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.
560R means 560
2K7 means 2.7k
39K means 39k
1M0 means 1.0M
Resistor colour code
Resistor values are usually 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 shows the tolerance (precision) of the resistor
but this may be ignored for almost all circuits.
This resistor has red (2), violet (7), yellow (4 zeros) and gold bands so its value is
270000 = 270k
(usually shown as 270K on circuit diagrams).
This calculator can be used to identify resistor values.
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 calculators per sheet):
To make the calculator: cut out the three discs and fasten them together with a brass paper fastener.
The Black & White version must be coloured manually and it is easiest to do this before cutting out.
Tolerance of resistors
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
between 390 - 39 = 351
and 390 + 39 = 429 (39 is 10% of 390).
A special colour code is used for the fourth band tolerance:
If no fourth band is shown the tolerance is ±20%
Tolerance may be ignored for almost all circuits because a precise resistor value is rarely required
and where it is a variable resistor will usually be used.
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.
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 has 6 values for each multiple of ten, it is used for resistors with 20% tolerance.
The values are: 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 has 12 values for each multiple of ten, it is used for resistors with 10% tolerance.
The values are 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
have a tolerance of ±10%.
Power Ratings of Resistors
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 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 can be worked out using these equations:
P = V² / R or P = I² × R
P = power developed in watts (W)
I = current through resistor in amps (A)
R = resistance of resistor in ohms ()
V = voltage across resistor in volts (V)
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 is 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 (or more) is required.
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