A 

resistor is a two-terminal passive electronic component which implements electrical resistance as a

circuit element. When a voltage V is applied across the terminals of a resistor, a current I will flow through
the resistor in direct proportion to that voltage. This constant of proportionality is called conductance, G.
The reciprocal of the conductance is known as the resistance R, since, with a given voltage V, a larger
value of R further "resists" the flow of current I as given by Ohm's law:

Ohm's law
The behavior of an ideal resistor is dictated by the relationship specified in Ohm's law:

Ohm's law states that the voltage (V) across a resistor is proportional to the current (I) passing
through it, where the constant of proportionality is the resistance (R).

Equivalently, Ohm's law can be stated:

Series and parallel resistors

Main article:  Series and parallel circuits

In a series configuration, the current through all of the resistors is the same, but the voltage across each
resistor will be in proportion to its resistance. The potential difference (voltage) seen across the network is
the sum of those voltages, thus the total resistance can be found as the sum of those resistances:

As a special case, the resistance of N resistors connected in series, each of the same
resistance R, is given by NR.

Resistors in a parallel configuration are each subject to the same potential difference (voltage),
however the currents through them add. The conductances of the resistors then add to
determine the conductance of the network. Thus the equivalent resistance (Req) of the network
can be computed:
Power dissipation
The power P dissipated by a resistor (or the equivalent resistance of a resistor network) is calculated

as: 

Variable resistors
[edit]Adjustable resistors
A resistor may have one or more fixed tapping points so that the resistance can be changed by moving
the connecting wires to different terminals. Some wirewound power resistors have a tapping point that
can slide along the resistance element, allowing a larger or smaller part of the resistance to be used.

Potentiometers
Main article:  Potentiometer

A common element in electronic devices is a three-terminal resistor with a continuously adjustable

tapping point controlled by rotation of a shaft or knob. These variable resistors are known
aspotentiometers when all three terminals are present, since they act as a continuously
adjustable voltage divider. A common example is a volume control for a radio receiver.[13]

Standards
[edit]Production resistors
Resistor characteristics are quantified and reported using various national standards. In the US, MIL-
STD-202[15] contains the relevant test methods to which other standards refer.

There are various standards specifying properties of resistors for use in equipment:

 BS 1852
 EIA-RS-279
 MIL-PRF-26
 MIL-PRF-39007 (Fixed Power, established reliability)
 MIL-PRF-55342 (Surface-mount thick and thin film)
 MIL-PRF-914
 MIL-R-11
 MIL-R-39017 (Fixed, General Purpose, Established Reliability)
 MIL-PRF-32159 (zero ohm jumpers)

There are other United States military procurement MIL-R- standards.

Four-band resistors
Four-band identification is the most commonly used color-coding scheme on 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. The first three bands are equally
spaced along the resistor; the spacing to the fourth band is wider. 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
560,000 Ω at ±2% tolerance accuracy. 560,000 Ω 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

Orang
3 3 ×103 15 ppm
e
Yello
4 4 ×104 25 ppm
w

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)

There are many mnemonics for remembering these colors.edit

Industrial type designation

Format: [two letters]<space>[resistance value (three digit)]<nospace>[tolerance code(numerical - one
digit)] [19]

Power Rating at 70 °C
Power
rating MIL-R-11 MIL-R-39008
Type No.
(watts Style Style
)
BB ⅛ RC05 RCR05
CB ¼ RC07 RCR07
EB ½ RC20 RCR20
GB 1 RC32 RCR32
HB 2 RC42 RCR42
GM 3 - -
HM 4 - -
 Tolerance Code
Industrial
MIL
type Tolerance
Designation
designation
5 ±5% J
2 ±20% M
1 ±10% K
- ±2% G
- ±1% F
- ±0.5% D
- ±0.25% C
- ±0.1% B

The operational temperature range distinguishes commercial grade, industrial grade and military grade
components.

 Commercial grade: 0 °C to 70 °C
 Industrial grade: −40 °C to 85 °C (sometimes −25 °C to 85 °C)
 Military grade: −55 °C to 125 °C (sometimes -65 °C to 275 °C)
 Standard Grade -5 °C to 60 °C