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What Are the Causes of Resistor Reading Differences?

Resistors protect sensitive electronic components from excessive current. Found in radios, computers, televisions and electronic appliances, resistors have three to five colored bands. These colored bands indicate their resistance, tolerance and temperature coefficients. Other factors that determine resistor readings include age, the resistor type and its composition. Resistance is measured in ohms, kilohms (1,000 ohms) or megohms (1,000,000 ohms).

Resistor Color Code
- The first band on a resistor represents the first digit in its resistance, the second band represents the second band and the third band represents the multiplier (the number of zeroes). The fourth and fifth bands represent tolerance and temperature coefficient, respectively. The colors correspond with the following numbers: black -- 0; brown -- 1; red -- 2; orange -- 3; yellow -- 4; green -- 5; blue -- 6; violet -- 7; gray -- 8; and white -- 9. A resistor with black, brown and yellow bands would be 100,000 ohms (100K ohms).
Tolerance
- The percent tolerance determines how much a resistors rating may deviate in percentage. The colors correspond with these values: brown -- 1 percent; red -- 2 percent; gold -- 5 percent; silver -- 10 percent; no band -- 20 percent. A 100,000 ohm resistor with no band ranges between 90,000 and 110,000 ohms. Tighter-tolerance resistors are more expensive.
Temperature Coefficient
- Temperature coefficients are measured in parts per million per degrees Celsius (ppm/degree C). A resistor with a temperature coefficient of 500 ppm per degree Celsius will change by 500 ohms per rated megohm for every degree in temperature change. While changes in resistance caused by temperature are undesirable, thermistors (thermal resistors) exploit this property for use in temperature detection circuits.
Power Dissipation
- Resistors are rated in power dissipation -- 1/8, 1/4, 1/2, 1, 2, 5, 10, 25, 50 and 100 watts. The power dissipation is caused by the current flowing through the resistor itself. Mismatching resistors and frequent voltage surges may overheat resistors and permanently change their resistance.
Frequency Response
- Imperfections in a resistor cause changes in its resistance readings. As the frequency of a signal flowing through a resistor changes, the effective resistance may change. This resistance tends to reduce as the frequency increases. Wirewound resistors are not well-suited for high frequencies, while carbon composition and carbon film resistors respond well.

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