Function
Resistors restrict the flow of electric current, for example a resistor is placed in series with a light-emitting diode (LED) to limit the current passing through the LED.Connecting and soldering
Resistors may be connected either way round. They are not damaged by heat when soldering.Colour Code | |
| Colour | Number |
| Black | |
| Brown | |
| Red | |
| Orange | |
| Yellow | |
| Green | |
| Blue | |
| Violet | |
| Grey | |
| White | |
Resistor values - the resistor colour code
Resistance is measured in ohms, the symbol for ohm is an omega
. 1
is quite small so resistor values are often given in k and M. 1 k
= 1000 1 M = 1000000 . Resistor values are normally shown using coloured bands.
Each colour represents a number as shown in the table.
Most resistors have 4 bands:
- The first band gives the first digit.
- The second band gives the second digit.
- The third band indicates the number of zeros.
- The fourth band is used to shows the tolerance (precision) of the resistor, this may be ignored for almost all circuits but further details are given.
This resistor has red (2), violet (7), yellow (4 zeros) and gold bands.
So its value is 270000
= 270 k. On circuit diagrams the
is usually omitted and the value is written 270K. Small value resistors (less than 10 ohm)
The standard colour code cannot show values of less than 10. To show these small values two special colours are used for the third band: gold which means × 0.1 and silver which means × 0.01. The first and second bands represent the digits as normal. For example:
red, violet, gold bands represent 27 × 0.1 = 2.7
green, blue, silver bands represent 56 × 0.01 = 0.56
Tolerance of resistors (fourth band of colour code)
The tolerance of a resistor is shown by the fourth band of the colour code. Tolerance is the precision of the resistor and it is given as a percentage. For example a 390 resistor with a tolerance of ±10% will have a value within 10% of 390, between 390 - 39 = 351 and 390 + 39 = 429 (39 is 10% of 390). A special colour code is used for the fourth band tolerance:
silver ±10%, gold ±5%, red ±2%, brown ±1%.
If no fourth band is shown the tolerance is ±20%.
Tolerance may be ignored for almost all circuits because precise resistor values are rarely required.
Resistor shorthand
Resistor values are often written on circuit diagrams using a code system which avoids using a decimal point because it is easy to miss the small dot. Instead the letters R, K and M are used in place of the decimal point. To read the code: replace the letter with a decimal point, then multiply the value by 1000 if the letter was K, or 1000000 if the letter was M. The letter R means multiply by 1.
For example: - 560R means 560
2K7 means 2.7 k = 2700 39K means 39 k 1M0 means 1.0 M = 1000 k Real resistor values (the E6 and E12 series)
You may have noticed that resistors are not available with every possible value, for example 22k and 47k are readily available, but 25k and 50k are not!
Why is this? Imagine that you decided to make resistors every 10 giving 10, 20, 30, 40, 50 and so on. That seems fine, but what happens when you reach 1000? It would be pointless to make 1000, 1010, 1020, 1030 and so on because for these values 10 is a very small difference, too small to be noticeable in most circuits. In fact it would be difficult to make resistors sufficiently accurate.
giving 10, 20, 30, 40, 50 and so on. That seems fine, but what happens when you reach 1000? It would be pointless to make 1000, 1010, 1020, 1030 and so on because for these values 10 is a very small difference, too small to be noticeable in most circuits. In fact it would be difficult to make resistors sufficiently accurate.
To produce a sensible range of resistor values you need to increase the size of the 'step' as the value increases. The standard resistor values are based on this idea and they form a series which follows the same pattern for every multiple of ten.
The E6 series (6 values for each multiple of ten, for resistors with 20% tolerance) 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.
The E12 series (12 values for each multiple of ten, for resistors with 10% tolerance)
10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82, ... then it continues 100, 120, 150 etc.
Notice how this is the E6 series with an extra value in the gaps.
The E12 series is the one most frequently used for resistors. It allows you to choose a value within 10% of the precise value you need. This is sufficiently accurate for almost all projects and it is sensible because most resistors are only accurate to ±10% (called their 'tolerance'). For example a resistor marked 390 could vary by ±10% × 390 = ±39, so it could be any value between 351 and 429.
could vary by ±10% × 390 = ±39, so it could be any value between 351 and 429. Resistors in Series and Parallel
Resistance
Resistance is the property of a component which restricts the flow of electric current. Energy is used up as the voltage across the component drives the current through it and this energy appears as heat in the component.Resistance is measured in ohms, the symbol for ohm is an omega
. 1
is quite small for electronics so resistances are often given in k and M. 1 k
= 1000 1 M = 1000000 . Resistors used in electronics can have resistances as low as 0.1
or as high as 10 M. Resistors connected in Series
When resistors are connected in series their combined resistance is equal to the individual resistances added together. For example if resistors R1 and R2 are connected in series their combined resistance, R, is given by: Combined resistance in series: R = R1 + R2
This can be extended for more resistors: R = R1 + R2 + R3 + R4 + ...
Note that the combined resistance in series will always be greater than any of the individual resistances.
Resistors connected in Parallel
When resistors are connected in parallel their combined resistance is less than any of the individual resistances. There is a special equation for the combined resistance of two resistors R1 and R2: | Combined resistance of two resistors in parallel: | R = | R1 × R2 |
| R1 + R2 |
For more than two resistors connected in parallel a more difficult equation must be used. This adds up the reciprocal ("one over") of each resistance to give the reciprocal of the combined resistance, R:
| 1 | = | 1 | + | 1 | + | 1 | + ... |
| R | R1 | R2 | R3 |
Note that the combined resistance in parallel will always be less than any of the individual resistances.
Conductors, Semiconductors and Insulators
The resistance of an object depends on its shape and the material from which it is made. For a given material, objects with a smaller cross-section or longer length will have a greater resistance.
Materials can be divided into three groups: - Conductors which have low resistance.
Examples: metals (aluminium, copper, silver etc.) and carbon.
Metals are used to make connecting wires, switch contacts and lamp filaments. Resistors are made from carbon or long coils of thin wire. - Semiconductors which have moderate resistance.
Examples: germanium, silicon.
Semiconductors are used to make diodes, LEDs, transistors and integrated circuits (chips). - Insulators which have high resistance.
Examples: most plastics such as polythene and PVC (polyvinyl chloride), paper, glass.
PVC is used as an outer covering for wires to prevent them making contact.
Power Ratings of Resistors
 |
 |
| High power resistors (5W top, 25W bottom) |
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 high) a large current may pass making the resistor become noticeably warm. The resistor must be able to withstand the heating effect and resistors have power ratings to show this.
Power ratings of resistors are rarely quoted in parts lists because for most circuits the standard power ratings of 0.25W or 0.5W are suitable. For the rare cases where a higher power is required it should be clearly specified in the parts list, these will be circuits using low value resistors (less than about 300) or high voltages (more than 15V).
The power, P, developed in a resistor is given by: ) or high voltages (more than 15V). | P = I² × R or P = V² / R | where: | P = power developed in the resistor in watts (W) I = current through the resistor in amps (A) R = resistance of the resistor in ohms ( ) V = voltage across the resistor in volts (V) |
Examples:
- A 470 resistor with 10V across it, needs a power rating P = V²/R = 10²/470 = 0.21W.
In this case a standard 0.25W resistor would be suitable. - A 27 resistor with 10V across it, needs a power rating P = V²/R = 10²/27 = 3.7W.
A high power resistor with a rating of 5W would be suitable.
Resistor Colour Code Calculator
Colour Code | |
| Colour | Number |
| Black | |
| Brown | |
| Red | |
| Orange | |
| Yellow | |
| Green | |
| Blue | |
| Violet | |
| Grey | |
| White | |
The Resistor Colour Code Calculator can be used to identify resistors. It consists of three card discs showing the colours and values, these are fastened together so you can simply turn the discs to select the value or colour code required. Simple but effective! There are two versions to download and print on A4 white card (two per sheet): - Coloured (for a colour printer)
- Black and White (for a black only printer)
This version must be coloured manually, it is easiest to do this before cutting out.
The calculator design is copyright but it may be freely copied for educational purposes.
