A capacitor stores electricity, as you will see in this experiment. It behaves somewhat like a tiny rechargeable battery, but it works in a different way.
- 1 470µƒ Electrolytic Capacitor
- 1 100µF Electrolytic Capacitor
- 1 Red LED, Low-Current
- 1 1K Resistor
- 1 10K Resistor
- 3 Alkaline AA Batteries
- 1 Three Battery Holder
- 1 Double-Throw Switch
- 1 Red Alligator Wire
- 2 Black Alligator Wires
- 3 Green Alligator Wires
Experiment 7: Comprehending Capacitors
This type of capacitor is called electrolytic. This one’s storage capacity, known as capacitance, is 470µF — but I’ll explain that in a moment. 50V is its maximum voltage, but for the experiments in this book, a capacitor rated 10V or higher is okay.
The short lead is the negative side, also identified with minus signs. Never connect an electrolytic capacitor to a power supply the wrong way around.
This is a larger circuit, so you can build it in two parts. This first 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 five seconds for the capacitor to recharge. Now you can discharge it 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. Or check the schematic at the end of this lesson.
Capacitors such as the one shown here 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 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.
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.
Caps and Batteries
There are two symbols for capacitors: a polarized capacitor, such as an electrolytic, and a nonpolarized capacitor, such as a ceramic. Some people use the nonpolarized symbol 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!
THINK ON THIS
How Did 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 plate. You can think of the plates as having positive and negative charges that attract each other.
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.