A resistor is a two-terminal electronic component designed to oppose an electric current by producing a voltage drop between its terminals in proportion to the current, that is, in accordance with Ohm's law: V = IR. The resistance R is equal to the voltage drop V across the resistor divided by the current I through the resistor.
Resistors are characterized primarily by their resistance and the power they can dissipate. Other characteristics include temperature coefficient, noise, and inductance. Practical resistors can be made of resistive wire, and various compounds and films, and they can be integrated into hybrid and printed circuits. Size, and position of leads are relevant to equipment designers; resistors must be physically large enough not to overheat when dissipating their power. Variable resistors, adjustable by changing the position of a tapping on the resistive element, and resistors with a movable tap ("potentiometers"), either adjustable by the user of equipment or contained within, are also used.
Resistors are used as part of electrical networks and electronic circuits.
There are special types of resistor whose resistance varies with various quantities, most of which have names, and articles, of their own: the resistance of thermistors varies greatly with temperature, whether external or due to dissipation, so they can be used for temperature or current sensing; metal oxide varistors drop to a very low resistance when a high voltage is applied, making them suitable for over-voltage protection; the resistance of a strain gauge varies with mechanical load; the resistance of photoresistors varies with illumination; the resistance of a Quantum Tunnelling Composite can vary by a factor of 1012 with mechanical pressure applied; and so on.
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| Potentiometer | ||
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| Resistor | Variable Resistor |
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| Potentiometer | ||
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| Resistor | Variable resistor |
Production resistors
There are various standards specifying properties of resistors for use in equipment:
- BS 1852
- EIA-RS-279
- MIL-PRF-26
- MIL-PRF-39007
- MIL-PRF-55342
- MIL-PRF-914
- MIL-R-11
- MIL-R-39008
- MIL-R-39017
There are other United States military procurement MIL-R- standards.
Four-band axial resistors
Four-band identification is the most commonly used color-coding scheme on all resistors. It consists of four colored bands that are painted around the body of the resistor. The first two bands encode the first two significant digits of the resistance value, the third is a power-of-ten multiplier or number-of-zeroes, and the fourth is the tolerance accuracy, or acceptable error, of the value. Sometimes a fifth band identifies the thermal coefficient, but this must be distinguished from the true 5-color system, with 3 significant digits.
For example, green-blue-yellow-red is 56×104 Ω = 560 kΩ ± 2%. An easier description can be as followed: the first band, green, has a value of 5 and the second band, blue, has a value of 6, and is counted as 56. The third band, yellow, has a value of 104, which adds four 0's to the end, creating 560,000Ω at ±2% tolerance accuracy. 560,000Ω changes to 560 kΩ ±2% (as a kilo- is 103).
Each color corresponds to a certain digit, progressing from darker to lighter colors, as shown in the chart below.
| Color | 1st band | 2nd band | 3rd band (multiplier) | 4th band (tolerance) | Temp. 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 | ×10-1 | ±5% (J) | |||
| Silver | ×10-2 | ±10% (K) | |||
| None | ±20% (M) |
SMT resistors
Surface mounted resistors are printed with numerical values in a code related to that used on axial resistors. Standard-tolerance Surface Mount Technology (SMT) resistors are marked with a three-digit code, in which the first two digits are the first two significant digits of the value and the third digit is the power of ten (the number of zeroes). For example:
| "334" | = 33 × 10,000 ohms = 330 kilohms |
| "222" | = 22 × 100 ohms = 2.2 kilohms |
| "473" | = 47 × 1,000 ohms = 47 kilohms |
| "105" | = 10 × 100,000 ohms = 1 megohm |
Resistances less than 100 ohms are written: 100, 220, 470. The final zero represents ten to the power zero, which is 1. 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.
Resistances less than 10 ohms have 'R' to indicate the position of the decimal point (radix point). 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 kilohm |
| "4992" | = 499 × 100 ohms = 49.9 kilohm |
| "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.
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