What can change the resistance in a circuit
The word "resistance" has a twofold meaning in electronics. On the one hand, the property of materials that inhibits the electric current is called resistance; on the other hand, a “resistor” is used to describe a component specially manufactured for this purpose.
A resistor usually consists of an insulating porcelain body that is coated with a thin layer of carbon or metal and a protective varnish.  Carbon film resistors are usually ocher-colored, metal film resistors are painted blue. Finally, the resistance value is printed on the protective varnish in the form of colored rings so that it can be read from all sides even without a magnifying glass.
Carbon and metal layer resistors differ essentially in their tolerance values, i.e. in the possible deviation of the actual resistance value from the printed value. Metal film resistors can be manufactured more precisely and thus have a lower tolerance (usually ), but are also slightly more expensive. For most purposes in the hobby area, the cheaper and somewhat less precise carbon film resistors (mostly or Tolerance) completely.
Notation of resistance values
Resistance values are often given with two special features:
- Since the values of different resistors can be very different (from around one ohm to a few million ohms), resistance values are often given as powers of ten, for example instead of . The -Sign is even often left out.
- In circuit diagrams, it is also common to write possible decimal places after the power factor; a resistance specification of thus corresponds to a resistance value of or. . In this way, it can be ruled out that a decimal point could be "skipped" by mistake.
Although the colors of the printed colored rings always have the same meaning, the carbon and metal layer resistors differ in the number of rings printed on them. On carbon film resistors, as listed in the table, the color table (carbon film resistors) is always displayed four Rings printed:
- The first two rings represent the first two digits of the resistance value.
- The third ring defines the number of zeros as a multiplier.
- The fourth ring, which is usually silver or gold in the case of carbon film resistors, is offset slightly to the right and indicates the tolerance value; the distance to the other rings can be used to ensure that the resistance value is read from the correct side.
As shown in the color table (metal layer resistors), the more precise metal layer resistances are always applied five Rings printed. The resistance value is shown with four rings, with the first three rings representing the first three digits of the resistance value and the fourth ring as a multiplier indicating the number of zeros to be added. In this way, there are also finer gradations of the resistance values such as representable.
A common resistance value is - A resistor of this size is required, for example, to connect a normal light-emitting diode to a -Battery or a corresponding rechargeable battery must be connected (see LED with series resistor). Assuming that it is a carbon film resistor, the first digit corresponds to a yellow ring, the second digit a purple one. The value encoded in this way a zero has to be appended; this can be achieved by printing the third ring in brown color.
If the fourth ring of the carbon layer resistor is gold, then there is a tolerance of allowed - the actual value of the resistance is between and Ohm.
Almost all electrical components also have a usually slightly higher resistance at higher temperatures; However, there are also (semiconductor) materials in which this temperature dependency is very pronounced. As special resistors, these special materials are often used as temperature sensors.
"PTC thermistors" or PTC resistors ("PTC" = "Positive Temperature Coefficient") are substances whose resistance increases with increasing temperatures.
Typical thermistor materials are, for example, carbon, semiconductors, or certain metal oxides; They are used, among other things, as fast and robust temperature sensors and for voltage stabilization.
“NTC” or “NTC” resistance (“NTC” = “Nositive Temperature Coefficient”) are substances whose resistance decreases with increasing temperatures.
Typical PTC thermistor materials are, for example, pure metals or semiconducting materials made from titanate ceramics such as or ; they are used, among other things, as temperature sensors, thermostats and for current stabilization.
Photo resistors - also called "LDR" (Light Dependend Resistor) - have a resistance of a few hundred when they are fully illuminated . With decreasing light intensity, the resistance increases - depending on the design - up to a few at.
The change in resistance of a photoresistor happens comparatively slowly; it can easily be observed with the help of an ohmmeter at different illuminance levels. At the same time, due to their "inertia", photoresistors are not suitable for fast optical signal transmission (e.g. infrared remote controls). Instead, photodiodes and phototransistors are used for this.
Varistors (also known as "Voltage Dependend Reistor" or VDR for short) are electrical resistances whose value depends on the applied voltage.
The characteristic curve of a varistor looks something like this:
Up to a tension of just under varistors have a very high resistance value; from such a high voltage, however, their resistance value drops considerably. Varistors are therefore installed as surge protection in power supply units and socket strips in order to protect sensitive electronic devices such as computers from possible voltage peaks, such as those that can occur in the event of a lightning strike.
Since there is no experimentation with mains voltages in the field of hobby electronics for safety reasons, varistors are rarely used in handicraft projects.
An adjustable resistor is a resistor whose resistance value can be changed (usually) from zero to the specified maximum value via a sliding contact. A distinction is made between potentiometers that can be regulated during operation and permanently adjustable trimming resistors.
A potentiometer ("Poti" for short) enables the resistance value to be set externally with the aid of a rotary knob or slide.
Each potentiometer has three connections, with the full resistance value indicated on the potentiometer occurring between the two outer connections. If one end and the middle tap of the potentiometer are connected to the circuit, the resistance can be changed between zero and the maximum value using the rotary knob.
Potentiometers are often used as rotary or slide controls, for example when setting the volume on a radio.
A trimming resistor ("adjustment potentiometer") can only be adjusted with the aid of a screwdriver. Since trimming resistors are usually permanently installed components of a circuit, readjustment during operation is not normally provided.
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