Resistors
Fixed and variable resistors, the color code, standard values, power ratings, thermistors, light sensors, pull-ups and pull-downs.
What a resistor does
A resistor limits current and sets voltages. It turns electrical energy into heat in proportion to the current squared. Almost every circuit uses them: to limit the current through an LED, to divide a voltage, to pull an input to a known level, to set the gain of an amplifier, and to set time constants with a capacitor. The relationships are in Circuit Basics.
V = I × R P = I² × R = V² ÷ R
Types of fixed resistor
| Type | Typical tolerance | Best for | Notes |
|---|---|---|---|
| Carbon film | 5 % | General use, cheap | Noisier and drifts more than metal film; the common “kit” resistor |
| Metal film | 1 % (0.1 % available) | Dividers, amplifiers, precision | Low noise, 50–100 ppm/°C; worth the small extra cost |
| Thick-film chip (SMD) | 1 % or 5 % | Almost all modern boards | Sizes 0402 to 2512; the code is printed on the part (see below) |
| Wirewound | 1–5 % | High power, current sensing | Inductive: poor at high frequency unless non-inductive wound |
| Metal-oxide / power film | 5 % | 1–5 W and up, flameproof | Runs hot; keep it clear of other parts |
| Current-sense (shunt) | 1 % | Measuring current | Milliohm values; use four-wire (Kelvin) connection for accuracy |
The color code and other markings
- Four bands: digit, digit, multiplier, tolerance. Five bands (precision parts) add a third digit. A sixth band, if present, is the temperature coefficient.
- SMD codes. Three digits: the first two are the value and the third is the number of zeros, so 103 = 10 × 103 = 10 kΩ and 472 = 4.7 kΩ. An “R” marks the decimal point: 4R7 = 4.7 Ω. Precision parts use four digits (1002 = 10.0 kΩ) or the EIA-96 two-digit-plus-letter code.
- When unsure, measure. Take the resistor out of the circuit (or at least isolate one leg) and use a multimeter in the ohms range. Parallel parts on the board give a lower reading.
Standard values (E-series)
Resistors are not made in every value. They follow the E-series of preferred numbers, spaced so that each value is within the tolerance of the next. Each decade (10 to 100, 100 to 1 000 and so on) repeats the same set.
| Series | Values per decade | Tolerance | Values (one decade) |
|---|---|---|---|
| E6 | 6 | 20 % | 10, 15, 22, 33, 47, 68 |
| E12 | 12 | 10 % | 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 |
| E24 | 24 | 5 % (and 1 %) | 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 | 96 | 1 % | 10.0, 10.2, 10.5, 10.7, 11.0, … (96 values) |
Worked example: making a value that does not exist
- You need 7.5 kΩ and have only E12 parts. 7.5 is in the E24 series but not E12; pick 6.8 kΩ and add 680 Ω in series: 6 800 + 680 = 7 480 Ω, which is within 0.3 % of 7.5 kΩ.
- Two equal resistors in parallel give half: two 10 kΩ in parallel make 5 kΩ, and they also share the power, so two ¼ W parts act as a ½ W resistor.
- For a critical value (a precision divider), use the E96 (1 %) series rather than combining parts.
Power rating and tolerance
Every resistor has a power rating, the heat it can lose without damage. Resistors that run at the rating get very hot: derate them to half the rating or less, and keep the ambient temperature in mind because the rating falls above about 70 °C.
| Package | Typical rating | Package | Typical rating |
|---|---|---|---|
| Axial ⅛ W | 0.125 W | SMD 0402 | 0.063 W |
| Axial ¼ W | 0.25 W | SMD 0603 | 0.1 W |
| Axial ½ W | 0.5 W | SMD 0805 | 0.125 W |
| Axial 1 W, 2 W | 1 W, 2 W | SMD 1206, 2512 | 0.25 W, 1 W |
Worked example: 5 V across 100 Ω
- P = V² ÷ R = 25 ÷ 100 = 0.25 W. A ¼ W resistor is running at its limit and will be too hot to touch.
- Use a 1 W part, which then runs at 25 % of its rating. Alternatively, four 400 Ω ¼ W resistors in parallel make 100 Ω and share the power.
- Tolerance says how far the real value may differ from the marked value. A 5 % 10 kΩ resistor can be anything from 9.5 to 10.5 kΩ. Use 1 % where accuracy matters (dividers, references, amplifier gain).
- Temperature coefficient (ppm/°C) says how much the value drifts with temperature: 100 ppm/°C moves a resistor by 0.5 % over a 50 °C swing. Precision work needs 25–50 ppm/°C or better.
- Voltage rating. Chip resistors have a maximum working voltage (50–200 V); a high-value resistor across a high voltage can arc even if its power is low.
Variable resistors
- Potentiometer (pot). A resistive track with a sliding contact (the wiper). Wired as a voltage divider with three terminals, it gives an adjustable voltage from 0 V to the supply: the usual volume or position knob.
- Linear and log taper. A linear (B) pot changes resistance in proportion to rotation. A log or audio (A) pot changes slowly at first, matching the ear’s response to loudness.
- Rheostat. A pot using two terminals (wiper and one end) as a variable resistor. Its rating is for the whole track; a low-power pot cannot control a big current.
- Trimmer. A small pot adjusted once with a screwdriver during set-up. Multi-turn trimmers give fine adjustment.
- Digital potentiometer. A chip whose wiper is set by a microcontroller through I²C or SPI. They have limited voltage range and current, so check the datasheet.
Resistors that sense
Some resistors change value with temperature or light, which turns them into sensors read through a voltage divider.
| Part | Behavior | Typical use |
|---|---|---|
| NTC thermistor | Resistance falls as temperature rises (about 4 %/°C near 25 °C) | Temperature sensing, inrush current limiting |
| PTC thermistor / polyfuse | Resistance rises sharply above a trip temperature | Resettable fuse, heater, motor protection |
| LDR (photoresistor) | Resistance falls as light increases (roughly 1 MΩ dark to 1 kΩ bright) | Simple light switching; slow and not precise |
| Strain gauge, force-sensitive resistor | Resistance changes with strain or pressure | Load cells, touch sensing; need an amplifier or bridge |
R(T) = R0 × eB × (1 ÷ T − 1 ÷ T0) (T in kelvin; R0 at T0 = 298.15 K)
Worked example: a 10 kΩ NTC thermistor (B = 3950)
- At 25 °C: 10 kΩ. At 0 °C: 33.62 kΩ. At 50 °C: 3.59 kΩ. At 100 °C: 697.5 Ω.
- Put the NTC from the 3.3 V supply to an ADC pin and a 10 kΩ resistor from the pin to ground. At 50 °C the pin sits at 3.3 × 10 ÷ (10 + 3.59) = 2.43 V; at 25 °C it sits at 1.65 V.
- The beta equation is an approximation over a limited range; for accuracy use the manufacturer’s table or the Steinhart-Hart equation. Self-heating is small if you keep the current low (use a 10 kΩ divider, not a 100 Ω one).
Pull-up and pull-down resistors
A digital input that is connected to nothing “floats” and picks up noise, reading random highs and lows. A pull-up resistor ties the input to the supply, and a pull-down ties it to ground, so the input has a defined level until something else drives it.
- Value: 4.7 kΩ to 100 kΩ. 10 kΩ is the usual choice for a button. The current while pressed is V ÷ R = 3.3 ÷ 10 000 = 0.33 mA. Large values save power but are more easily upset by noise and slow the signal edge with any wire or pin capacitance.
- Built-in pull-ups. Most microcontrollers have internal pull-ups (typically 20–50 kΩ) that you can enable in software, so a button between the pin and ground needs no external part. See Embedded Systems.
- I²C buses need external pull-ups (commonly 2.2–4.7 kΩ to the supply) because the devices only pull the line low.
- Open-drain and open-collector outputs also need a pull-up; they can only pull low.
Related
- Electronics Fundamentals: the overview of this guide.
- Circuit Basics: Ohm’s law, series and parallel, and dividers.
- Capacitors: resistor-capacitor timing and filters.
- Soldering: fitting resistors to a board.
FAQ
Can I replace a ¼ W resistor with a ½ W one?
Yes, if the value and tolerance are the same: a higher power rating is always fine electrically. It may be physically larger, and in a precision circuit you should also match the temperature coefficient.
What is the difference between 1 % and 5 % resistors?
The value is held tighter (±1 % against ±5 %), and 1 % metal-film parts also have lower noise and drift. They cost only slightly more, so many designers stock only 1 % parts.
Why did my resistor burn or discolor?
It is dissipating more power than its rating, usually because the value is too small, the voltage is higher than planned or something is shorted. Check P = V² ÷ R. A resistor running warm to the touch is already above a comfortable derating.
What value of pull-up should I use?
10 kΩ is the safe default for buttons. Use 2.2–4.7 kΩ for I²C at 100–400 kHz. A lower value gives a faster, more noise-resistant edge but draws more current.
How do I read a resistor with a worn color code?
Remove it (or lift one leg) and measure with a multimeter in the ohms range. Compare the reading with the nearest E-series value.
References
- IEC 60062, Marking codes for resistors and capacitors; IEC 60063, Preferred number series for resistors and capacitors (E-series).
- Horowitz, P. and Hill, W., The Art of Electronics, 3rd ed., Cambridge University Press.
- Manufacturer datasheets for thick-film chip resistors (for example Vishay, Yageo, Panasonic): power derating, voltage ratings and temperature coefficients.
- Thermistor manufacturer notes (beta and Steinhart-Hart equations), for example Murata and TDK.
Last reviewed: October 2026. Values are typical: check the datasheets of the parts you use.