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Introduction: Capacitors

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Some of the earliest capacitors were simply glass jars filled with salt water and wrapped in metal foil. These capacitors - called Leyden Jars - were crude devices which stored high voltage electric charge. They helped early experimenters gain a grasp on electricity and how to harness it. Over the years the science behind capacitors has obviously become more refined, and jars have been phased out. Capacitors can fit onto the smallest of circuit boards and hold varying amounts of charge. In this class we will learn about capacitors and make a small vibrobot (a robot which moves using vibration).


What Is a Capacitor?

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To put it simply, a capacitor is a component that stores electricity and then discharges it back into the circuit when there is a drop in voltage.

It is important to stress that capacitors do not equal batteries.

The difference between a battery and a capacitor is that in a battery power is generated through a chemical reaction, and in a capacitor a charge is stored and maintained in an electrical field. A battery can yield much more energy for a longer period than a capacitor. On the other hand, a capacitor - even with much less power storage - can discharge considerably more energy exponentially faster than a battery. This is ideal when you need a lot of energy fast.

A capacitor consists of conductive metal plates separated by an insulating material called a dielectric. The dielectric can be anything such as ceramics, plastics, oxidizing metals, glass, and paper. For instance, if you crack open an electrolytic capacitor you will find it's just two metal plates wrapped in a chemically coated film. When electricity is applied, the metal conductors polarize the electrons in the film and store an electric field.

If you were to look at this old-fashioned adjustable capacitor you will notice that it is just a series of metal plates that slide between one another without touching. In this scenario, the air is the dielectric. If it seems weird that air can be charged to hold an electrical field, think of lightning and the Jacob's Ladder project.

Depending on how much physical overlap there is between plates determines how large of an electrical field can be held. The more surface area that the two sets of plates shares between them, the greater the electrical field will be, and the more capacitance it will have.

On account of their unique ability to store charge and maintain an electrical field, capacitors tend to try to resist changes in voltage within a circuit. As such, capacitors are often connected between power and ground to maintain a steady power supply and filter out intermittent voltage spikes and drops.

Capacitors are measured in Farads, which simply is symbolized with a capital F. Although, keep in mind that the values that you will typically encounter are in the picofarad (pF), nanofarad (nF) or microfarad (uF) ranges.

The two schematic symbols associated with capacitors are pictured above. The less complex one on the left is for non-polarized capacitors and the more complex one on the right is for polarized electrolytic capacitors.


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Capacitors can be either polarized or non-polarized. Put simply, a polarized capacitor has a positive lead which needs to be connected to power, and a negative lead which needs to get connected to ground. Nonpolarized capacitors can be connected any which way.

Ceramic disc and mylar film capacitors are non-polarized.

Electrolytic capacitors are polarized.


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Just like resistors, capacitors have markings that need to be deciphered.

Ceramic disc capacitors have two to three digits printed upon them. The first two numbers describe the value of the capacitor and the third number (should it exist) is the number of zeros in the multiplier. When the first two numbers are multiplied against the multiplier, the resulting solution is the value of the capacitor in picofarads. If there is no multiplier, you just read the value of the first two numbers in picofarads.

For instance, if a capacitor says 10 upon it, it has a multiplier of 1 and is simply read as 10pF. On the other hand, if it followed by a 1, it gets multiplied by 10 to become 100pF. Each subsequent number shifts the multiplier over one decimal point. By the time you get to 104, 10 is being multiplied by 10,000.

The one tricky bit about capacitors is that they can be described in picofarads (pF), nanofarads (nF) and microfarads (uF). The measurement you use is determined by whatever makes the most sense to describe the value. You would not describe a 10pF capacitor as a 0.00001uF capacitor any more than you would describe a 0.1uF capacitor as a 100,000pF capacitor. As a general rule, any number that requires more than two zeros to express it is excessive.

Reading electrolytic capacitors is a bit easier. You just literally read them. They have their capacitance and voltage rating printed directly upon them. We have yet to touch upon voltage rating. Suffice it to say that the voltage rating is only important in that your supply voltage should never exceed it. This is unlikely for the type of electronics we are doing.

On electrolytic capacitors, the most important thing to pay careful attention to is the stripe marked with a minus sign indicating the capacitor's negative lead.


High Voltage Vs. Supercapacitor

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Capacitors typically are rated for a small amount of charge and voltage. However, there are two special types of capacitors that buck this trend.

High voltage capacitors - as the name would imply - are capable of storing a large amount of voltage. These are often found in camera flashes and some electronic appliances that plug into the wall, such as old-fashioned tube televisions. You need to be careful when encountering these because they can maintain charge for quite some time after being charged and will shock the heck out of you if you touch the leads.

Supercapacitors on the other hand typically are very low voltage, but store a heckuva lot of charge. For instance, the smallest capacitor you might encounter in working with electronics is 1pF. If we were to convert this unit to Farads, it would be equivalent to 0.000000000001F. In short, a Farad is really - really - big. Thus, the 15F supercapacitor we are going to use in the vibrobot project stores a deceptively large amount of energy.

While you can touch the leads of a supercapacitor without worrying about electric shock, if you were to accidentally cross the leads of a supercapacitor with something conductive, it could rapidly release enough energy to melt metal. You may not get shocked, but if you are careless, you could get badly burned.

CLASS PROJECT

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