Resistor (fixed)
Resistor
A component that limits current and divides voltage by providing a fixed opposition (resistance) to current flow. The most common part in any circuit.
Schematic symbol
Resistor
How resistor (fixed) appears in a circuit diagram.
Types & variants
Key specs
Resistance
Ohm (Ω)How strongly it opposes current. Set by Ohm's law: R = V / I.
Tolerance
%How far the real value can stray from the marked value (e.g. ±1%, ±5%).
Power rating
Watt (W)Max heat it can dissipate without burning. P = I^2 x R. Pick at least 2x the expected power.
Temperature coefficient
ppm/°CHow much resistance drifts with temperature. Lower = more stable.
Laws that govern it
- Ohm's lawV = I × R
The resistor's whole job: it fixes the ratio of volts across it to amps through it, so V = I × R sets every current you design around it.
- Watt's lawP = V × I = I²R = V²/R
P = I²R is what the wattage rating is protecting. A ¼ W part passing 30 mA through 470 Ω is fine; through 4.7 kΩ it is smoking.
- Laws of resistanceR = ρ × L / A
Explains why the value exists at all — R = ρL/A of the carbon or metal film, and why the value drifts as the part warms.
- Joule's law of heatingH = I² × R × t
H = I²Rt is the heat you must get rid of, and the reason a resistor's body temperature can hit 100 °C well inside its rating.
Each law is stated in full — with its diagram, variables and worked meaning — on the electrical laws page.
Markings
Through-hole: 4 or 5 colour bands (use the built-in color-code calculator). SMD: 3-4 digit code (e.g. 103 = 10 x 10^3 = 10k).
Standard values
E12 / E24 / E96 series. Common: 220Ω, 330Ω, 1k, 4.7k, 10k, 100k.
How to choose
1) Calculate needed resistance (Ohm's law). 2) Pick nearest E-series value. 3) Pick power rating >= 2x expected. 4) Choose tolerance (±5% is fine for most hobby use, ±1% for precision).
Pinout & package
2 leads (non-polar, can go either way). Packages: axial (through-hole), 0402/0603/0805/1206 (SMD).
Example circuits
- Current-limiting resistor for an LED
- Voltage divider
- Pull-up / pull-down on a logic pin
Common failures
Goes open-circuit (burns) from overpower; value drifts with heat/age. Burn marks or cracks are visible signs.
How to test
Set multimeter to ohms, measure across leads (out of circuit). Should read close to marked value within tolerance.
Substitutes
Use same value & power rating or higher power. Series/parallel combos can make non-standard values.