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Skill Builder: Comprehending Capacitors

I wrote my book Easy Electronics to help beginners get acquainted with electronics more simply, quickly, and affordably than ever before. A dozen hands-on experiments show you the basics, and each one takes half an hour or less. You won’t need any tools at all.

Some capacitors have colored cans.
Others don’t. The color is not important.

You will need: 

  • AA batteries (3) with holder
  • alligator jumper wires (3)
  • 470µF capacitor
  • 100µF capacitor
  • 1kΩ resistor
  • through-hole 5mm LED
  • SPDT slide switch.

EXPERIMENT:

The type of capacitor shown here is called electrolytic. Its storage capacity, known as capacitance, is 470µF—but I’ll explain that in a moment. 50V is its maximum voltage, but for this experiment, a rating of 10V or higher is okay.

The short lead is the negative side, also identified with minus signs. That’s because this capacitor has polarity — never connect an electrolytic capacitor to a power supply the wrong way around.

You can build this circuit in two steps. This part just charges the capacitor with electricity when the slide switch moves to upper-left.
Some of the voltage from the battery transfers to the capacitor, although you can’t see any sign of it yet.

Add the 1K resistor and the LED, with the negative side of the LED sharing the negative leg of the capacitor.

Now move the switch to the lower-right. The capacitor discharges itself through the LED.

Move the switch to the upper-left and wait 5 seconds for the capacitor to recharge. Now you can discharge it again and light the LED again!

If this diagram looks complicated to you,
try sketching a copy of it, replacing the alligator wires with simple lines to connect the components.

HOW DOES IT WORK?

Inside the capacitor you used are two pieces of metal film known as plates. They are separated by paste called an electrolyte, which is why this capacitor is called electrolytic.

When electrons flow into one plate, they try to create an equal, opposite charge on the other. You can think of the plates as having positive and negative charges that attract each other.

TIMING

The 1K resistor was needed because you charged the capacitor with 4.5V from the battery pack, and the LED can only handle about 1.8V. The resistor prevents the LED from being damaged.

The resistor also controls how fast the capacitor discharges. Substitute a 10K resistor (brown, black, orange) and the LED is dimmer than before and takes much longer to fade out.

Here’s another thing to try. Go back to using the 1K resistor. Remove the 470µF capacitor and substitute a 100µF capacitor. Push the switch to and fro, and now the LED lights up very briefly.

Electricity moves fast, but a capacitor and a resistor can make things happen slowly.

CERAMICS

Capacitors such as the one shown above
are less than 1/2″ wide. They are dipped in a ceramic compound.

Most ceramic capacitors do not have polarity.

Many ceramic capacitors have a code printed on them instead of their actual capacitance.

Some ceramic capacitors are shaped like circular discs.

In simple circuits of the type you have been building, usually you can substitute a ceramic instead of an electrolytic if you wish. Note that for values around 10µF and above, ceramics may be more expensive.

UNITS

Capacitance is measured in farads, abbreviated with letter F. But a 1F capacitor is very large. In hobby electronics we mostly use capacitors rated in microfarads, abbreviated µF. The µ symbol is the Greek letter mu, but often µF is printed as uF.

There are 1,000,000 microfarads in 1 farad, 1,000 nanofarads (nF) in 1 microfarad, and 1,000 picofarads (pF) in 1 nanofarad

SCHEMATICS

There are two symbols for capacitors.

A polarized capacitor, such as an electrolytic, is on the left.

A nonpolarized capacitor, such as a ceramic, is on the right.

Some people use the symbol on the right everywhere in a schematic, and let you decide if you want to use an electrolytic capacitor, and if so, which way around it should be.

YOU MIGHT THINK:
A capacitor may seem similar to a battery. After all, they both store electricity.

A battery, however, uses chemical reactions, and even a rechargeable battery wears out after a limited number of charging and discharging cycles.

A capacitor does not use chemical reactions, and can still work as well after several years.

In this schematic showing the circuit that you just built, the double-throw switch has completed a circuit with the battery, so that the battery charges the capacitor.

In this schematic, the double-throw switch is in its other position, completing a circuit from one plate of the capacitor, through the resistor and the LED, back to the other plate. 

This article is adapted from the book Make: Easy Electronics, available at the MakerShed and other booksellers. A complete kit of all components used in the book is available here.

[Feature Photo by Michael Dziedzic on Unsplash ]

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Charles Platt

Charles Platt is a contributing editor to Make magazine, which has published more than 50 of his articles. Six of his books are available from MakerMedia:

Make: Electronics, an introductory guide, now available in its second edition.

Make: More Electronics, a sequel that greatly extends the scope of the first book.

Encyclopedia of Electronic Components, volumes 1, 2, and 3 (the third written in collaboration with Fredrik Jansson).

Make: Tools, which uses the same teaching techniques as Make: Electronics to explore and explain the use of workshop tools.

View more articles by Charles Platt