Reading Resistor and Capacitor Values

Resistors

Physically most resistors look like the one shown below (except much smaller), with 3 colored bands that give the value of the resistor, and a fourth band (gold or silver) that gives the precision of the resistor. If the fourth band is gold the resistor is within 5% of the specified value; silver means 10%. The other three bands determine the specified value.  Each band represents a number using the mapping:

resistor.GIF (8630 bytes)

0=Black
1=Brown
2=Red
3=Orange
4=Yellow
5=Green
6=Blue
7=Violet
8=Gray
9=White

 

The value of the resistor is read using the color code and the formula:

R=Band1 Band2 * 10Band3

So if the three color bands are (from left to right) brown, red, orange, then the value of the resistance is:

R=12 * 103 = 12000 Ω = 12 kΩ.

The fourth band, the tolerance, is silver and denotes that the resistor is accurate to within 10% (gold would mean 5%).

The colors yellow, violet, orange would be 47*103=47000 Ω=47 kΩ(kilo Ohm).

Resistor Diameter

Typical
Rated Power

1/16" 1/4 W

1/8"

1/2 W

3/16"

1 W

1/4"

2 W

Note: 1% resistors use the same color code, but have a fourth band for the resistor value.

This links to a JavaScript  Resistor Calculator..


Standard values

The 5% resistors that we have in the lab come in 17 different values for every decade of value (i.e., there are 17 resistance values between 100Ω and 1kΩ, and 17 between 100kΩ and 1MΩ). These values have as their two digits the numbers 10, 12, 15, 18, 20, 22, 27, 30, 33, 39, 47, 51, 56, 68, 75, 82, 91.  So for example between 100Ω and 1kΩ resistors have the values 100, 120, 150, 180, 200, ... 820, 910.  Between 100kΩ and 1MΩ the values become 100k, 120k, ... 910k.  Although these may seem like a strange set of values, they were chosen so that consecutive numbers in the sequence have a difference of between 10% and 20%.  If possible, learn the color codes for the standard values.

Also check out http://www.resistorguide.com/.

Capacitors

Reading capacitors is just like reading resistors, only different. It is not much more difficult, but unfortunately there are several common conventions for marking capacitors. Some capacitors will have the value written on the side (eg, 10μF (microFarad, 10-6 F), or 220μF). Some will have a number 0.01 or 0.003, in this case the unit is usually assumed to be μF. The third common convention is much like that for resistors and consists of three number followed by a letter. The letter is the tolerance, and the three numbers denote the value and are read in the same way as that for resistors except the base unit is pF (picoFarad, 1 pF = 10-12 F = 10-6 μF). So a marking of 154K is 15*104=1.5*105 pF=150000pF=0.15 μF.  The letter K represents the tolerance (+/-10%).  Other common letters for tolerance are M (+/- 20%), J (+/- 5%), G (+/- 2%), F(+/- 1%) and Z (+80%/-20%).  The latter is used for filter capacitors on power supplies where you don't care if the capacitor is bigger than specified, but definitely don't want it smaller.  The use of the letters K and M are unfortunate since they are also used for kilo and micro (or mega).

A common mistake is to think of a pF as 10-3 μF instead of as 10-6 μF.  However, that is a nF (nanoFarad, 10-9 F) and the term nF is seldom used.  Instead 1 nF is either expressed as 0.001 μF, or as 1000 pF.


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