Valores normalizados de resistencias

Por qué existe una resistencia de 470 Ω pero no de 450: los valores se espacian por proporción, así que cada uno empieza donde acaba la tolerancia del anterior.

Los valores que se pueden comprar de verdad

Todas las resistencias de inserción y de chip que se venden salen de alguna serie E. Cada serie divide una década de resistencia en pasos de proporción constante y luego repite las mismas cifras a 10x, 100x, 1000x y así sucesivamente. El 22 de la E12 te da 22 Ω, 220 Ω, 2,2 kΩ, 22 kΩ y 220 kΩ, y nada intermedio.

E12, ±10%, doce por década

10 · 12 · 15 · 18 · 22 · 27 · 33 · 39 · 47 · 56 · 68 · 82

E24, ±5%, veinticuatro por década

10 · 11 · 12 · 13 · 15 · 16 · 18 · 20 · 22 · 24 · 27 · 30 · 33 · 36 · 39 · 43 · 47 · 51 · 56 · 62 · 68 · 75 · 82 · 91

E96, ±1%, noventa y seis por década

100 · 102 · 105 · 107 · 110 · 113 · 115 · 118 · 121 · 124 · 127 · 130 · y 84 más

La E24 contiene todos los valores de la E12 más uno entre cada par; la E96 vuelve a subdividir. Ese anidamiento explica por qué un surtido de valores E12 sigue cubriendo casi todo lo que pide un esquema.

Por qué los números parecen arbitrarios

Cada paso de la E12 está un 21% por encima del anterior, que es justo el punto en que se tocan dos tolerancias del 10%. Una resistencia marcada 47 podría medir entre 42 y 52; el siguiente valor, 56, cubre de 50 a 62. Sin huecos preocupantes, con solapamiento mínimo y doce valores por década. Cuando lo ves, el 4,7 deja de parecer un número raro para normalizar.

Los tipos que te vas a encontrar

El mismo valor con distinta construcción. Lo que cambia es la tolerancia, la estabilidad, el ruido y cuánta potencia puede disipar.

Capa de carbón resistor — **Capa de carbón**5%, céntimos cada una The default hobby resistor: cream or beige body, four color bands, a gold tolerance band. Cheap, everywhere, and good enough for LEDs, pull-ups and anything that does not care about a percent either way. It gets noisy and drifts with temperature, which only matters in audio and precision work.

Capa metálica resistor — **Capa metálica**1%, cuerpo azul Blue or pale green body with five bands, because three digits need an extra ring. Quieter and far more stable with temperature than carbon film, which is what you want in a divider feeding an ADC, a voltage reference, or an audio signal path.

Bobinada resistor — **Bobinada**vatios enteros Resistance wire wound on a ceramic core, potted in a white cement block, rated in watts rather than fractions. Built for dummy loads, current sensing and bleeder duty. It is slightly inductive, so keep it out of fast switching or RF paths.

Chip (SMD) resistor — **Chip (SMD)**impresa, sin bandas A ceramic brick with the value printed on top instead of banded: 1002 means 100 x 10^2, so 10 kΩ. Sizes are named by footprint in hundredths of an inch, 0805 being 0.08 by 0.05. Decode the print in the SMD code lookup.

Potenciómetro resistor — **Potenciómetro**tres patillas y un eje A resistive track with a wiper riding on it. The outer legs are the ends of the track, the middle leg is wherever the knob has reached. Marked with a code like 103 for 10 kΩ, plus a letter for the taper: B is linear, A is logarithmic.

Trimmer resistor — **Trimmer**se ajusta una vez, con destornillador A potentiometer with a screw slot instead of a knob, meant to be set at build time and left alone. Multi-turn types such as the 3296 take about 25 turns end to end, which is what makes fine calibration possible.

Bandas de tolerancia

Band	Tolerancia	Series	Where you meet it
Silver	±10%	E12	old stock and cheap kits
Gold	±5%	E24	the everyday resistor
Brown	±1%	E96, five bands	dividers, references
Red	±2%	E48	occasional metal film
Green	±0.5%	E192	instrumentation
Blue	±0.25%	E192	precision references

Reading the bands on a part already in your hand is the color code calculator's job.

Potencia y tamaño del cuerpo

Through-hole resistors come in standard wattages and the body grows with the rating. The bench rule: pick one rated at least twice the power it will really dissipate, because the printed rating assumes free air and a cool room.

Rating	Body length	Where it shows up
1/8 W	3.2 mm	dense signal boards
1/4 W	6.3 mm	the default hobby resistor
1/2 W	9 mm	LED strings, higher voltage
1 W	11 mm	power dividers, bleeders
5 W	22 mm, ceramic block	dummy loads, current sense

Tamaños de chip

SMD resistors are named by footprint in hundredths of an inch: an 0805 is 0.08 by 0.05 inches. Below 0603 they stop being comfortable to hand solder, and below 0402 they stop being easy to see.

Code	Size	Typical rating	Hand soldering
1206	3.2 × 1.6 mm	0.25 W	easy
0805	2.0 × 1.25 mm	0.125 W	comfortable
0603	1.6 × 0.8 mm	0.1 W	fiddly but fine
0402	1.0 × 0.5 mm	0.063 W	tweezers and patience
0201	0.6 × 0.3 mm	0.05 W	machines only

Conseguir un valor que no tienes

Two standard parts in parallel reach almost anything: 5.1 kΩ beside 5.1 kΩ gives 2.55 kΩ, closer to 2.5 k than any E12 value gets. In series they simply add, which is usually the tidier answer. When the exact figure genuinely matters, buy 1% parts instead of chasing combinations, because tolerance stacks faster than intuition suggests.