BASIC 
ELECTRONICS COURSE 
Page 4
INDEX

RESISTORS

Resistors are one of the most valuable components in electronics. They are used in almost EVERY circuit. The one thing that makes an electronics person skilled is his ability to read the coloured bands on resistors. 
This is one of the hardest things to do and the best way to master the code is to have a handful of resistors and find the value of each using the colour-code calculator below. 
It's best to work with actual devices as you get to know what to do much faster.  Resistors come in a values from less than one ohm to more than 22 million ohms. 

Obviously we can't have 22 million resistors and so the manufacturers have produced about 60 resistors to cover all the values you will need. These resistors can be placed in groups called decade groups and the first groups is 10 ohms to 82 ohms, the second is 100 ohms to 820, the third is 1,000 ohms to 8,200 ohms, then 10,000 to 82,000 and 100,000 to 820,000 and finally 1,000,000 to 22,000,000. 
Each range is called a decade range and basically the numbers in each range repeat. This means you have to learn "pairs of numbers," plus the value for each of the ten colours. 

Let's go over this in more detail. 

Each range from say 10 to 82 or 100 to 820 is a "decade value" and within each of these ranges, there are twelve resistors. The first two digits for the twelve resistors are the same for each range, namely:  "10," "12," "15," "18," "22," "27," "33," "39," "47," "56," "68," and "82." To increase the value in the next range, zero's are added to the two digits to produce values such as 1000 or 15 000 or 470 000. The result is always called OHMS. 
An ohm is a very small value and for electronic work we need values from one ohm (and less)  to millions of ohms. It would be impossible to produce resistors of every value and so a set of values has been chosen that cover the whole range. In the early days resistors were made in very rough batches and given a tolerance value of 10%. To make sure every resistor in the batch could be banded with a value, the 10% "incremental staircase" was produced. It resulted in "funny" values such as 22 ohms and 68 ohms but we have to live with the decision. 

It's fortunate that these "unusual values" were chosen as it has made electronics a very specialised field. Most people look at a circuit board filled with "coloured devices" and don't know where to start.  
By learning the resistor colour-code, you break-through this barrier. 
Now we know resistors come in funny values, we have to be able to ready them accurately. You must not make a mistake as the wrong number of zero's will make the resistor TEN TIMES more or TEN TIMES less than its true value - or even 100 times too much!
The colour-code calculator below will get it right for you but there's one thing you have to remember when using the calculator. 

The calculator provides a decimal point in the answer. But when drawing circuit diagrams, it is best to eliminate decimal points as they do not reproduce very well if the circuit diagram has to be faxed or photocopied. To get around this we place a letter called a multiplier (R, k or M) in the position of the decimal point. This prevents any mis-reading of values. 
Three letters are used to display the values of a resistor, they are:
R = ohms,     k = kilo ohms   and      M = meg ohms.  
This produces values such as 0R1, 4R7, 10R, 100R, 1k, 2k2, 4k7, 10k, 27k, 39k, 120k, 470k, 1M, 2M2, 4M7 22M plus all the other values within this range. The letters R, k and M are not normally bold. This is just to show how they are used. 
For very low values of resistance (between zero ohms and about ten ohms) the letter R acts as the decimal point. E.g: 0R1 for "point one ohms"   2R2 for two point two ohms, 4R7, 6R8 etc.
To determine the value of a resistor, it should be held so that the tolerance band gold or silver (+/-5% or +/-10%) is on the right-hand side. This gives us three coloured bands - then the gold or silver band.

Take each resistor and hold it to match the colour-code calculator below and "dial-up" the appropriate colours. The answer will appear in the window. While you are doing this, remember the "pairs of digits" so you don't get wrong answers such as 230R or 42k, these numbers don't go together. 
There is one unusual feature of the banding system. The third band can be coloured to produce the number of zeros in the answer or it can be gold. If it is gold, the first two digits are "DIVIDED BY 10" to produce values from 1 ohm to 8.2 ohms. If the third band is silver, the first two digits are divided by 100. This gives resistance values from .1 ohms to .82 ohms. Putting it all together you have values from 0R1 to 22M. 
The way to remember the two divisor bands is this: Silver has more letters in the name and thus the division is 100. Gold has less letter and the division is 10.

TOLERANCE 
As we have said, no "100 ohm" resistor is exactly 100 ohms and  the tolerance band (silver or gold) indicates the value may be 10% (silver) or 5% (gold) higher or lower than the stated value. If you require a very accurate value, 1% resistors are available. These have 5-bands (4 bands for the value of resistance and 1 band for the tolerance (brown=1%). A separate 1% resistor colour-code calculator is included in this course to identify these resistors. 
The colour-code calculator presented below covers the "standard" or "NORMAL" 4-band resistors (5% and 10%).

Now, you can try the calculator:

Here are some samples. Use the Colour-Code calculator to find the resistance of:     

(a) red red black gold  (b) orange orange red silver (c) yellow purple yellow gold (d) brown green orange silver (e) red red gold gold (f) brown black red gold (g) green blue red gold (h) brown black green gold (i) red red green gold (j) brown black black gold (k) red red silver gold (l) yellow purple brown gold (m) brown black orange gold (n) blue grey black silver (o) brown green red gold

What are the colours for these? Use the Colour-Code calculator:

(a) 10k    5% (b) 12k    5% (c) 2k7    5% (d) 10R   10% (e) 1k     5% (f) R22    5% (g) 1M    10% (h) 120k  5% (i) 22R    5% (j) 82R    5% (k) 47k    5% (l) 3k3     5% (m) 680k 10% (n) 1k2    5% (o) 100R  5%