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A resistor is a current limiting, power dissipating, device designed to limit the flow of electrons to a known controlled value. Resistors come in all sizes and shapes, but as far as the leaded types go (ones with wire leads) the most common by far is the 1/4 watt metal film type.

To the right is a picture of several resistors all of the same wattage (1/4 watt) but different values of resistance.

Next is a picture of resistors of the same value of resistance but with different power ratings.

The largest one has a rating of 10 watts. Here is some insider information, for many years now resistors of the metal film type have been popular because of their temperature stability and low noise.

The older resistors were made of carbon and their resistance would tend to increase over the years due to a number of reasons but the main one being moisture absorption.

When you come across a older carbon resistor be sure to measure it's value with an ohmmeter to be sure it's still what it says it is.

So how do you know what the resistance of a resistor is? The value of resistance is color coded on the unit. Colored bands are used to denote the ohmic value.

A-First significant number B-Second significant number C-Decimal multiplier D-Tolerance in percent.

Here is the resistor color code chart for the bands. These colors have been used since the beginning of electronics, almost a hundred years.

Let's look at an example below to see how this all works.

The first band (A) is brown so looking at the chart above we see that it equals 1. The next band (B) is black and equals 0. So far we have 10. The next band (C) is orange which equals 3 so we multiply 10 by 1000 The resistors value is 10,000 ohms. Another way to look at the multiplier is to simply add that many zeros. Since orange is three, we add three zeros to 10 and get 10,000. The last band (D) is gold which means this resistor has a tolerance of 5% of its rated value.

One more example. Look at the resistor on the right and see if you can tell it's value. We have orange, white, red and a silver tolerance band. Orange=3, White=9, Red=2. So it's 39 with two zeros or 3900 ohms. Silver=10% tolerance.
By the way, both these resistors are the older carbon type. So I would check them with an ohm meter before use, especially the higher resistance one of 10K ohms.

Just a few more things about resistors. There's a type that is variable so the user can set the value. Its real name is potentiometer or 'pot' as it is called for short. (No you can't smoke it -- but I have seen them smoke!) You and I know it as a volume control. Now the next important thing to know is the schematic symbol for the resistor. This will be your introduction to reading electronic schematics.

The device on the left is, yes, the battery (duh!) and the resistor is on the right labeled (R). The connecting wires are just straight lines. Remember these two symbols, the battery and the resistor.

By the way, as you know batteries are made up of cells, the total voltage of the battery is dependent on the total number of cells. That is why the schematic symbol is drawn the way it is.

Now if each cell of this battery has a voltage of 2.4 volts, what would the voltage be across this battery? What would be the voltage across the resistor? (Answer: The same as the battery voltage. 12 volts.)

If the resistor had a value of 6 ohms what would the current flow in the circuit be? How much power in watts is dissipated by the resistor?
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The Lost Secrets To Learning Basic Electronics