Resistor FAQ (Frequently Asked Questions)

By John Patrick (j.s.patrick@ieee.org)

  1. About this FAQ
  2. What are resistors?
    1. What are their symbols on a schematic?
    2. How do you read the schematic types and values?
  3. What are...
    1. ...pots/potentiometers/rheostats?
    2. ...thermistors?
    3. ...photo resistors?
  4. What types of resistor packages are available?
  5. What is the difference between carbon composition, carbon film, metal film, and wire-wound resistors?
  6. What are those markings, and how do I read them?
    1. What do the colors mean?
      1. Is there an easy way to remember the colors?
    2. This resistor doesn't have colors, but funny numbers. How do I read them?
  7. What are the standard values of resistors?
    1. Why are resistor values like 1.0k, 1.2k, 1.5k, 1.8k, 2.2k, etc., and not just simply 1k, 2k, 3k, 4k, etc.?
    2. What is a RETMA series (E12, E24, E48, E96, E192)?
  8. What does the power rating mean?
  9. I think this is a resistor, but it only has one black/red/silver band on it. What is it?
  10. Where can I buy a resistor of XXXX type/value? Who makes XXXX type/value of resistor?
  11. References
  12. Contributors and helpers

1. About this FAQ

This Frequently Asked Questions (FAQ) was written by John S. Patrick for beginners and students in electronics and electrical engineering. Seasoned professionals could also find interesting tid-bits of information here. I hope you'll find this FAQ useful and educational.

If you use this FAQ, let me know what you think. If there's a question that isn't listed, let me know, too, and I'll add it. Feel free to link to this FAQ, and let me know.

2. What are resistors?

Resistors are small electronic devices that resist the flow of electrical current. A resistor obeys Ohm's Law, which states that the voltage (or potential) across a resistor is proportional to the current flow through the resistor. In formula form, V=IR, where V is the voltage across the resistor, R is the value of the resistor in ohms, and I is the current flow through the resistor in amperes (amps).

Note that Ohm's Law is for "perfect" resistors. Since there is no such thing as a "perfect" resistor, Ohm's Law approximates what really happens. It doesn't take into consideration thermal effects, noise, etc. For most calculations, the approximation of Ohm's Law is good enough.

2.1 What are their symbols on a schematic?

[Resistor Schematic Images] The basic schematic diagrams of resistors are shown at right.

They are:

a) Fixed value resistor
b) Variable resistor
c) Potentiometer (adjustable resistor)
d) Thermistor (temperature variable resistor)
e) Photo resistor

2.2 How do you read the schematic types and values?

When reading a schematic, you need to determine four things about the resistor:

  1. Resistor type -- from the schematic diagram drawn (see above).
  2. Value -- Most include the value as 33M, 2.2k, 220, etc. with the units being implied as ohms. European schematics may be marked differently, see below.
  3. Tolerance -- Most diagrams will have a footnote (e.g. All resistors are 5% tolerance unless noted), and resistors that contradict the note will have their tolerance value next to the value
  4. Power rating -- Most diagrams will have a footnote (e.g. All resistors are 1/4 Watt unless noted), and resistors that contridict the note will have a value, often designated in Watts (e.g. 1/2 W, 1 W).

3. What are...

3.1 ...pots/potentiometers/rheostats?

A potentiometer (also known as a pot, rheostat or adjustable resistor) is a form of resistor that allows its resistance value to be varied. This is usually accomplished by rotating a contact wiper along an open resistor, thus changing the amount of resistor that the current has to flow through.

Generally speaking, a rheostat is a larger potentiometer that can handle dissipating greater amounts of power (as heat). They are generally used in AC power circuits, such as an old-fashion home light dimmer. (Modern home light dimmers use a different, less heat-generating circuit.)

3.2 ...thermistors?

A thermistor is a resistor whose resistance varies linearly with temperature. Thermistors come in either negative temperature coefficient (NTC) or positive temperature coefficient (PTC). An NTC thermistor has its resistance go down when the temperature goes up, and its resistance go up when the temperature goes down. For a PTC thermistor, the resistance changes in the same direction as the temperature.

3.3 ...photo resistors?

A photo resistor is a resistor whose resistance varies with the mount of light hitting it. Photo resistors are often made with Cadmium Sulfide (CdS).

4. What types of resistor packages are available?

Fixed-value resistors come in many physical forms, such that it is generally difficult to list every type available. They do, however, fall into two different physical categories:

Through-hole

Through-hole resistors come in small, cylinder-like packages with axial wire leads (the wires come out the main axis of the cylinder). They are marked with either colored bands or numerically to determine their resistance. To read the value, go here.

Some through-hole resistors are not cylindrical, but rectangular. These resistors are know as power resistors due to their ability to handle large power loads.

Surface Mount

Surface mount resistors come in small, "chip-like" packages, typically 2mm by 3mm in size. Instead of wire leads, their ends are metalic, while the rest is ceramic. Surface mount resistors are marked in the same method as the color bands of through-hole resistors, but use numbers instead of the color scheme.

Resistors also come in "network" packages (available in both through-hole and SMT), that have several resistors in a single part. The through-hole variety comes in a SIPP (Single Inline Pin Package) or DIP (Dual Inline Package) form, and either have several resistors with a common connection, or several independent resistors. SMT varieties of resistor networks also exist.

Variable resistors come in many forms.

5. What is the difference between carbon composition, carbon film, metal film, and wire-wound resistors?

There are four basic types of resistor composition: carbon composition, carbon film, metal film, and wire-wound.

Carbon Composition


Carbon composition resistors are made from a molded carbon powder that has been mixed with a phenolic binder to create a uniform resistive body. It is then surrounded in a insulating case after attaching end leads.

Carbon composition resistors are used in applications in which the initial tolerance does not need to be any closer than +/- 5% of the value (i.e. they are not high tolerance devices). In the long term, the tolerance of carbon composition resistors may be up to +/- 20%.

Moisture can be absorbed into the resistor, as the phenolic plastic coating is not fully moisture-proof. Moisture problems can change the value by as much as +/- 15% to +/- 20%. This can be cured by baking the parts in a dry oven, at 100 +/- 5 degrees C, with the amount of time dependant on the size of the resistor. See the manufacturers recommended procedures for baking resistors.

In order to reduce the changes created by humidity, the carbon composition resistors should be operated at least at 20% of their rated power.

High voltages can cause breakdown in the insulation of fixed composition resistors, and manufacturers have recommended procedures for high voltage usage.

Carbon Film


Metal Film


Wire-wound


6. What are those markings, and how do I read them?

The markings on a resistor show its resistance value in ohms. The traditional through-hole resistor is marked with color bands, as shown below. Some resistors show their value in numbers, including Mil-spec resistors, which have their own special code.

6.1 What do the colors mean?

[Resistor]Each of the color bands have a specific meaning. The first two colors give the base value of the resistor; the next gives the multiplier, and the final band the tolerance. If a resistor has five bands, it is a more precise resistor, in which the first three bands represent the base value, and the fourth and fifth represent the multiplier and tolerance, respectively.

In order to read the color markings, you need to know what colors stand for what numbers. Here is a table showing you each color and it's respected value/multipler:

Color Value Multiplier Tolerance
BlackZero (0)x 1--
BrownOne (1)x 101%
RedTwo (2)x 1002%
OrangeThree (3)x 1,000--
YellowFour (4)x 10,000--
GreenFive (5)x 100,0000.5%
BlueSix (6)x 1,000,0000.25%
VioletSeven (7)x 10,000,0000.10%
GreyEight (8)--0.05%
WhiteNine (9)----
Gold--x 0.15%
Silver--x 0.0110%
None----20%
 In our example above, the first band is red, which represents the number two (2). The next band is violet, which is seven (7). The multiplier is yellow which is x 10,000, while the tolerance is silver, which represents a tolerance of 10%. Therefore, the value of the resistor is 27 x 10,000, or 270,000, or 270k, with a tolerance of +/-10%.

6.1.1 Is there an easy way to remember the colors?

People often remember the colors by using mnemonics--a saying with a seperate word representing each color in the code. These sayings are often crude, and possibly sexual in nature (Aside: People tend to remember crude sayings better that clean sayings. Why? Ask a psychologist!)

Here are a couple mnemonics for the resistor color code:

Bad Booze Rots Our Young Guts But Vodka Goes Well

Barbara Brown Runs Over Your Garden But Violet Grey Won't (courtesy of Mike Tomlinson, from Usenet)

Bad Boys Rape Our Young Girls But Violet Gives Willingly -- Get Some Now
(This one is special in that it includes the standard tolerance codes of Gold, Silver, and None.)

Bachelor Boys Rush Our Young Girls But Veronica Goes Wild for Silver and Gold
(This one also includes Silver and Gold, although some might argue in the wrong order.)

Bad Boys Rape Our Young Girls Behind Victory Garden Walls

Here are some more mnemonics (including the above): look under "engineering". There is also another mnemonics list here.

6.2 This resistor doesn't have colors, but funny numbers. How do I read them?

Some resistors, especially precision resistors, SMT resistors, and Mil-spec resistors use a numbering scheme to value their resistance. Some use just the value numerically, such as 2.2k 5%, which would put the resistance at 2,200 ohms, +/- 5%. Some European numberings put the metric multiplier where the decimal point would be, so the example above would be 2k2 5%, for 2.2 kilohms. 2M2 would be 2,200,000 ohms, or 2.2 megohms, while 2R2 would be 2.2 ohms.

Surface mount (SMT) resistors use the same numbering systems as the color bands above, but instead of using colors, they use the number. So for a 2.2 kilohm resistor, the number would be 222. For the example we used in the color section, the SMT resistor of the same value would read 274. All resistors are assumed to be 5% tolerance, 1/4 watt resistors unless otherwise noted. Smaller SMT resistors may be 1/8 watt.

Military specified, or Mil-spec, resistors use a special coding system for their values. Such a resistor may be designated as:

RC 30 J 33K5 G R

where the first field shows the type,
the second is the power rating,
the third is the temperature coefficient,
the fourth is the value numerically, as the European system above,
the fifth is the tolerance, and
the sixth is the failure rate.

The tables below show the codes for each category.

TypePower
Code Meaning Code Value, W
RAVariable, wirewound precision 20 2
25 3
30 4
RB Fixed, wirewound precision 08 0.5
16 0.666
17 1
18 1.5
19 2
52 1
53 0.5
55 0.25
56 0.125
57 1
58 2
70 0.5
71 0.25
RC Fixed, composition 05 0.125
07 0.25
09 0.5
20 0.5
30 1
32 1
41 2
42 2
RD Power, film, non-inductive 31 7
33 13
35 25
37 55
39 115
60 1
65 2
70 4
RE Power, wirewound, with heatsink 60 7.5
65 20
70 25
75 50
77 100
80 200
RL Fixed, film 07 0.25
20 0.5
32 1
42 2
RN Fixed, film, high stability 05 0.125
50 0.05
55 0.1
60 0.125
65 0.25
70 0.5
75 1
RP Variable, power, wirewound 10 25
11 12
15 50
16 25
20 75
25 100
30 150
35 225
40 300
45 500
50 750
55 1000
RV Variable, composition 01 0.25
04 2
05 0.5
06 0.333
RW Fixed, power, wirewound 55 5
56 10
67 5
68 10
69 2.5
70 1
74 5
78 10
79 3
80 2.25
81 1

If the letter R follows the type code, then it means that the component meets the established military reliability levels.

Temperature Coefficient Tolerance Failure Rate
Code Coefficient, ppm/deg. C Code Tolerance, % Code Rate, %/1000 h
J +/- 25 F +/- 1 M 1
E +/- 25 G +/- 2 P 0.1
H +/- 50 J +/- 5 R 0.01
C +/- 50 K +/- 10 S 0.001
K +/- 100
O +/- 100

So, our Mil-spec resistor example above is a fixed, composition resistor of 33.5 kOhms, with a power rating of 1 Watt, +/- 25 ppm/deg. C temperature coefficent, +/- 2% tolerance, and a failure rate of 0.01% per 1000 hours of use.

7. What are the standard values of resistors?

7.1 Why are resistor values like 1.0k, 1.2k, 1.5k, 1.8k, 2.2k, etc., and not just simply 1k, 2k, 3k, 4k, etc.?

7.2 What is a RETMA series (E12, E24, E48, E96, E192)?

8. What does the power rating mean?

The power rating of a resistor describe the amount of power that the resistor can dissipate safely. The amount of power dissipated by a resistor is simply determined as P = IV, or the product of the voltage across the resistor and the current running through the resistor.

If we take Ohm's Law (V = IR) we also see that the power dissipated can also be described as P = I2R or P = V2/R.

9. I think this is a resistor, but it only has one black/red/silver band on it. What is it?

If you have a part in a clear case, with either a single red or black band, what you most likely have is a germanium (red) or silicon (black) diode. If it has a black case, with a single silver band, it is a power (rectifying) diode. For more information about diodes, see the Diode FAQ.

Also, you may find what looks like a resistor, however it has only one black band. This is a zero ohm resistor. Why have a zero ohm resistor? It is useful for automatic placement machines to use as a jumper. A single wire jumper requires a special interface, while a zero ohm resistor only requires the machine to be able to place resistors on the board. A zero ohm resistor is quite an interesting object!

10. Where can I buy a resistor of XXXX type/value? Who makes XXXX type/value of resistor?

11. References

12. Contributors and helpers

A special thanks go out to the following:

Any corrections should be addressed to John Patrick, at j.s.patrick@ieee.org. John is quite busy, so please don't e-mail asking for clarifications on the information, or more details. If you want further information, look here, or try your local library under Electronics.

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John S. Patrick (j.s.patrick@ieee.org)
Copyright © 1999-2005 by John Patrick
Last Revision: 28 January 2005