Monday, June 30, 2008

Basic Definitions and Concepts

Basic Definitions and Concepts


Welcome to the exciting world of electronics. Before we can build anything we need to look at a couple of things. Anytime you have an electrical circuit, you have voltage and current. We build circuits to control voltage and current.


Current is what flows through a wire. Think of it as water flowing in a river. The current flows from one point to another point just like water in a river. Current flows from points of high voltage to points of low voltage. Current can be shown in circuit diagrams by using arrows as in Figure 1. The arrow shows which way the current is flowing. An I is usually included beside the arrow to indicate current.

Figure 1

The unit of measurement for current is the Ampere, or Amp for short, and abbreviated as A. (The name Ampere comes from Mr. Ampere who played with electricity as a small boy in Vermont.) Common currents are 0.001 Amps (0.001A) to 0.5 Amps (0.5A). Since currents are usually small, they are usually given in the form of milliAmps (abbreviated mA.) The milli means divided by 1000, so 0.001 Amps equals 1 milliAmp (1 mA) since 1 / 1000 = 0.001. Also, 0.5 Amps equals 500 milliAmps (500mA) since 500 / 1000 = 0.5.


Voltage indicates the power level of a point. Voltage is measured in volts. If we continue the river comparison, a point at the top of a hill would be at a high voltage level and a point at the bottom of a hill would be at a low voltage level. Then, just as water flows from a high point to a low point, current flows from a point of high voltage to a point of low voltage. If one point is at 5 volts and another point is at 0 volts then when a wire is connected between them, current will flow from the point at 5 volts to the point at 0 volts.

A measurement of voltage is much like a measurement of height. It gives you the difference in voltage between those two points. If point A is at 10 volts and point B is at 2 volts then the voltage measured between A and B is 8 volts (10 -2). This is similar to measuring height. We measure the height of hills the same way. We say the sea level is at zero feet and then compare other points to that level. On top of Mary’s Peak you are 4000 ft high (compared to sea level). In the same way we call the lowest voltage in a circuit zero volts and give it the name ground. Then all other points in the circuit are compared to that ground point. Rivers always flow towards sea level and currents always flow towards ground.

A battery is similar to a dam. On one side is a lot of stored up energy. When a path is formed from that side to the other side then current flows. If there is no path then current does not flow and the energy just stays there waiting for a path to form to the other side. The path can be a big path with lots of current flowing or a small path with just a little bit of current flowing. With a dam, a little bit of water flow could go on for a long time, but flow through a big path that lets all the water go at once would only last a short while. A battery is the same. If there is big path from the high voltage side to the low voltage side then the battery will not last long.

There are two special cases that we give names. One is when the current is zero (open circuit) and the other is when the voltage is zero (short circuit).

Open Circuit

An open circuit is when two points are not connected by anything. No current flows and nothing happens. If a wire in your vacuum cleaner breaks it can cause an open circuit and no current can flow so it does not do anything. There may be a voltage between those two points but the current can not flow with out a connection.

Short Circuit

A short circuit (or short) is when two points with different voltage levels are connected with no resistance (see resistors) between two points. This can cause a large amount of current to flow. If a short circuit happens in your house, it will usually cause a circuit breaker to break or a fuse to blow. If there is no device to limit the current, the wires may melt and cause a fire. This situation is something like a dam breaking. There is a large amount of energy suddenly free to flow from a high point to a low point with nothing to limit the current.

Series Connection

A series connection is when two components are joined together by a common leg and nothing else is connected to that point as shown in Figure 2.

Figure 2

Parallel Connection

A parallel connection is when two components are joined together by both legs as shown in Figure 3.

Figure 3

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