected bulb (right) remains bright from the power supply.

Figure 3: Discharge phase: with the power supply off, the capacitor discharges through both bulbs in series. Both glow with equal, diminishing brightness.

0.68 F Capacitor

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Danger: High Voltage and Capacitor Discharge Hazard

Warning: Capacitor Polarity Critical

Warning: Post-Demonstration Capacitor Discharge

Purpose

Safety Warnings

Danger: High Voltage and Capacitor Discharge Hazard

Warning: Capacitor Polarity Critical

Warning: Post-Demonstration Capacitor Discharge

Purpose