A resistor is a component with a controlled resistance. In the ideal version, the relationship between voltage and current is linear: double the voltage across the resistor and the current doubles. Real resistors are not perfect, but they are predictable enough to be one of the most useful building blocks in electronics.

V = I x R P = I^2 x R P = V^2 / R
Schematic symbol for a resistor in series with a circuit path R1 Current through the resistor creates a voltage drop. I Node A Node B
Resistor symbol. The zig-zag symbol shows a part placed in series with a path. The current through it and the resistance value set the voltage across it.

Resistance value

Resistance is measured in ohms. Small values such as 0.1 ohm are often used for current sensing. Values from hundreds of ohms to tens of kilohms are common for LEDs, pull-ups, filters, and signal circuits. Megohm values are useful when a circuit must draw very little current, but they are more sensitive to leakage, noise, and measurement loading.

Resistor values usually come from standard series such as E12, E24, E96, and so on. You often calculate an exact value, then choose the nearest standard value that gives the safer or more useful result. For an LED current limiter, rounding the resistor up is often fine. For an amplifier gain resistor, the direction of the rounding may affect calibration.

Tolerance is part of the design

A 10 kohm resistor with 5 percent tolerance may be anywhere from 9.5 kohm to 10.5 kohm before temperature and ageing are considered. That does not make it bad. It means the circuit must still work over that range.

Specification What it means When it matters
Resistance The nominal ohm value. Current, voltage division, gain, timing, sensing.
Tolerance How far the real value may be from nominal. Measurement circuits, dividers, filters, calibration.
Power rating How much heat the part can safely dissipate. LEDs, loads, dividers on high voltage, current shunts.
Temperature coefficient How value changes with temperature. Precision measurement and stable references.

Power rating keeps the part alive

Resistors turn electrical power into heat. If that heat is too high, the resistor value can drift, the board can warm up, or the part can fail. A small surface-mount resistor may be rated for 0.1 W or less. A through-hole resistor may be 0.25 W, 0.5 W, 1 W, or more.

The rating is not a target. A resistor running near its maximum can be hot enough to damage plastic, discolor a PCB, or change nearby measurements. In practical design, leave margin and consider the surrounding airflow, board copper, enclosure temperature, and whether the resistor is part of a fault path.

Pull-ups and pull-downs

Digital inputs must not be left floating. A floating input can read high one moment and low the next because it is picking up noise. A pull-up resistor connects the signal weakly to the positive rail. A pull-down resistor connects it weakly to ground. 

Button input with pull-up:

VCC -> resistor -> input pin -> button -> ground

When the button is open, the resistor gives the input a defined high state. When the button is pressed, the input is connected to ground. Values around 4.7 kohm to 100 kohm are common, depending on noise, power consumption, input leakage, and switching speed.

Resistors do not simply use up voltage

It is common to hear that a resistor "drops voltage". That can be true, but the voltage drop depends on current. If no current flows, an ideal resistor has no voltage drop. This distinction matters when debugging, because a resistor that measures a voltage in one circuit state may measure something different when a load is connected.

Practical mental model

A resistor gives a circuit a relationship between voltage and current. It does not decide either one alone. The surrounding circuit decides the operating point.

Current sensing

A low-value resistor can turn current into a small voltage. For example, 1 A through 0.1 ohm produces 0.1 V. That voltage can be measured by an ADC or amplifier. This is useful in power supplies, motor drivers, chargers, and fault detection.

Current sense resistors need special care: power rating, tolerance, temperature coefficient, PCB copper, and measurement routing all affect accuracy. For very small voltages, the PCB traces and solder joints become part of the measurement unless Kelvin connections are used.

Common mistakes

  • Choosing the value but forgetting to check power.
  • Using very high resistance values where leakage and noise matter.
  • Using a voltage divider as if it were a power supply.
  • Leaving digital inputs floating because the internal pull-up was assumed but not enabled.
  • Ignoring tolerance in measurement circuits and thresholds.

A practical checklist

  1. What job is the resistor doing: current limit, bias, pull-up, divider, sense, filter, or load?
  2. What voltage and current can appear across it in normal and fault conditions?
  3. Is the power comfortably below the rating?
  4. Does tolerance affect the result enough to matter?
  5. Will temperature, leakage, or noise make the chosen value unreliable?
  6. Is the physical package suitable for assembly, heat, voltage, and availability?
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