Preferred values
The system of preferred values, which is used for resistors, capacitors and inductors, was developed to provide a logical progression from one value to the next, where each value represents an increase by an approximately constant percentage.
Depending on the tolerance of the particular components, there can be between 3 and 192 preferred values in each decade.
The more common series are shown in the tables below. Values given for each series are repeated in every decade.
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3 Per decade (50% tolerance)
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| 10 |
22 |
47 |
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12 Per decade (10% tolerance)
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| 10 |
12 |
15 |
18 |
22 |
27 |
33 |
39 |
| 47 |
56 |
68 |
82 |
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|
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24 Per decade (5% tolerance)
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| 10 |
11 |
12 |
13 |
15 |
16 |
18 |
20 |
| 22 |
24 |
27 |
30 |
33 |
36 |
39 |
43 |
| 47 |
51 |
56 |
62 |
68 |
75 |
82 |
91 |
| Decimal multipliers
Decimal multiplier prefixes are in common use to simplify and shorthen the notations of quantities such as component values.
Capacitance, for example, is measured in Farads, but the Farad is far too large a unit to be of practical use in most cases. For convenience, we use sub-multiples tosave a lot of figures.
For example, instead of writing 0.000000000001 Farads, we write 1pF (1 picofarad).
The more common prefixes and the relationships to one another are as follows.
| Abbrev. | Prefix | Multiply by | or |
| p | pico | 0.000000000001 | 10-12 |
| n | nano | 0.000000001 | 10-9 |
| u | micro | 0.000001 | 10-6
|
| m | milli | 0.001 | 10-3 |
| - | UNIT | 1 | 100 |
| k | kilo | 1000 | 103 |
| M | mega | 1000000 | 106 |
| Units |
| 1000 pico units | = | 1 nano unit |
| 1000 nano units | = | 1 micro unit |
| 1000 micro units | = | 1 milli unit |
| 1000 milli units | = | 1 unit |
| 1000 units | = | 1 kilo unit |
| 1000 kilo units | = | 1 mega unit |
Tolerance
All components differ from their marked value by some amount.
Tolerance specifies the maximum allowed deviation from the specified value.
Tolerances are normally expressed as a percentage of the nominal value.
For example, a component with a marked value of 100 and a tolerance of 5% could actually be any value between 5% below the marked value (95) and 5% above the marked value (105).
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Resistors
Most resistors are so small that it is impractical to print their values on them using normal numeric characters.
Instead, they are marked using a code of coloured bands.
Resistors made to tolerance of 5% and 10% are marked with 4 bands while higher precision types, such as 2%, 1% or better, may be marked with 5 bands to allow for an extra digit of precision.
How to read 4-band codes:
At one end of the resistor there will be a gold, silver or brown tolerance band.
This band is usually spaced apart from the other three bands. Start with the band nearest to the other end.
Its colour represents the first digit of the resistor's value, as shown in the colour code chart.
The next band represents the second digit of the resistor's value.
The third band represents the decimal multiplier, that is, the number of zeroes that we have to put after the first two digits to arrive at the resitor's value.
The final band gives us the tolerance of the resistor, silver for 10% types, gold for 5% types, brown for 1% types.
Let's take the resistor shown at the top of the colour chart as an example.
It's first band is yellow, representing "4" and the second band is violet, representing "7".
The third band, the multiplier, is orange which tells us to add 3 zeroes to the number we already have.
This is the same as multiplying it by 1,000.
Thus the value of the resistor is 47,000 -forty seven thousand ohms or 47k-ohms.
Finally, the fourth band, being gold, indicates that the resistor has a 5% tolerance, that is, its actual value will be somewhere between 44650 ohms and 49,350 ohms.
Some special high-voltage resistors use a yellow band in lieu of gold.
This is simply because the metal particles in the gold paint might compromise the resistor's voltage rating.
Reading 5-band resistors:
Because the final band on these resistors is usually brown or red, it can be a bit more difficult to know which end to start from.
In most cases the first four bands are grouped a bit closer together than the fourth and fifth bands.
The first two bands are read the same as they are on the 4-band types.
The third band supplies a third digit to the value.
The fourth band now becomes the multiplier and the fifth represents the tolerance.
For example, if the 5 bands are from first to fifth, red/yellow/white/gold,brown, then the three significant digits of the value would be "249",
the multiplier would be 0.1 and the tolerance 1%.
Hence, this is the code for a 24.9 ohm 1% resistor.
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