Resistances are created
Fixed resistor / fixed resistors
Fixed resistors get their name from their fixed resistance value, which is not adjustable. The resistance value has the unit Ω (ohm) and the symbol R for resistor. Fixed resistors differ in their design. There are film resistors and wirewound resistors.
Fixed resistors are not available with every resistance value. They are subject to an international color code with four-fold or five-fold rings. The rings allow the determination of the resistance value in Ω. They are manufactured exclusively in accordance with the internationally valid IEC series of standards. If you still want a certain resistance value, then an adjustable resistor must be used.
Fixed resistors are linear resistances. Linear resistances are also called ohmic resistances. They have a linear I-U characteristic. Current and voltage are proportional to each other. This means that when the voltage increases, the current also increases. If the current increases, so does the voltage drop across the resistor. Ohm's law applies to the calculation.
The resistance values of fixed resistors and also adjustable resistors are given for a temperature of 20 ° C. The change in resistance with a change in temperature is only very small, so that an almost constant resistance value can be assumed.
The value of resistors with metal layers increases with increasing temperature. In the case of layers of coal, the value decreases.
Like all other electronic components, resistors are subject to a certain manufacturing tolerance. A resistor marked 100 ohms may only be 99 ohms or 101 ohms. Depending on the resistor type, this is still within the tolerance.
The tolerances range from 0.1 to 10%. Depending on the application, you have to pay more or less attention to the tolerance.
In the case of film resistors, a thin layer of carbon, metal or metal oxide is sprayed or vaporized (in a vacuum) onto cylindrical ceramic or hard glass.
The resistance value (tolerance up to 5%) is determined by the layer thickness and the spraying time. Resistance values with a lower tolerance are produced by grinding in the layers. However, these lead to a higher inductance of the resistor.
Film resistors differ between two types of design and material. Carbon film resistors are particularly suitable in the HF range. Metal film resistors combine the properties of wire and carbon film resistors. These resistors have a low tolerance.
Wirewound resistors consist of a temperature-resistant ceramic or plastic body on which a wire made of a metal alloy is wound. The wire winding creates a relatively high inductance. The reason lies in the similar design of a coil. A bifilar winding is used to reduce the inductance. The resistor carrier is wound twice. The windings lying next to one another are then traversed by the current in opposite directions. The resulting magnetic fields cancel each other out. They reach a great age without loss of resistance, are very resilient and are suitable up to a frequency of 200 kHz.
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