Resistor
A resistor sets a controlled relationship between voltage and current. Drop a known number of volts across it and a known current flows; force a known current through it and a known voltage appears.
A resistive material limits current for a given applied voltage and turns electrical energy into heat. Construction varies: film resistors use a resistive coating, wirewound types use resistance wire on a bobbin, and carbon-composition types use a bulk mixture. Material and geometry affect resistance, temperature behaviour and frequency response.
In plain terms
Imagine a wide pipe with one narrow section. Water still flows, but the narrow part forces a pressure drop. A resistor is that narrow section for electric charge.
Also called: R, ohm part.
Why designers use it
- Limit current so the next part survives — most famously through an LED.
- Build a voltage divider that tells an analog input what a sensor or knob is doing.
- Pull a digital line up or down so floating inputs settle to a known state.
- Set the gain or feedback of an op-amp or transistor stage.
- Terminate a transmission line so reflections don't garble fast signals.
Best for
- Current limiting
- Voltage dividers
- Pull-ups / pull-downs
- Biasing transistors
- Op-amp gain setting
Key specifications
- Resistance and tolerance: Use the nominal value and permitted initial variation
- Temperature coefficient: Check resistance drift over the operating temperature range
- Power and pulses: Use series-specific ratings, mounting conditions and derating
- Working voltage: Check separately from the power rating
- High-frequency behaviour: Parasitic inductance and capacitance depend on construction
When not to use it
- When a series voltage drop would waste too much power or vary too much with load. Consider an appropriate regulated supply.
- When you need to block steady DC while coupling an AC signal. A suitably selected capacitor can do that; a resistor alone cannot.
- When the user needs adjustable resistance. Consider a potentiometer rather than a fixed resistor.
Common mistakes
- Checking resistance but not power dissipation, working voltage, pulse limits and temperature derating.
- Ignoring tolerance and temperature drift when setting a gain, threshold or divider ratio.
- Assuming every resistor in the same package has the same power rating.
Where you will find it
- A resistor in series with an indicator LED helps set its current. Choose the value and power rating for the supply and LED operating conditions.
- Two resistors can form a voltage divider. A connected load changes the divider behaviour and must be included in the calculation.
- A current-sense resistor produces a small voltage drop that a measurement circuit can read. The resistor is one part of the measurement, not a complete battery-charge controller.
A short history
Making resistance measurable and repeatable was an important engineering task. A British Association committee established in 1861 worked on resistance standards, and standard resistors were fabricated in 1865. The surviving standards also reveal a practical lesson: a physical resistor's value can drift over time.
Good to know
- Not every resistor hides the same structure: a film coating, a coil of wire and a carbon mixture are three different ways to build resistance.
- A resistor network puts several resistors into one package. The package count and the resistor count are not necessarily the same!