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