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Resistors

Index

    Symbol
    Definition
    Colour Codes
    In Use
    Resistor Convention
    Body Colour
    Standard Series Values (5%)


Symbol

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Definition
The unit of measurement for resistance is the ohm. One ohm is the resistance between two points when a constant current of 1 Amp flows as the result of an applied voltage of 1 Volt between the two points.

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Colour Codes
In electronics, resistor (and sometimes other components) values are shown by a sequence of colours. Now there are lots of methods of trying to remember the number/colour sequence - but most beginners seem to find the colours difficult to remember. In truth most skilled engineers don't go through the steps of working out the colours any more than you would work out c-a-t when looking at 'cat': you simply see the word as a whole. In the same way an experienced engineer simply recognises yellow-violet-red as being 4K7: we don't (need to) spell it out.

In exactly the same way as there are common words in English, so there are common values in electronics.

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In Use
To use it, take a common resistor: this will have a gold (possibly a silver) band at the end with 3 other bands. Ignore the gold band - hold the resistor with this on the right, and start reading from the left end so you read towards the gold band. The first two bands should be one of the combinations shown in the chart. The third band shows the 'decade value' of the resistor.

This is the multiplier - and it tells you which decade range the resistor is in.

So for the above example of a four band resistor we would read... 6, 8, 000. Put it all together and you get 68000 or 68K with a 5% tolerance (gold band).

You would read a five band resistor in the same way except you now have three figure bands before the multiplier band. Look at the above example 470K with a 2% (red) tolerance.

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Resistor Convention
When we write resistor values we use a convention. I mentioned at the start a resistor value of 4K7. So why put the K in the middle? Why not say 4.7K?. This comes down to the days of bad photocopiers. Take a circuit diagram and photocopy it. Then copy the copy. Soon you end up with a dirty copy with stray dots all over it. Is that a 47K or a 4.7K?

This doesn't happen when it is written as 4K7. And after all you say 'four thousand, seven hundred' so as K is a thousand, 4K7 is totally logical.

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Body Colour
Resistor body color markings do not normally have much meaning. Sometimes they tell you about the resistors temperature coefficent. But there are two resistor body colors which you should know what they mean if youre fixing an electronic circuit.
Resistors with body colors of white or blue are used to identify non-flammable resistors and fusible resistors. If you encounter this type of resistor in a circuit do not replace it with normal reistor because this could cause a fire hazard if something goes wrong with the circuit. Non-flammable resistors and fusible resistors are designed so that they do not catch fire when they overheat. When a fusible resistor overheats it blows internaly like a fuse with no external visable signs.

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Standard Series Values (5%)
Standard value series of resistors are named the 'E' series. For an 'E1' series of resistors there would be one value per decade, i.e. 10R, 100R, 1K, 10K, 100K etc. For 'E3' there would be three values per decade, ie 1R, 2R2 & 4R7. A little restricted, I am sure you will agree but do not worry there are more sets, E6, E12, E24, E48 and E96 series.
Below I have detailed the E24 series so that you can get a better idea.

 1R0     10      100     1K0     10K     100K    1M0     10M
 1R1     11      110     1K1     11K     110K    1M1     11M
 1R2     12      120     1K2     12K     120K    1M2     12M
 1R3     13      130     1K3     13K     130K    1M3     13M
 1R5     15      150     1K5     15K     150K    1M5     15M
 1R6     16      160     1K6     16K     160K    1M6     16M
 1R8     18      180     1K8     18K     180K    1M8     18M
 2R0     20      200     2K0     20K     200K    2M0     20M
 2R2     22      220     2K2     22K     220K    2M2     22M
 2R4     24      240     2K4     24K     240K    2M4
 2R7     27      270     2K7     27K     270K    2M7
 3R0     30      300     3K0     30K     300K    3M0
 3R3     33      330     3K3     33K     330K    3M3
 3R6     36      360     3K6     36K     360K    3M6
 3R9     39      390     3K9     39K     390K    3M9
 4R3     43      430     4K3     43K     430K    4M3
 4R7     47      470     4K7     47K     470K    4M7
 5R1     51      510     5K1     51K     510K    5M1
 5R6     56      560     5K6     56K     560K    5M6
 6R2     62      620     6K2     62K     620K    6M2
 6R8     68      680     6K8     68K     680K    6M8
 7R5     75      750     7K5     75K     750K    7M5
 8R2     82      820     8K2     82K     820K    8M2
 9R1     91      910     9K1     91K     910K    9M1

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