What is a Resistor
Resistors are devices which can be used in a circuit to control the current. There are many different types of resistors which are used in different situations. The resistance of a conductor is taken to be the ratio of the potential difference across the resistor to the current through the resistor. The figure below shows two standard circuit symbols for a resistor:
Physically, resistors typically look like the one shown below:
Broadly speaking, resistors can be divided into two types: fixed resistors and variable resistors. As their names suggest, the resistance in fixed resistors does not change, whereas a variable resistor’s resistance could be altered easily. In this article, we will look at how different types of resistors are constructed. First, we will look at types of fixed resistors.
Types of Fixed Resistors
Carbon Composition Resistors
Carbon composition resistors are made of granules of graphite and ceramics bound in a binding material. These types of resistors are the cheapest to make.
Carbon Film Resistors
Carbon film resistors consist of a “core” of insulating material, around which a “strip” of carbon winds around as a helix. This strip of carbon acts as a narrow conducting path for electrons.
Metal Film Resistors
Metal film resistors are similar, where a strip of metal winds around the insulating material.
Wirewound resistors consist of a wire wound around an insulating core. These types of resistors are usually quite large, but they are comparatively more stable.
Types of Variable Resistors
Potentiometers are devices with three terminals. With two of the terminals connected to an electric circuit, a potentiometer could be used as a variable resistor.
These consist of a long, thin wire wound around an insulator. By changing the position of a moveable contact, current can be made to flow through different lengths of the wire, giving different values for resistance.
Nonlinear resistors are resistors whose resistance changes in response to a physical quantity. Examples include thermistors and light-dependent resistors.
Thermistors are resistors whose resistance changes in response to temperature. In negative temperature coefficient (NTC) thermistors, the resistance decreases as the temperature increases. In positive temperature coefficient (PTC) resistors, the resistance increases as the temperature increases. Thermistors are used in circuits that regulate temperature. The symbol for a thermistor is shown below:
Light-dependent resistors (LDRs) are resistors whose resistance decreases when the intensity of light increases. The symbol for an LDR is shown below:
Its ability to change resistance in accordance to lighting makes them useful in lighting circuits, as shown below:
As the light intensity decreases (the environment becoming darker), the resistance across the LDR increases. This causes the LDR to draw a higher proportion of voltage from the cell. As a result, the voltage and current across the lamp increases, causing it to grow brighter.
“The IEC Symbol for a Resistor, with the specified 3:1 aspect ratio (IEC 60617)” by Markus Kuhn (Made in Inkscape from scratch) [Public Domain], via Wikimedia Commons (Modified)
“Some old carbon? resistors in an old valve radio…” by Ozguy89 (Own work) [Public Domain], via Wikipedia
“Charles Wheatstone’s 1843 Rheostat with a moving whisker” by Wheatstone, Charlesn (Wheatstone, Charles: “An Account of several new Instruments and Processes for determining the Constants of a Voltaic Circuit”, Philosophical Transactions of the Royal Society of London, Volume 133, 1843, pp. 308-309.) [Public Domain], via Wikimedia Commons