- Resistor Definition: A resistor is a component that provides resistance to the flow of current in a circuit.
- Types of Resistors: Various types of resistors include carbon composition, thermistor, wire wound, metal film, carbon film, variable, varistor, and light dependent.
- Carbon Composition Resistor: These are low-cost, durable resistors made of carbon clay, but they are temperature sensitive.
- Thermistors and LDRs: Thermistors change resistance with temperature, while LDRs change resistance based on light intensity.
- Applications of Resistors: Different types of resistors are used based on cost, power dissipation, temperature sensitivity, and specific needs in electronic projects.
What is an Electrical Resistor?
A resistor is a passive component in a circuit which provides resistance to the flow of current. There are many different types of resistors. These resistors vary in their construction, power dissipation capacities, and tolerance to various parameters (such as temperature and light). The types of resistors include:
- Carbon Composition Resistor
- Thermistor
- Wire Wound Resistor
- Metal Film Resistor
- Carbon Film Resistor
- Variable Resistor
- Varistor
- Light Dependent Resistor
Carbon Composition Resistor
A carbon composition resistor uses a solid carbon-based resistive element rather than a film or wound wire.
The element is a moulded mixture of carbon and an insulating binder. Leads connect to the element, and an insulating body protects the assembly.
Carbon composition construction has little winding inductance and can tolerate some short, high-energy pulses better than many small film resistors. Its suitability still depends on the device rating and pulse waveform.
Available values, tolerances and power ratings depend on the product series. General-purpose film resistors are more common in current designs and in many of the best Arduino starter kits.
Compared with precision film types, carbon composition resistors usually have wider tolerance, more noise and greater resistance change with temperature, humidity and ageing.
Those limits may be acceptable in non-critical circuits, but they matter in precision, low-noise and long-life designs.
The granular resistive element can produce more excess electrical noise than a stable metal-film element.
Choose this type only when its pulse behaviour, resistance range, tolerance, noise and environmental stability match the circuit.
Axial resistors often use coloured bands to show nominal resistance, multiplier, tolerance and sometimes temperature coefficient.
Thermistor
A thermistor is a temperature-sensitive resistor with a characterised relationship between temperature and resistance.
Most commercial thermistors have a negative temperature coefficient, so resistance decreases as temperature rises. Positive-temperature-coefficient types increase resistance and are often used for control or protection.
Thermistors commonly use semiconductor ceramics or polymers. Resistance range, useful temperature range and response time depend on the material and package.
Their high sensitivity can resolve small temperature changes, but accurate temperature measurement requires calibration and control of self-heating from the measuring current.
Wire Wound Resistor
In a wire wound resistor, resistance-alloy wire is wound around an insulating core. The alloy and construction determine the temperature coefficient of resistance, so resistance variation with temperature can be low but is never absent.
An enamel, cement or moulded coating can provide insulation and mechanical protection. It does not remove ambient-temperature or self-heating effects.
Wire length, diameter, alloy, winding pattern, core and enclosure determine resistance, power handling and thermal performance.
Wirewound products cover many resistance and power ratings, but the available range varies by construction and manufacturer.
Precision wirewound resistors can offer tight tolerance and stability. Power wirewound resistors can dissipate substantial heat when mounted and cooled as specified.
They are used for precision, current sensing and power dissipation when their size, cost and frequency response suit the application.
The wound structure is inherently inductive. At high frequency, inductance and parasitic capacitance can make impedance differ from the marked DC resistance.
Bifilar or other non-inductive winding patterns can reduce net inductance. They do not make every parasitic effect zero.
For fast pulses or high-frequency circuits, use the manufacturer’s impedance data rather than assuming that any wirewound resistor behaves as a pure resistance.
Metal Film Resistor and Carbon Film Resistor
A film resistor deposits or applies a resistive layer, such as carbon, metal or metal oxide, to an insulating substrate.
The manufacturer sets the resistance through film composition and geometry, often trimming a helical groove or patterned path in the layer.
In a leaded cylindrical part, metal end caps contact the resistive film and connect it to the lead wires.
Metal film resistors and carbon film resistors are compact and available in many resistance and power ranges. The exact limits depend on the product series.
Film construction usually has less inductance than a conventional wirewound part, but helical trimming, leads and packaging still create parasitic inductance and capacitance.
Metal-film types are widely used when a circuit needs tighter tolerance, lower noise or better temperature stability than a general-purpose carbon-film resistor.
Carbon-film performance depends on formulation and construction. Compare each part’s tolerance, voltage coefficient, noise, pulse rating and temperature coefficient of resistance rather than selecting by material name alone.
Variable Resistor
A variable resistor changes resistance through a movable contact. A three-terminal potentiometer normally acts as a voltage divider, while a two-terminal connection acts as a rheostat.
Moving the wiper changes the effective resistive path. Select the track material, taper, adjustment life, wiper current and power rating for the intended use.
A variable resistor can use a rotary or linear mechanism. The mechanical form does not by itself determine its electrical taper or power capability.
Non Linear Resistor or Varistor
A varistor has a strongly non-linear voltage-current characteristic. Its effective resistance falls as applied voltage rises, so it is not an ohmic device with the constant ratio described by Ohms law.
Modern surge-suppression varistors usually use zinc oxide. Older designs also used silicon carbide.
Common and historical constructions include:
- Silicon carbide disc varistors
- Silicon carbide rod varistors
- Zinc oxide metal-oxide varistors
Light Dependent Resistor
A Light Dependent Resistor (LDR), or photoresistor, changes resistance with incident light. Cadmium sulfide is a common photosensitive material, although material and construction vary.
Absorbed photons increase the number of mobile charge carriers in the photoconductor. Conductivity therefore rises and resistance falls under illumination. Spectral response, response time, dark resistance and temperature sensitivity must match the sensing task.
