Resistor
From BEAM Robotics Wiki
A resistor is a two-terminal passive component designed to oppose an electric current. As a result of this opposition a voltage drop will occur across the resistor in proportion to the current flowing through it. That is to say, it is in accordance with Ohm's law: V = I * R, where V equals the voltage drop across the resistor, I equals the current flowing through the resistor, and R equals the resistance of the resistor.
Resistors are characterized primarily by their resistance value and the power they can dissipate. A 5% accuracy is good enough for typical BEAM circuits, and resistors rated for 1/4 Watt are generally more than adequate. In fact 1/8 Watt resitors are physically smaller and are usually still quite sufficient for most BEAM applications. Although they tend to be a bit tougher to find.
Resistors are pretty darned cheap, so it would be a good idea to keep a broad selection of them on-hand. Values that are commonly used in BEAM circuits include -- 470, 1.0K, 2.2K, 4.7K, 47K, 100K, 300K, 360K, 680K, 1.5M, 2.2M, 3.9M, 6.2M --.
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[edit] Resistor part codes
[edit] Through-hole resistors
Most resistors you'll see are marked with a part code, in the form of colored bands painted onto the resistor. These can be read via a defined standard like this:
Let's make an example of this resistor: 
First find the tolerance band, it will typically be gold or silver (in this case, it's silver).
Starting from the other end, identify the first band, and write down the number associated with that color. In this case the color is red, so you write down "2."
Now the next color is green, so write down a "5" (so far, you've got "25").
Now read the third or 'multiplier' band (blue = 1M) and multiply the value by it. Here, the indicated resistance value is 25 MOhm.
The tolerance band tells you how accurate the resistor's marked value is; here, it's silver -- so it's a 10% resistor (kinda' cheap). In all, this resistor will have a resistance of 25 ± 2.5 MOhm.
Note that 1% and 2% resistors will have 3 digits of resistance value.
[edit] Surface-mount resistors
Based on a list posting by Vincent Chin...
Some surface-mount resistors are marked with exactly the same color code as shown above for through-hole resistors.
More often, though, surface-mount resistors are marked with a basic three symbol code (See Table 1 for examples) -- the first two marked digits are the first two digits of the value, and the third digit is a multiplier (power of ten). An "R" or a "K" in the second position indicates a decimal point. All the values marked are in Ohms.
A two symbol system (See Table 2) is also occasionally used (although this is more common for capacitors). The first symbol in the two digit system is a letter representing a number value; the second symbol is the multiplier (power of ten). All the values marked are in Ohms.
Also See:
[edit] External Refferences
- Resistors available from Solarbotics.com
