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The electric resistors are the most common and numerous passive components present in any electronic equipment.
Some of them are of such small size that is chosen conventionally to indicate the ohmic value, and other parameters,
using a color code. Deciphering the resistance's ohmic value is quite easy, and with exercise you can find it
mentally at glance. The most common resistors have 4 bands of color, 3 to indicate the resistance value and 1
to show the tolerance. The first thing to do is to find the ring of color that identifies the first digit. In
many types of resistance that is very close to one side of the resistance itself. But not all manufacturers use
the same scheme, so when the identification of the first ring is not possible with this method, you should try
to identify the last ring that identifies the tolerance which is gold or silver. Once established the sequence
of rings, replace the first two colors with the corresponding value and multiplie the figure by the value corresponding
to the third ring. For an example: if the sequence of the 3 colors is red-yellow-brown, the inner resistance is
120,000 ohms because the brown corresponds as the number "1", the red as "2", and yellow corresponds to a multiplication
factor of 10000. For the generality of the resistors you could simply juxtapose the values "1", "2", and "0000".
In the electric scheme of electronic devices, resistors are represented by the follow symbols:
Below, a computer module is proposed to determine the value of a resistance, four and six rings code, starting from
the colors that are printed on it. Selecting from the drop-down menu of the module the colors of the resistance of
which one wants to determine the value and pressing the button calculates. Automatically you will get: the resistance
and tolerance of the resistor and, on the right, a colored diagram.
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| modulo calcolatore resistenza a 4 anelli - 6 anelli |
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For general use, resistors are manufactured in standard values for that reason you can not buy any possible values
as it is possible to select with the resistors calculator above. For some applications, where greater precision is
required, are on the market non-standard resistors, of course, more expensive. It's the case, for example, in resistors
used to build a multimeter and in meter instruments in general. For this type of resistors six rings are also used:
4 for the resistance value, 1 for tolerance, and 1 to identify the temperature coefficient. The latter becomes an
important parameter for this kind of resistors. There are, also, resistors with 5 color bands, 4 of which have the
same meaning as the rings of a 4-band one, with the addition of the fifth ring that identifies the "fail rate"
for 1000 hours of operation under full wattage.
The following table shows the values corresponding to each color in the specific location:
| Colour | I digit | II - III digit | Moltiplicator | Tollerance | Fail Rate X 1000 hours | Temperature Coefficient |
| Black | - | 0 | 1 | - | - | - |
| Brown | 1 | 1 | 10 | ±1% (*) | 1 | 100 ppm/°C |
| Red | 2 | 2 | 100 | ±2% (*) | 0,1 | 50 ppm/°C |
| Orange | 3 | 3 | 1000 | ±3% (*) | 0,01 | 15 ppm/°C |
| Yellow | 4 | 4 | 10000 | - | 0,001 | 25 ppm/°C |
| Green | 5 | 5 | 100000 | ±0,5% (*) | - | - |
| Blue | 6 | 6 | 1000000 | ±0,25% (*) | - | - |
| Purple | 7 | 7 | - | ±0,1% (*) | - | - |
| Gray | 8 | 8 | - | ±0,05% (*) | - | - |
| White | 9 | 9 | - | - | - | - |
| Gold | - | - | 0,1 | ±5% | - | - |
| Silver | - | - | 0,01 | ±10% | - | - |
| None | - | - | - | ±20% | - | - |
(*) = Non standard
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