Resistors
Resistance is the property of an electrical circuit that impedes current flow. It is one of the basic electrical properties, and it's pretty damn important too! Resistance is measured in ohms, represented by the greek letter Ω. One ohm is the resitance through which one amp will flow with a pressure of one volt.
All circuits contain an inherit amount of parasitic resistance because wires are not 100% conductive. Usually this resitance is very small and doesn't make a big impact, so to add resistance to a circuit we use a component called a resistor. Resistors are the most basic passive electrical component, and nearly every circuit has some sort of resistor in it. Although they all preform the same function, there are many different types of resistors one may encounter.
The most common type of resistor is a film resistor, and it consists of a carbon or metal film wrapped around a tube. These resistors are
cheap, reliable and easy to produce, but they can't handle much power before burning up. Common film resistor powers are 1/8W, 1/4W, 1/2W and
1W. To the left you can see these four resistors –notice that as power rating increases the physical size of the resistor increases. To the right I have
a picture of a very old glass resistor where the spiral film can easily be seen.
Another type of resitor is the wirewound resistor. These resistors can handle more power than film resitors, and as such they are
used in high power applications. As the name implies, a wirewound resistor is just a high resistance wire that has been wound around a spool
of some sort, and then encased in ceramic [left]. It might also be put in a fancy aluminum housing [right] where it can be mounted on a heatsink.
Wirewound resistors are very tough, and can withstand higher temperatures than the aforementioned resistor.
A once common resistor was the carbon composition resistor. A carbon comp resistor is a cardboard tube filled with pressed carbon powder mixed with a binder, and the ratio of carbon to nonconductive binder determined the resistance. Film resistors are more stable than carbon comp ones, so they have mostly replaced carbon comp ones in the majority of applications. These are still often used for high voltage applications though because they can withstand higher voltages than film or wirewound resistors.
You might have noticed I mentioned heat a few times, but why?
As you know, when electricity is pushed through a resistor the resitor limits the current. It does this by dropping voltage, and when voltage is lost in a resistor the energy gets turned into heat. This heat needs to be dissipated, and if you try to push 40W through a resistor the size of a tic-tac the thing is going to burn up!
Color Codes
Larger resistors are printed with text that describes their resistance, wattage etc, but small resistors don't have any room for text. Because of this colors are used to describe the resistor, with the colors corrosponding to numbers. Typically a four band color code is used because it is a simple and memorable system. On the right is a table resistor color codes. The first and the second bands are called the identifier bands, while the third band is the multiplier band.
For example, if you have a resistor painted yellow-violet-red-gold, one would look at the table and notice that yellow means 4 and violet represents 7; the identifier would then be 47. The third band is red and that represents x100, so 47 multiplied by 100 gives you a 4700Ω resistor.
The last band is the tolerance band. Manufacturers don't make perfect resistors, but they garuntee that the resistance will be within a certain percentage of the painted value. In this case the tolerance band is gold, so that means this resistor has a tolerance of ±5%, and thus can measure 5 percent over or 5 percent under its painted value. If it is greater than 5% out of spec then the resistor is considered no-good.
Resistors in Series
When resistors are placed in series their resitance simply adds up. If you put a 100Ω resistor in series with a 150Ω resistor, congratulations you just made a 250Ω resistor!
Resistors in Parallel
When resistors are put in parallel things start to get more complicated. One would assume that you just average the resistances but that is not the case. Instead you have to use a fancy formula known as the resistors in parallel formula.
1 / ( R1-1 + R2-1 + R3-1) R3 is optional, as well as R4, R5 or any more resistors you'd like to add.
Say you have the same resistors plus another in parallel; 100, 150 and 60 ohms. In order to figure out their total resistance you would use 1 / (150-1 + 100-1 + 60-1), and after some crazy maths you'd figure out that it would be 30 ohms.
To the right beyond the mess of zigzags and lines I've included a parallel resistor calculator, and it can calculate the resistance of up
to 9 parallel resistors so you don't have to.One thing to notice is that the total resistance of parallel resistors will always be lower than the resistance of the smallest resistor. If you have a 100k resistor in series with a 30Ω resistor and you calculate 37 kilohms of resistance, you're doing it wrong!
Ohm's Law
See: Ohm's Law
