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TABLE OF CONTENTS Robotics
Chapter 3.2.1 Resistors
Fixed Resistors
Resistors are one of the most commonly used components in electronics. As its name implies, resistors resist the flow of electrons. They are used to add resistance to a circuit. The color bands around the resistors are color codes that tell you its resistance value. Recall that a unit of resistance is an ohm.
One last important note about resistors is their wattage rating. You should not use a 1/4-watt resistor in a circuit that has more than 1/4 watt of power flowing.
For example, it is not okay to use a 1/4-watt resistor in a 1/2-watt circuit. However, it is okay to use a 1/2-watt resistor in a 1/4-watt circuit.
The tolerance band indicates the accuracy of the values. A 5% tolerance (gold band) for example, indicates that the resistor will be within 5% of its value. For most applications, a resistor within 5% tolerance should be sufficient.
FIGURE 3.3 Shows the way to calculate the value of a resistor.
FIGURE 3.4 A variable resistor.
To get the value of a resistor, hold the resistor so that the tolerance band is on the right.
The first two color bands from the left are the significant figures simply write down the numbers represented by the colors. The third band is the multiplier it tells you how many zeros to put after the significant figures. Put them all together and you have the value.
There are resistors with more bands and other types for specific applications.
However, 4-band resistors (the ones discussed here) are the most common and
should work for most projects.
Variable Resistors
Variable Resistors, or potentiometers, often have three terminals and can change resistance easily.
Horizontally Adjustable Presets
These are miniature versions of the standard variable resistor. They are designed to be mounted directly onto the circuit board and adjusted only when the circuit is built. For example, to set the frequency of an alarm tone or the sensitivity of a light-sensitive circuit, a small screwdriver or similar tool is used to adjust the presets.
Preset Symbol
Presets are much cheaper than standard variable resistors so they are sometimes used in projects where a standard variable resistor would normally be used.
Multiturn presets are used where very precise adjustments must be made. The screw must be turned many times (10+) to move the slider from one end of the track to the other, giving very fine control.
FIGURE 3.5 Symbol for preset.
A variable resistor is a potentiometer with only two connecting wires instead of three. However, although the actual component is the same, it does a very different job. The pot allows us to control the potential passed through a circuit. The variable resistance lets us adjust the resistance between two points in a circuit.
A variable resistance is useful when we don t know in advance what resistor value will be required in a circuit. By using pots as an adjustable resistor we can set the right value once the circuit is working. Controls like this are often called presets because they are set by the manufacturer before the circuit is sent to the customer. They re usually hidden away inside the case of the equipment, away from the fingers of the users!
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