Definition
A resistor is a passive two-terminal component designed to oppose electric current by converting electrical energy into heat. Its principal property is resistance, measured in ohms (Ω).
Learning objectives
- Read a resistor value from its color code or marking and state its tolerance.
- Calculate current, voltage and power in a resistor using Ohm’s law.
- Reduce series and parallel resistor networks to one equivalent resistance.
- Choose a power rating and test a resistor safely with a meter.
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Progressive explanation
Start with the plain-language idea, then move into the engineering detail. Each level stands on its own, so you can stop at the depth you need.
Beginner The core idea
A resistor is the simplest way to control current in a circuit. When a voltage is applied across it, the resistor allows a current that is proportional to that voltage and inversely proportional to its resistance. That relationship is Ohm’s law, V = I × R.
Resistors appear everywhere: they bias transistor bases, set the current in a light-emitting diode, divide a voltage for a sensor input, terminate a communication bus and discharge a capacitor when power is removed. The physical size is not arbitrary — a larger resistor can usually dissipate more heat, which is why power rating matters as much as resistance value.
Resistors appear everywhere: they bias transistor bases, set the current in a light-emitting diode, divide a voltage for a sensor input, terminate a communication bus and discharge a capacitor when power is removed. The physical size is not arbitrary — a larger resistor can usually dissipate more heat, which is why power rating matters as much as resistance value.
Intermediate How it behaves in a circuit
A real resistor is specified by resistance, tolerance, power rating and temperature coefficient. Tolerance is the permitted deviation from the nominal value, so a 100 Ω resistor with ±5 % tolerance may measure anywhere from 95 Ω to 105 Ω when new and at the reference temperature.
In series, resistances add: the same current flows through each resistor and the voltage divides in proportion to resistance. In parallel, the voltage is common and the currents add, so the reciprocal of the equivalent resistance is the sum of the reciprocals. A resistor converts electrical energy into heat at a rate P = I²R = V²/R, and its body temperature rises until that heat leaves through the leads, the board and the surrounding air. Always compare the calculated dissipation with the rated power, then derate for a hot enclosure.
In series, resistances add: the same current flows through each resistor and the voltage divides in proportion to resistance. In parallel, the voltage is common and the currents add, so the reciprocal of the equivalent resistance is the sum of the reciprocals. A resistor converts electrical energy into heat at a rate P = I²R = V²/R, and its body temperature rises until that heat leaves through the leads, the board and the surrounding air. Always compare the calculated dissipation with the rated power, then derate for a hot enclosure.
Advanced Limits, assumptions, engineering detail
At high frequencies a resistor is not purely resistive: wirewound and film parts carry parasitic inductance and capacitance, so impedance rises or falls with frequency and pulse response becomes important. Precision and low-noise circuits also care about thermal noise, whose available noise power is proportional to absolute temperature and bandwidth, and about excess noise that appears when current flows through a resistive material.
Voltage coefficient, humidity, solder heat and long-term drift change the value over life. Pulse applications must respect the resistor’s pulse-energy capability rather than only its continuous power rating, because a short pulse can create a local hot spot before the body has time to spread heat. Four-terminal (Kelvin) connections are used for shunts and very low values so that lead and contact resistance do not corrupt the measurement.
Voltage coefficient, humidity, solder heat and long-term drift change the value over life. Pulse applications must respect the resistor’s pulse-energy capability rather than only its continuous power rating, because a short pulse can create a local hot spot before the body has time to spread heat. Four-terminal (Kelvin) connections are used for shunts and very low values so that lead and contact resistance do not corrupt the measurement.
Formulas and variables
Ohm’s law
V = I × R
V Voltage across the resistor · V
I Current through the resistor · A
R Resistance · Ω
12 V across 220 Ω gives I = 12 / 220 = 0.0545 A, about 54.5 mA.
Power dissipated
P = I² × R = V² / R
P Power converted to heat · W
I Current · A
V Voltage · V
R Resistance · Ω
The same 220 Ω resistor at 12 V dissipates 12² / 220 = 0.655 W.
Series and parallel equivalents
Rseries = R1 + R2 + … ; 1/Rparallel = 1/R1 + 1/R2 + …
Rseries Equivalent series resistance · Ω
Rparallel Equivalent parallel resistance · Ω
R1, R2 Individual resistances · Ω
100 Ω in parallel with 220 Ω gives 1/R = 1/100 + 1/220, so R ≈ 68.8 Ω.
Units of measurement
| Quantity | Symbol | Unit |
|---|---|---|
| Resistance | R | ohm (Ω) |
| Conductance | G | siemens (S) |
| Power | P | watt (W) |
| Temperature coefficient | α | per kelvin (1/K) or ppm/K |
Worked examples
Worked example
Current and dissipation in a series resistor
Problem. A 12 V supply is connected across a 220 Ω resistor. Find the current and the power dissipated, then decide whether a 0.25 W part is acceptable with no derating.
Solution. I = V / R = 12 / 220 = 0.0545 A. P = I²R = 0.0545² × 220 = 0.655 W. A 0.25 W resistor would be overloaded by a factor of about 2.6.
I ≈ 54.5 mA and P ≈ 0.655 W; use at least a 1 W resistor, preferably with margin.
Worked example
Equivalent parallel resistance
Problem. A 100 Ω and a 220 Ω resistor are connected in parallel. What is the equivalent resistance?
Solution. 1/R = 1/100 + 1/220 = 0.0100 + 0.004545 = 0.014545 S. R = 1 / 0.014545 = 68.75 Ω.
R ≈ 68.8 Ω, which is lower than either resistor — a useful sanity check.
Practical applications
- Limiting LED current and setting transistor bias points
- Dividing voltage for sensor and feedback inputs
- Terminating communication buses and discharging capacitors
- Sensing current as a low-value shunt, and dissipating braking or bleed energy
Automotive applications
In a vehicle, resistors bias sensors, pull up or down digital inputs, set LED brightness, terminate CAN wiring and act as current shunts in battery-monitoring systems. They also appear in series with ignition and injector circuits where the control module needs a defined load, and as heating elements in some auxiliary systems. Vibration, humidity and wide temperature swings mean the resistor must be rated for the environment, not only for steady-state power.
Industrial applications
Industrial panels use resistors for braking large motors, discharging DC-link capacitors, limiting inrush current, sensing current and creating reference loads. Wirewound and aluminium-housed resistors are common where several hundred watts must be dissipated; they are mounted on heat sinks and protected by thermal switches. In hazardous areas the surface temperature and enclosure rating become part of the safety case.
Measurement procedure
- Isolate and verify dead the circuit before testing; a resistance measurement on a live circuit gives a meaningless result and can damage the meter.
- Disconnect at least one resistor terminal so parallel paths do not change the reading.
- Compare the measured value with the nominal value and tolerance; a reading outside tolerance suggests drift or damage.
- Use a four-wire method for values below a few ohms so lead and contact resistance do not dominate.
- Check for discoloration or cracking and measure again after the part has cooled; heat can shift the value temporarily.
Common mistakes
- Choosing a resistor by resistance only and ignoring the power rating, which leads to overheating or failure.
- Measuring resistance in-circuit and assuming the lower reading is the resistor value instead of a parallel path.
- Assuming a color code reading is infallible; ambiguous bands must be confirmed with a meter.
- Connecting a resistor directly across a low-impedance supply and expecting it to limit current without checking dissipation.
Safety considerations
- A resistor dissipates heat: keep clearance from flammable materials and respect the enclosure temperature rating.
- Treat mains-connected resistors and their terminals as live until the circuit is isolated and proven dead.
- High-voltage bleeder resistors may remain hot and charged for a long time; allow the specified discharge time and measure before touching.
- Do not use a resistor as a substitute for a fuse or a protective device; it does not provide safe interruption.
Frequently asked questions
Does a resistor reduce voltage or current?
It does both in a circuit, in the sense that it creates a voltage drop proportional to the current through it. In a series circuit the same current flows through the resistor and the load, and the resistor takes a share of the supply voltage; that is why it can limit the current reaching a load.
Can I replace a resistor with a higher power rating?
Usually yes, provided the resistance and tolerance are suitable and the larger body fits electrically and mechanically. A higher power rating gives more thermal margin but does not change the circuit behaviour. Do not change the resistance value without checking the design.
Why does my measured resistance differ from the color code?
The part may be outside tolerance or drifted, another path may be in parallel, your fingers may be across the leads, or the meter leads and contact resistance may add error at low values. Desolder one end and repeat the measurement.
When should I use a resistor instead of a regulator?
A resistor is acceptable only when the current is predictable and the wasted power is acceptable. For a varying load, a resistor produces a varying voltage, so a regulator or a proper current source is the correct engineering choice.