A component in an electrical circuit designed to introduce a known resistance to the flow of current. Resistance changes with temperature rise, increasing in metals, and falling in semiconductors. It also varies with the size of the conductor, rising as it becomes longer or thinner.
The electrical resistance is equal to the voltage drop across the resistor divided by the current through the resistor.
Nonideal characteristics
A resistor has a maximum working voltage and current above which the resistance may change (drastically, in some cases) or the resistor may be physically damaged (overheat or burn up, for instance).
Furthermore, all real resistors also introduce some inductance and a small amount of capacitance, which change the dynamic behavior of the resistor from the ideal.
Types of resistor
Fixed resistors
Some resistors are cylindrical, with the actual resistive material in the center (composition resistors, now obsolete) or on the surface of the cylinder (film) resistors, and a conducting metal lead projecting along the axis of the cylinder at each end(axial lead).
Variable resistors
The variable resistor is a resistor whose value can be adjusted by turning a shaft or sliding a control.
Other types of resistors
A metal oxide varistor (MOV) is a special type of resistor that changes its resistance with rise in voltage: a very high resistance at low voltage (below the trigger voltage) and very low resistance at high voltage (above the trigger voltage). There are two kinds, classified according to the sign of their temperature coefficients: A Positive Temperature Coefficient (PTC) resistor is a resistor with a positive temperature coefficient.Identifying resistors
Most axial resistors use a pattern of colored stripes to indicate resistance.
Four-band axial resistors
Four-band identification is the most commonly used color coding scheme on all resistors. Coefficient
| Black | 0 | 0 | ×100 | ||
| Brown | 1 | 1 | ×101 | ±1% (F) | 100 ppm |
| Red | 2 | 2 | ×102 | ±2% (G) | 50 ppm |
| Orange | 3 | 3 | ×103 | 15 ppm | |
| Yellow | 4 | 4 | ×104 | 25 ppm | |
| Green | 5 | 5 | ×105 | ±0.5% (D) | |
| Blue | 6 | 6 | ×106 | ±0.25% (C) | |
| Violet | 7 | 7 | ×107 | ±0.1% (B) | |
| Gray | 8 | 8 | ×108 | ±0.05% (A) | |
| White | 9 | 9 | ×109 | ||
| Gold | ×0.1 | ±5% (J) | |||
| Silver | ×0.01 | ±10% (K) | |||
| None | ±20% (M) |
Note: red to violet are the colors of the rainbow where red is low energy and violet is higher energy. Subsequently, if you buy 100 resistors of the same value with a tolerance of +/- 10%, you won't get some resistors with the correct value, some off by a little and the worst off by 10%;
E24 preferred values, includes E12 values and : 11, 13, 16, 20, 24, 30, 36, 43, 51, 62, 75, 91
5-band axial resistors
5-band identification is used for higher tolerance resistors (1%, 0.5%, 0.25%, 0.1%), to notate the extra digit. ROY of Great Britain had a Very Good Wife Buffalo Bill Roamed Over Yellow Grass Because Vistas Grand Were God's Sanctuary Bully Brown Ran Over a Yodeling Goat, Because Violet's Granny Was Gone Snorkeling Buy Better Resistance Or Your Grid Bias May Go Wrong
Black Brown Red Orange Yellow Green Blue Violet Gray White (Gold Silver)
SMD resistors
Surface mounted resistors are printed with numerical values in a code related to that used on axial resistors. For example:
| "334" | = 33 × 10,000 ohms = 330 kiloohms |
| "222" | = 22 × 100 ohms = 2.2 kiloohms |
| "473" | = 47 × 1,000 ohms = 47 kiloohms |
| "105" | = 10 × 100,000 ohms = 1 megaohm |
Resistances less than 100 ohms are written: 100, 220, 470. For example:
| "100" | = 10 × 1 ohm = 10 ohms |
| "220" | = 22 × 1 ohm = 22 ohms |
Sometimes these values are marked as "10" or "22" to prevent a mistake. For example:
| "4R7" | = 4.7 ohms |
| "0R22" | = 0.22 ohms |
| "0R01" | = 0.01 ohms |
Precision resistors are marked with a four-digit code, in which the first three digits are the significant figures and the fourth is the power of ten. For example:
| "1001" | = 100 × 10 ohms = 1 kiloohm |
| "4992" | = 499 × 100 ohms = 49.9 kiloohm |
| "1000" | = 100 × 1 ohm = 100 ohms |
"000" and "0000" sometimes appear as values on surface-mount zero-ohm links, since these have (approximately) zero resistance.
|
Power rating (Watts) |
MIL-R-11 Style |
MIL-R-39008 Style |
|
|---|---|---|---|
| BB | 1/8 | RC05 | RCR05 |
| CB | ¼ | RC07 | RCR07 |
| EB | ½ | RC20 | RCR20 |
| GB | 1 | RC32 | RCR32 |
| HB | 2 | RC42 | RCR42 |
| GM | 3 | - | - |
| HM | 4 | - | - |
| Industrial type designation | Tolerance | MIL Designation |
|---|---|---|
| 5 | ±5% | J |
| 2 | ±20% | - |
| 1 | ±10% | K |
| - | ±2% | G |
| - | ±1% | F |
| - | ±0.5% | D |
| - | ±0.25% | C |
| - | ±0.1% | B |
The operational temperature range distinguishes commercial grade, industrial grade and military grade components.
Commercial grade: 0°C to 70°C Industrial grade: −40°C to 85°C (sometimes −25°C to 85°C) Military grade: −55°C to 125°CCalculations
Ohm's law
The relationship between voltage, current, and resistance through a metal wire, and some other materials, is given by a simple equation called Ohm's Law:
V = IRwhere V (or U in some languages) is the voltage (or potential difference) across the wire in volts, I is the current through the wire in amperes, and R, in ohms, is a constant called the resistance—in fact this is only a simplification of the original Ohm's law (see the article on that law for further details).
Power dissipation
The power dissipated by a resistor is the voltage across the resistor multiplied by the current through the resistor:
All three equations are equivalent.
The total amount of heat energy released is the integral of the power over time:
If the average power dissipated exceeds the power rating of the resistor, then the resistor will first depart from its nominal resistance, and will then be destroyed by overheating. For two resistors,
The current through resistors in series stays the same, but the voltage across each resistor can be different.
Types of resistors:
Carbon composition Carbon film Metal film Metal oxide Wirewound (usually has higher parasitic inductance) Cermet Phenolic TantalumFoil resistor
Foil resistors have had the best precision and stability ever since they were introduced in 1958 by Berahard F.
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