From Electromechanical Design Handbook, Third Edition
6.6 ELECTRICAL COMPONENTS
6.6.1 Resistors
Resistors are circuit elements that limit the flow of current in a circuit. The basic equation W = I 2 R relates the amount of heat dissipated by a resistor to the current flowing through the resistor. Thus, a resistor that passes 1.5 A of current and whose rating is 500 ? dissipates (1.5) 2 500 = 1125 W power.
There are various common forms of resistors in use:
-
Carbon composition
-
Metal film
-
Wirewound (precision and power types)
-
Filament or ribbon (power types)
-
Cast-alloy element (power type)
Refer to Fig. 6.3 and the accompanying listing for an explanation of the color code for carbon composition resistors.
Figure 6.3: Carbon composition resistors and color code.
| Color | 1st digit (A) | 2d digit (B) | Multiplier (C) | Tolerance (D) |
|---|---|---|---|---|
| Black | 0 | 0 | 1 | |
| Brown | 1 | 1 | 10 | |
| Red | 2 | 2 | 100 | |
| Orange | 3 | 3 | 1000 | |
| Yellow | 4 | 4 | 10000 | |
| Green | 5 | 5 | 100000 | |
| Blue | 6 | 6 | 1000000 | |
| Violet | 7 | 7 | 10000000 | |
| Gray | 8 | 8 | 100000000 | |
| White | 9 | 9 |
| |
| Gold |
|
| 0.1 | 5% |
| Silver |
|
| 0.01 | 10% |
| No color |
|
|
| 20% |
Precision wirewound and carbon-film resistors have the resistance value and tolerance (usually 1 percent) marked on them.
Maximum Continuous Working Voltage across a Resistor
where P = power, W ( I 2 R); R = resistance, ?.
Example. How many...
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