We striped its skin with its own name, so no dust or flood could wash the label away. The stripes are still on every bench. Learn to read them.
⟨ transmission ends ⟩By the end of this page you will read any resistor’s value straight off its body, order the values that actually exist, and pick a wattage so the part runs warm instead of dead. This is the whole craft of the world’s most common part.
A resistor wears its value as colored stripes. Read them from the end whose stripe sits closest to it: the first two stripes are digits, the third says how many zeros follow, and the last is the guarantee: gold means the true value is within 5% of the printed one, silver within 10%. The chart beside the bench gives each color’s digit. Big values shorten with a prefix: k is kilo, a thousand, so 2 200 Ω is written 2.2 kΩ. Below, three parts wait to be read: set the three wheels to what the stripes say.
Three real 5% parts: yellow-violet-brown is 470 Ω, red-red-red is 2.2 kΩ, brown-black-black is 10 Ω. The black third stripe means no zeros at all: black is 0 everywhere.
Parts too small for stripes print the same system in digits. On a chip resistor the last digit counts the zeros: 471 is 47 followed by one zero, 470 Ω. When the value needs a decimal point, an R stands in for it: 4R7 is 4.7 Ω. Below, each chip prints its code; name the value it means.
Drawn about 12× life size; the small outline next to each chip is its true 2 mm footprint. 471 and 222 are the very parts you just read as stripes, in their surface-mount skin. 103 is new: 10 with 3 zeros.
Now the strange part: you cannot buy every number. No shop stocks a 50 Ω resistor, yet every shop stocks 47 Ω. The reason is the guarantee. Each value is only promised to within its tolerance, so manufacturers space the values just far enough apart that the guarantee ranges cover the whole number line, each range reaching toward the next. Slide the tolerance below and watch the family of values grow.
The families have names: 12 values per decade is the E12 series (10% parts), 24 is E24 (5% parts). Ever-finer series exist: 1% parts use E96, 96 values per decade. The dashed 50 never appears: it always sits inside a neighbor’s guarantee. Look closely at the reach line: the rounded values leave hairline slivers between some guarantees, widest near 14 Ω at ±5%; the finer families cover what the coarse ones miss.
So the bench habit has two steps: compute the number, then buy the nearest value that exists. In The Three Numbers the recipe asked for 473 Ω and the bench used 470 Ω; that was this habit at work. The drawer below holds the neighboring E24 values. The seat is the same 9 V LED loop, aiming for 15 mA.
The loop from The Three Numbers: 9 V supply, the LED keeps 1.9 V. Seated, 470 Ω carries 15.1 mA, as close to the 15 mA aim as the drawer can land; 430 Ω runs 16.5 mA and 510 Ω runs 13.9 mA.
One choice is left, and it is the one that burns parts: how much heat the resistor can shed. Everything a resistor fights becomes heat in its body, and the heat has a simple price:
With V = IR from The Three Numbers the same price reads P = I² × R or P = V² ÷ R, whichever pair you know. Every resistor is sold with a rating: 1/8 W, 1/4 W, 1/2 W, 1 W and up, bigger body for bigger rating. The rating is a ceiling, not a comfort zone: a part run at its ceiling gets very hot and its value drifts. The bench habit: work out the heat, double it, and buy the next rating above that. The seat below drops 10 V at 19.6 mA; pick its body.
12 V supply, the LED keeps 2 V, so the 510 Ω resistor gets 10 V and 19.6 mA: 0.196 W. The 1/8 W body sits at 157% of its ceiling and burns open; heating is shown over a few seconds, faster than a real part fails. The 1/4 W body survives at 78%, running hot enough to drift.
When you are ready, test yourself:
A resistor is banded brown-black-orange, then gold. Its value?
A chip resistor prints 334. Its value?
A resistor must drop 6 V at 50 mA. Which rating do you buy?
You compute 250 Ω and shop a drawer of 5% parts. Which do you order?
You can now read, choose and size the part that appears in nearly every circuit ever built. What you cannot yet do is check one: measure what a real resistor, wire or battery is actually doing. That instrument, the multimeter, is the next fragment, The Meter.
Lesson 3 done.
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