Why are measured values of resistors different from the stated values?Peerpe51 - 6 Answers
Resistors are manufactured with a tolerance which is a percentage of the resistance value which is understood to be plus or minus that percentage. This is due to the variances in materials used in manufacturing process. Another thing that needs to be considered when measuring a resistor is if it is in the circuit or not. When reading resistors in a circuit it is best to lift one lead of the resistor to break the circuit and ensure you are actually reading the resistor. If he resistor is in a complete circuit then gou would be reading the resistance of the circuit which may or not be the same as the resistor depending on how the circuit is designed and the remaining circuit resistance....
Because they have a tolerance band and depending on which color band they have it will have a different variance of how close it is to the actual resistance.
If you want a resistor that is almost exact then you need to buy a precision component. I've used these in the air force as a radar technician. The precision resistors have a 1 percent tolerance but can often cost hundreds of dollars.
If you want a cheap precision one, either sort through a batch of regular resistors and measure their actual value until you find one that is exactly what you want. Or you can get a variable resistor (potentiometer) and set it to the value you want.
Stated values are the average you can expect for the type of resistor you have. Resistors are also provided with a tolerance, given as a percent. The difference between the actual value and stated value should be no more than the tolerance (as a fraction) multiplied by the stated value.
Put another way: If the stated value is "S" and the tolerance in % is "T" then the actual value "R" will be S-T*S/100 <= R <= S+T*S/100.
For more information, see http://www.doctronics.co.uk/resistor.htm
Resistor values will differ from the stated value due to variations in materials and the accuracy and control of the manufacturing process. If more stringent techniques are used during manufacturing, resistors can be made with lower tolerance. Common tolerances for resistors are 5% and 1%.
The accuracy of meters used to measure resistance can cause a discrepancy also. A digital multimeter may have an accuracy of 0.5 % on DC voltage but the accuracy might be only 1.1 to 1.5 % on the resistance ranges.
Slight inconsistencies in materials and manufacturing techniques are the main reason for slight differences in values. To keep to very close tolerances, a manufacturer must spend more time/effort in quality control which results in a somewhat higher price for resistors with more precise values....
Measured values of resisters different from the stated values due to tolerance which is the materials properties by which the resisters are made....