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Resistor Colour Codes

If you go to the resistor store, you can buy different types of resistors in different sizes. Some fancy types will have the resistance stamped right on them, but most just have bands of different colours around them as is shown in Figure 12.1.

Figure 12.1: A typical moulded composition resistor.

The bands are used to indicate a couple of pieces of information about the resistor. In particular, you can find out both the resistance of the device, as well as its tolerance.

Firstly, let's talk about what the tolerance of a resistor is. Unfortunately, it's practically impossible to make a resistor that has exactly the resistance that you're looking for in your circuit. Instead of guaranteeing a particular value such as 1 k$\Omega$, the manufacturer tells you that the resistance is approximately 1 k$\Omega$, $\pm$ some percentage. For example, you'll see something like 1 k$\Omega$, $\pm$20%. Therefore, if you measure the resistor, you'll see that it has a value between 800 $\Omega$ and 1.2 k$\Omega$. If you want better precision than that, you'll have to do one of two things. You'll either have to spend more money to get resistors with a tighter tolerance, or you'll have to hand-pick them yourself by measuring one by one.

So, how do we tell what the specifications of a resistor are just by looking at it? Take a look at Figure 12.2. You'll see that each band has a label attached to it. Reading from left to right (the right side of the resistor doesn't have a colour band) we have the 1st and 2nd digits, the multiplier and the tolerance. Using Table 12.2 we can figure out what the shown resistor is.

Figure 12.2: A typical moulded composition resistor showing the meanings of the coloured bands.

Table 12.2: The corresponding meanings of the colour bands on a resistor [Mandl, 1973]. (GMV stands for Guaranteed minimum value

Colour	Digit	Multiplier	Carbon $\pm$	Film-type
			Tolerance	Tolerance
Black	0	1	20%	0
Brown	1	10	1%	1%
Red	2	100	2%	2%
Orange	3	1,000	3%
Yellow	4	10,000	GMV
Green	5	100,000	5% (alt)	0.5%
Blue	6	1,000,000	6%	0.25%
Violet	7	10,000,000	12.5%	0.1%
Gray	8	0.01 (alt)	30%	0.05%
White	9	0.1 (alt)	10% (alt)
Silver		0.01 (pref)	10% (pref)	10%
Gold		0.1 (pref)	5% (pref)	5%
No colour			20%

The colour bands on the resistor in Figure 12.2 are red, green, violet and silver. Table 12.3 shows how this combination is translated into a value.

Table 12.3: Using Table 12.2, the colour bands can be used to find that the resistor in Figure 12.1 has a value of 250,000,000 $\Omega$ or 250 M$\Omega$, $\pm$10%.

1st digit	2nd digit	Multiplier	Tolerance
red	green	violet	silver
2	5	10,000,000	10%