Electronics Puzzler: Super Capacitor

By Forrest M. Mims III
Demonstration Circuit

What's the missing component?

Super capacitors have orders of magnitude of more storage capacity than conventional electrolytic capacitors. While the capacity of electrolytic capacitors is rated in microfarads, super capacitors are rated in farads. They can also be charged much faster than electrolytic capacitors and have a much longer lifetime.

Electronics instructor Ollie Circuits planned to show his class of freshman electrical engineering students how to use a Jameco Super Capacitor as a memory back-up capacitor, but first he wanted to show the students how they could make their own super capacitor and then demonstrate its charge/discharge cycle with the simple circuit shown in Fig. 1.

Most of the components were already on his workbench, including an LED, some resistors, a pair of 2-inch square blank PC boards to which he had soldered connection leads, some paper towels, a dish and a fresh lemon. The homemade super capacitor would be made from several layers of lemon juice-soaked paper towel interleaved between several layers of a mystery material to form a multi-layer stack. The stacked layers would then be sandwiched between the two copper-clad PC boards and held together with a rubber band. All Ollie needed was some mystery material, so he rushed to a nearby pet shop to purchase some. What did he buy?

The figure above shows the homemade super capacitor (the puzzle piece) connected to an LED and a 6-volt battery. The capacitor is charged by toggling SPDT switch S1 to position 1 for a minute or so. When S1 is toggled to position 2, the LED will glow as the super capacitor is discharged through R2 and the LED.

The super capacitor was made from stacked layers of lemon-juice soaked paper towel and a mystery material sandwiched between the foil sides of two PC boards. Ollie bought the mystery material at a neighborhood pet shop. What did he buy?

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