Resistors
A resistor is an electrical/electronic passive component used to limit the flow of current. The figure represents the various types of resistors.
Specification of Resistors:
The specification of resistors are :
- Resistance value
- Tolerance
- Power rating
- Thermal stability
that implements electrical resistance as a circuit element.
Resistors act to reduce current flow, and, at the same time,
act to lower voltage levels within circuits. In electronic
circuits resistors are used to limit current flow, to adjust
signal levels, bias active elements, terminate transmission
lines among other uses. High-power resistors that can dissipate many watts of electrical power as heat may be used
as part of motor controls, in power distribution systems,
or as test loads for generators. Fixed resistors have resistances that only change slightly with temperature, time or
operating voltage. Variable resistors can be used to adjust circuit elements (such as a volume control or a lamp
dimmer), or as sensing devices for heat, light, humidity,
force, or chemical activity.
Resistors are common elements of electrical networks
and electronic circuits and are ubiquitous in electronic
equipment. Practical resistors as discrete components
can be composed of various compounds and forms. Resistors are also implemented within integrated circuits.
The electrical function of a resistor is specified by its
resistance: common commercial resistors are manufactured over a range of more than nine orders of magnitude. The nominal value of the resistance will fall within
a manufacturing tolerance.
Resistor Definition and Symbol
A resistor is a passive electrical component with the primary function to limit the flow of electric current.
The international IEC symbol is a rectangular shape with leads at each end as shown in the figure at left. In the USA, the ANSI standard is very common and represents a fixed resistor as a zigzag line (shown on the right).
IEC fixed resistor symbol |
ANSI fixed resistor symbol |
Overview of Types and Materials
Resistors can be divided by functional type as well as resistance material. The following breakdown for the types can be made:
- Fixed resistors
- Variable resistors, such as the:
- Potentiometer
- Rheostat
- Trimpot
- Resistors for which the resistance value is dependent on a physical quantity:
- Thermistors (NTC and PTC) – the resistance is a function of temperature
- Photo resistors – the resistance is a function of light level
- Varistors – the resistance is a function of voltage
- Magneto resistors – the resistance is a function of magnetic field
- Strain gauges – the resistance is a function of mechanical load
For each of these types, a standard symbol exists. Another breakdown based on the material and manufacturing process can be made:
- Carbon composition
- Carbon film
- Metal film
- Metal oxide film
- Wirewound
- Foil
The choice of material technology is specific to the resistor purpose. Often it is a trade-off between cost, precision, power, and other requirements. For example, carbon composition is a very old manufacturing technique that creates a low precision resistor, but is still used for specific applications where high energy pulses occur. Carbon composition resistors have a body made from a mixture of fine carbon particles and a non-conductive ceramic.
The carbon film technique creates resistors with a better tolerance (less resistance value variation) than carbon composition resistors. These are made of a non-conductive rod with a thin carbon film layer around it. This layer is treated with a spiral cut to increase and control the resistance value.
Metal and metal oxide film are widely used nowadays, and have better properties for stability and tolerance. Furthermore, they are less influenced by temperature variations. Like carbon film resistors, they are constructed with a resistive film around a cylindrical body. Metal oxide film is generally more durable.
Wirewound resistors are probably the oldest type and can be used for both high precision as well as high power applications. They are constructed by winding a special metal alloy wire, such as nickel chrome, around a non-conductive core. They are durable, accurate, and can have very low resistance value. A disadvantage is that they suffer from parasitic reactance at high frequencies.
For the highest requirements on precision and stability, metal foil resistors are used. They are constructed by cementing a special alloy cold rolled film onto a ceramic substrate.
Resistor Characteristics
Dependent on the application, the electrical engineer specifies different properties of the resistor. The primary purpose is to limit the flow of electrical current; therefore the key parameter is the resistance value. The manufacturing accuracy of this value is indicated with the resistor tolerance and is expressed as a percentage of the resistance value (for example ±5%). Many other parameters that affect the resistance value can be specified, such as long term stability or the temperature coefficient. The temperature coefficient, usually specified in high precision applications, is determined by the resistive material as well as the mechanical design.
In high frequency circuits, such as in radio electronics, the parasitic capacitance and inductance can lead to undesired effects. Foil resistors generally have a low parasitic reactance, while wirewound resistors are among the worst. For accurate applications such as audio amplifiers, the electric noise of the resistor must be as low as possible. This is often specified as microvolts noise per volt of applied voltage, for a 1 MHz bandwidth. For high power applications the power rating is important. This specifies the maximum operating power the component can handle without altering the properties or damage. The power rating is usually specified in free air at room temperature. Higher power ratings require a larger size and may even require heat sinks. Many other characteristics can play a role in the design specification. Examples are the maximum voltage or the pulse stability. In situations where high voltage surges could occur, this is an important characteristic.
Sometimes not only the electrical properties are important, but the designer also has to consider the mechanical robustness in harsh environments. Military standards sometimes offer guidance to define the mechanical strength or the failure rate.
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