The Laws of Circuit - you can learn and practice by just reading

Copyright. Charles Kim 2006

Resistor Specification

Value, Tolerance, and Power Rating
There are three important parameters that specify a resistor: value, tolerance, and power rating.
Have you ever wondered why the part store (or part cabinet) in your school does not have 115 ohm resistor you need in your implementation of a circuit on your breadboard?  If yes, probably you also wondered why these weird values of resistors in the store: 3.9 kohm, 4.7 kohm, 22 ohm, 33 ohm, but not 25 ohm.  Even though 25 ohm value is more reasonable than 33 or 22 ohm resistors?   To discuss this resistor value, we need to talk about the second parameter: tolerance.  Tolerance is simply error from the value specified.  In other words, if tolerance is 10%, then the expected value from a resistor of 10 ohm is between 9 (i.e., 10 - 1) ohm and 11 (i.e., 10 + 1) ohm.   Two most common tolerance values are 5% and 10%.  There is 1% tolerance also but you can rarely see in the cabinet.

Standard Resistor Values
The following are a part of standard resistor values in ohm for 5% and 10 % tolerance:

5% resistors: 1.0, 1.1, 1.2, 1.3, 1.5, 1.6, 1.8, 2.0, 2.2, 2.4, 2.7, 3.0, 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2, 9.1, 10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91, 100, 110, 120, 130, 150, 160, 180, 200, 220, 240, 270, 300, 330, 360, 390, 430, 470, 510, 560, 620, 680, 750, 820, 910, 1.0k, 1.1k, 1.2k, 1.3k, 1.5k, 1.6k, 1.8k, 2.0k, 2.2k, 2.4k, 2.7k, 3.0k, 3.3k, 3.6k, 3.9k, 4.3k, 4.7k, 5.1k, 5.6k, 6.2k, 6.8k, 7.5k, 8.2k, 9.1k, 10k, 11k, 12k, 13k, 15k, 16k, 18k, 20k, 22k, 24k, 27k, 30k, 33k, 36k, 39k, 43k, 47k, 51k, 56k, 62k, 68k, 75k, 82k, 91k.

10% resistors: 1.2,1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82, 120, 150, 180, 220, 270, 330, 390, 470, 560, 680, 820, 1.2k, 1.5k, 1.8k, 2.2k, 2.7k, 3.3k, 3.9k, 4.7k, 5.6k, 6.8k, 8.2k, 12k, 15k, 18k, 22k, 27k, 33k, 39k, 47k, 56k, 68k, 82k

Now we wonder why we have such, weird, so-called "standard" resistor values.   Who made these?  The EIA (Electronic Industries Association: http://www.eia.org/) did.  EIA specifies "standard values" for resistors, sometimes referred to as the "preferred value" system.  The preferred value system has its origins in the early years of the last century at a time when most resistors were carbon-graphite with relatively poor manufacturing tolerances.  The rationale is simple - select values for components based on the tolerances with which they could be manufactured.  Using 10% tolerance devices as an example, suppose that the first preferred value is 100 ohms.  It makes little sense to produce a 105 ohm resistor since 105 ohms falls within the 10% tolerance range of the 100 ohm resistor. The next reasonable value is 120 ohms because the 100 ohm resistor with a 10% tolerance is expected to have a value somewhere between 900 and 110 ohms. The 120 ohm resistor has a value ranging between 110 and 130 ohms. Following this logic, the preferred values for 10% tolerance resistors between 100 and 1,000 ohms would be 100, 120, 150, 180, 220, 270, 330 and so on.

E Series Resistors
EIA has 6 series of resistor values with different tolerances. The E6 series (6 values for each multiple of ten, for resistors with 20% tolerance) has the following values: 10, 15, 22, 33, 47, 68, ... then it continues 100, 150, 220, 330, 470, 680, 1000 etc.  Notice how the step size increases as the value increases. For this series the step (to the next value) is roughly half the value. On the other hand, the E12 series (12 values for each multiple of ten, for resistors with 10% tolerance) has the following resistor values: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82, ... then it continues 100, 120, 150 etc.

Power Rating
The power rating specifies the maximum power that can be dissipated by a resistor.  Electrical energy is converted to heat when current flows through a resistor. Usually the effect is negligible, but if the resistance is low (or the voltage across the resistor is high) a large current may pass and make the resistor becomes noticeably warm.  Remember the resistive heating is proportional to I2R.  The resistor must be able to withstand the heating effect and resistors have power ratings up to which they can stand.  Some typical ratings are 1/8 W(Watts), ¼ W, ½ W, 1W, and 2W.  Power ratings of resistors are rarely quoted in part list because for most circuits the standard power ratings of ¼ W or ½ W are suitable. For rare cases where a higher power is required, it should be clearly specified in the part list, specifically for circuits using low value resistors (less than about 300ohm) or serving high voltages (more than 15V).   Anyway, in selecting a resistor for some particular application, remember that one important criterion is the expected power dissipation.

Color Coding and Size
Resistor values and tolerances are color coded, and power ratings are usually recognizable by the physical size.  This information is readily available online or in print, so I do not dispaly it here.

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