Electronics and Small Circuit Basics
First of all, if you don't already know it, electricity can be very dangerous. Fatal electrocution has happened with as little as 42 volts. Most of the demonstrations here will be at much lower voltage but don't underestimate the power of electricity and always be aware of the voltages and currents you are working with.
Electricity is the flow of electrons through a wire or other conductive material. Certain materials allow the flow of electricity more easily than others. Copper and aluminum are two of the most popular. Electricity can flow through various other materials, even water, but not so much with materials like rubber or plastic. That is why plastic is used to cover copper wires so that it protects or 'insulates' someone from touching a bare wire and becoming 'electrocuted'. The most common source of electricity that most people are familiar with (besides the wall sockets in your house) is the battery. We know that for an electrical device to work it has to be hooked up to both terminals of the battery, that is, the positive and the negative terminals of the battery. This makes a complete 'circuit' for the electricity to flow through. There are 3 main terms used when discussing electricity: VOLTAGE, CURRENT and RESISTANCE. Let's define how these 3 characteristics of electricity work:
Voltage
You can make a rough analogy of these electrical terms and the way electricity flows through a wire in the same way water flows through a pipe. First, an electrical source such as a battery can be represented by a large tank of water. You can measure the pressure of the water in the tank even though there may be no water flowing through pipes. The same for electricity, you can measure the electrical pressure of a battery even though there may be no wires hooked up to the battery and no electricity flowing. This pressure is called the VOLTAGE of the battery. It is named after the Italian physicist Alessandro Volta (1745–1827)
Current
If we hook up some pipes coming out of the bottom of the water tank then we will begin to get a flow of water through the pipes. If we hook up a wire to the battery we can begin to get a flow of electricity through the wire, this is called an electrical current or just CURRENT. An important difference between a water tank and a battery is that water can flow from a pipe coming out of the bottom of the tank because water flows down. For electricity to flow, the wire must come out of one side of the battery and be hooked to the other side of the battery, in other words the positive and negative terminals of the battery. The electricity (or electrons) flows from the negative side of the battery to the positive side of the battery. So, electricity flows in a certain direction, the same as water flows in a certain direction. Electrical current flowing through a wire is measured in amperage (or amps). The word is derived from the name of the French physicist who discovered this principle in 1881: André-Marie Ampère.So electrical pressure is measured in volts, current is measured in amps.
Resistance
RESISTANCE in electricity can be represented in our water tank example as a difference in the size of the pipe coming out of the bottom of the tank. If we make the pipe very small then the flow of water coming out will be slowed down significally. It will also keep the tank of water from draining too quickly. In electricity we can use a device called a resistor. We can hook up a resistor in the circuit between the negative and positive terminals of the battery. Now the electricity flows through the wire and when it comes up to the resistor it is slowed down by the properties of the resistor, just like a small pipe slows down the flow of water. Resistance is important in electricity to slow down the flow of electricity, so that we don't drain the battery immediately. Just like we can drain a water tank of all the water quickly by using large pipes, if we don't slow down the flow of electricity in an electrical circuit, the battery will loose it's usefulness quickly. In other words, a dead battery. Not to mention, if we don't slow down the flow of electricity in a wire, the wire could become very hot and possibly start a fire. This is the way an electrical burner works on a stove, but the electricity is controlled so that the burners don't become too hot as to burn out. Resistance is measured in ohms. Named after German physicist Georg Simon Ohm 1827. So again, the electrical pressure of the battery or source is measured in volts, the current flowing through the wires is measured in amps and any resistance to this flow caused by a device hooked in this circuit is measured in ohms.
It should also be noted that to stop the flow of water, all we have to do is plug up the end of the pipe. To stop the flow of electricity, all we have to do is cut the wire and not have it hooked to anything. (This would be called an open circuit).
Ohm's Law
So in electricity we have voltage (the electrical pressure), the current (the amount of flow of electricity through a wire) and the resistance (anything in the electrical circuit that is slowing down the flow of electricity). These 3 items have a relationship with each other represented by a mathematical formula called Ohm's law. If we know two of the values of any of the three items, we can find the third item by just doing some math. For example if we know the voltage and the resistance in the circuit, we can find the current by using one of the following formulas. First we use the following letters to represent voltage, current and resistance:
v = voltage (measured in volts - sometimes voltage is represented by e for 'electromotive force').
i = current (measured in amps - note the odd use of 'i', used from the French 'intensite du courant')
r = resistance (measured in ohms - the ohm symbol is represented by the greek letter omega Ω, that is alt 234 on your keyboard)
So the first algebraic formula is:
i = v/r
That is, current 'i' equals voltage divided by resistance. (Don't get hung up on the use of 'i' to represent current). Since this is an algebraic equation the equation can also be represented by transforming the equation:
r = v/i
v = ir
Then, all three variations of Ohm's Law are:
i = v/r (current = voltage divided by resistance) r = v/i (resistance = voltage divided by current) v = ir (voltage = current times resistance)
So if we have a 12 volt battery with a only a 6 Ω resistor in the circuit then what would be the current (i) flowing through the circuit? If
i = v / r then we know that we will have 2 amps of electricity flowing through the wires. (current = voltage divided by resistance).
Power (Wattage)
Another common unit of measure used is the watt. The watt represents how much power is being consumed such as a 100 watt light bulb compared to a 25 watt light bulb. The 100 watt uses 4 times more power. The formula and it's variations for finding watts is:
p = vi (power 'watts' = voltage times current)
p = v² / r
p = i² x r
(To write the little '2' squared symbol, do alt 0178)
So in the example used above with the 12 volt battery and a 6 ohm resistor, what would be the power being consumed or used by the resistor in the circuit? If power equals voltage times current according to the above formulas and we know there is 2 amps of current flowing through the circuit then obviously that same current is flowing through the resistor. So the power being consumed by that resistor would be 24 watts, that is 12 volts times 2 amps.
Also, FYI, a resistor able to handle 24 watts of power would have to be a fairly large and costly resistor. Most resistors in today's electronic devices are only able to handle 1/4 or 1/2 of a watt. So when designing circuits, make sure to match the power handling capabilities of your components to the power that will be going through them.
Test your knowledge
If we have a standard 100 watt light bulb screwed into a lamp plugged into a standard home electric socket, what would be the current flowing through the lamp cord and what would be the resistance offered by the light bulb? Let's say that the voltage measured at the wall plug is 110 volts which is fairly standard for homes in the U.S.
The new popular LED light bulbs use only about 5 to 10 watts of power. What would be the current and resistance of a 5 watt bulb? Answers at bottom of this tutorial.
AC vs DC
AC stands for alternating current and DC stands for direct current. As mentioned before, electricity travels through a wire in a certain direction. In a battery, it travels from the negative post (minus) to the positive post (plus). This is known as direct current. Batteries only give direct current. Alternating current is where the electricity flows back and forth through the wire. For comparison take a length of pipe that is plugged on both ends with water in it. If you shake the pipe back and forth, the water will go back and forth through the pipe. In the same way, electricity goes back and forth through a wire in alternating current (AC). Batteries don't give alternating current. Alternating current has to be produced with oscillator circuits or other means like electric generator motors. Alternating current is what comes out of your wall sockets at your house to plug in your lamps, toasters, TV, etc. Alternating current is used because it is easier to maintain over miles and miles of electric lines on telephone poles. Alternating current is also used to produce radio waves, video, music synthesizer sounds and used in a host of other electrical devices. The number of times in which the current flows back and forth through the wire per second is known as the frequency. For a wall socket at your house, the number of times (or frequency) the electric changes direction is 60 times per second. or better known as 60 hertz. This is actually pretty slow compared to the frequency of radio waves which start in the thousands and goes up to the millions and billions. A AM radio wave is around one million hertz. That is, it is changing direction a million times in one second. A radio wave oscillating through an antenna produces a sort of vibrating magnetic field around it which can be detected miles away by a AM or FM radio.
All the devices that work off of radio waves such as AM, FM radio, TV, ham radio, even microwave ovens operate with different electric frequencies. AM is around one million hertz (1Mhz), FM starts at 88 million (88Mhz), TV goes higher and microwave ovens around 2,450 million hertz or (2.45 GHz). All radio stations operate at different frequencies to separate them from other stations. It is the job of the radio to isolate just the one frequency of the station that you want to listen to. When you change radio stations you are changing the frequency at which the radio is detecting. In conclusion, there are two main types of electricity used in electrical circuits, alternating current and direct current or better know by there abbreviations (and a famous rock-n-roll band) AC DC.
Ok, so enough of the very basic stuff. Electrical circuits are basically designed with a power source (battery) hooked up to a combination of resistors, capacitors, transistors, coils, IC's, etc, all of which are hooked up in series or parallel to each other to accomplish some purpose like being a radio or an amplifier, a TV or a computer, a cell phone or an iPad, etc. The different electrical components, such as capacitors or transistors, will be more fully described as we discuss different circuits and their functions.
First Circuits
As already mentioned, when designing circuits, we need to be aware of the power handling capabilities of the different components used in electrical circuits. For example, when designing a simple LED light indicator, like the ones that let you know if your monitor is on, we need to know how much power the LED can handle without blowing out. If we take a common LED and hook it up straight to a battery, there's a chance the LED will very soon burn out. So, we need to place a resistor in series to the LED to lower the amount of electrical current going to the LED. Also, we need to make sure that the resistor itself can handle the amount of current going through it.
If the maximum current the LED in this circuit can handle safely is 20 mA (that is, 20 milliamps or .020 amps) what size resistor do we need in the circuit?
Using ohm's law, r = v / i, and assuming a 9 volt battery, then
r = 9 / .02 or 450 ohms.
If power equals voltage times amperage, then the resistor would be handling .18 watts, less than a quarter of a watt.
(9 X .02 = .18) so it should be safe. (quarter watt resistors are common and cheap)
In the above example for the sake of simplicity, we've left off the fact that the LED has a voltage drop and causes resistance to the circuit also.
Voltage drops and resistors in series or parallel
Resistors in series
If we had a circuit with just one resistor hooked up to a battery, and we measured the voltage across the resistor, the voltage would be the same as the voltage of the battery. If we were to place another resistor in series with the first resistor in the circuit, then measured the voltage across one of the resistors, we would get a voltage drop. In other words the voltage would be less than the voltage of the battery. If we were to measure the voltage drops across each resistor separately, the two voltages would add up to be the same as that of the battery. If we were to put any number of resistors in the circuit, all in series, then measured all the voltage drops across each resistor, the total number would be the same as the voltage of the battery. This is known as Kirchhoff's Voltage law. It should be noted that this law does not apply to resistors in parallel to each other.
Resistors in parallel
If you were to measure the voltage across two or more resistors in parallel with each other in a circuit they will all measure the same voltage as the source battery, there will be no voltage drops. But, if the resistors are all different values, even though they measure the same voltage, they will all have different currents flowing through them. This leads up to another of Kirchhoff's laws, the sum of the currents going into a junction point will be equal to the sum of the currents going out of the junction point. You could have 3 resistors in parallel meet at a certain junction point in a circuit and have 5 resistors in parallel leave that junction. The sum of the currents of the 3 resistors will equal the sum of the currents of the 5 resistors. This is known as Kirchhoff's current law. These 'laws' are important to remember when designing or analyzing circuits so to make sure that all the components in the circuit can safely handle the amount of electricity flowing through them. All electrical components, such as resistors, capacitors, transistors, etc have a maximum amount of current or power they can safely handle. This information is given in the 'data sheets' provided by the manufacturer.
More Electrical Formulas
Resistors in series:
R total = R1 + R2 + R3 ...
Resistors in parallel:
R total = 1 / (1/R1 + 1/R2 + 1/R3 ...)
For a 100 watt bulb the current would be .91 amps so resistance would be 121 ohms (rounded).
For a 5 watt bulb the current would be .045 amps and resistance would be 2,444 ohms. (rounded)