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Reading Resistors

In the lab exercises, today, and ever after, you will want to be able to read resistor valueswithout pulling out a meter to measure the parts value (sometimes desperate students

resort to such desperate means). The process will seem laborious, at first; but soon, as

you get used to at least the common resistance values, you will be able to read manycolor codes at a glance. We will use resistors that are big enough to be readable. Thenow-standard surface-mount parts are so tiny that they normally are not labeled at all.

This spares you the trouble (and opportunity) of reading them. If you mix a few surface-mount parts on the bench, you can only sweep them up and start overunless you are

willing to measure each.

A Menagerie of 10k Resistors

The type that we usethe carbon composition resistors at the topare nearly obsolete,and relatively expensive. But we like them for lab work, because (on a good day-) we can

read them. The others are pretty nasty for breadboarding. The one with the value writtenout in numerals may appeal to you, if you dont want to learn the color codesbut it

really isnt much fun to work with, because if it happens to be mounted with the valuelabel down, youre out of luck.

Resistor Values and Tolerances

Below is a resistor of the sort we use in this courses labs: it is a 5% carbon

composition. 5% means that its actual value should lie within 5% of its nominal

value. If it is labeled 100 k (100,000 ohms), its actual value can be expected to liebetween about 95,000 ohms and 105,000 ohms. The first problem one confronts, on afirst day with resistors, is which way to orient the part:

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tolerance stripe?, you protest. How do I know which that is? The answer is, Find afourth stripe that is silver or gold. (if you are using 1% resistors, the tolerance value is

the fifth band. You need the first three bands to have enough significant figures for thevalue and the fourth band for the multiplier)

Tolerances

Only two tolerance values are common, for the 3-band carbon composition parts you are

likely to meet in the labs: 5% and 10%

silver: 10% gold: 5%

Value

Once youve oriented it properlytolerance to the rightyou can read off the value

colors, and then can translate those to numbers. Finally, with the three numbers in hand,you will have enough information to discover the value. The resistor we just looked at is

brown-black-yellow:

Brown-black-yellow is one-zero-four, and the fourth band, gold, says 5%.Band threeis an exponenta power of ten. So, this one is 100 k.

The Color Code

The colors represent numbers, as set out below. A variety of mnemonics have arisen,

most of them more-or-less offensive, in order to help engineers to memorize this code.One of the blander mnemonics is Big Boys Race Our Young Girls, But Violet Generally

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Wins. This mnemonic is not very clever. It fails to distinguish the trio Black, Brown andBlue and also between the pair Green and Gray. Blackis a plausible representation of the

value zero, since black represents the absence of color; the color Brown is close to thecolor Black; perhaps one might remember Grays position as next to White. None of

this is very satisfactory. It is suggested that you find a color chart you like and affix it

into your lab notebooks for reference.Here are the colors and the values they represent: black: zero

brown: one red: two

orange: three yellow: four

green: five blue: six

violet: seven gray: eight

white: ninethe next two are used as multipliersonly:

gold: 0.1 silver: 0.01

The set of 10% valuesaka, the E12 Series

It is hard to get used to the strange set of values that are standard in the lab. They are

not the nice, round values that one might expect. They seem weird and arbitrary at first.But their strangeness does make sense. Because of our uncertainty about the actual value

of a resistor, it doesnt make sense to specify distinct nominal values that are too closetogether; if we specify nominal values that are too close, their actual values are likely to

overlap. To avoid this, the nominal values are placed far enough apart so as to makeoverlap slight. A 10% resistor of nominal value 10 ohms, for example, could be as

large as 11 ohms. A 12 ohm resistor could be 10% smallera bit under 11 ohms. So,12 is about as close as it makes sense to place the next 10% value after 10. And so

onthe steps growing slightly (exponentially actually) as the values rise, producing suchunfamiliar numbers as 27, 39, 47, and so on. Here is the 10% set, also called the E12

series of resistors.

It is named such that E means exponential steps of 12 different values across a decade.

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Most of our laboratory circuits will use these 10% values (with appropriate multipliers:we rarely use 10 ohm resistors, but often use 10k ohm parts, for example). This is so even

though our lab resistors are better than 10% partsnormally they are good to 5%. Thereare other E series, e.g., E24 and E48 that make a given number of exponential steps

across a decade. Of course, the more steps...the more precise the resistor value.

Consequently, you only need to know the 12 step values across one decade to figure outwhat they are for other decades, e.g., 22 ohms is a value in the 10-100 decade. 2.2 ohmswould be the value in the 1 to 10 decade. 220 ohms would be the value in the 100 to 1000

decade. 2,200 would be the value in the 1000 to 10,000 decade, etc.

Power

Only now and then are we obliged to considerpower ratings of the components we use inthe labs. That is true because our signal voltages are modest and our currents are small.

Power in our components, therefore, is modest as well, since power is the product of thetwo: Voltage times Current. 10V X 10mA, for example, dissipates 100mWone tenth of

a Watt. But our standard resistors cannot handle much power: 1/4 Watt is the most theycan stand, sustained. So, recall this limitationor you may be reminded by burned

fingertips.Incidentally may need to remind yourselftill your intuition catches up with your book

knowledgethat it is low- valued resistors that are likely to overheat. 10V across 1k is noproblem; 10V across 100 ohms will hurt, if you touch that quarter-watt resistor.