Taking into consideration that in all electrical devices, electricity performs work, it seems unusual that the most common component in these devices is a resistor a component that "slows down" electric current. More precisely, a resistor is a component with two terminals that has a specific electrical resistance, a property that opposes the flow of current when a potential difference (voltage) is applied across its terminals. Electrical resistance is the ratio of applied voltage to the current flowing through the component and is measured in Ohms (Ω), a unit named after the German physicist Georg Simon Ohm.
As current flows through a resistor, the potential energy of electrons is lost (i.e., there is a voltage drop), converting this energy into the thermal energy of the resistor's molecules. This property is utilized in various ways in electrical circuits: limiting current, reducing signals, dividing voltages, and more. Therefore, it is important to know the exact resistance of each resistor, as well as how this resistance changes depending on the resistor's temperature. These values depend on the material properties of the resistor, its manufacturing method, and uncontrollable variations.
The resistance of any object, including a resistor, can be measured using an ohmmeter. However, it is often useful to determine the resistance by visually inspecting the resistor since the resistance of a component in a device cannot be precisely measured unless the component is removed from the device. Various markings indicating resistance are placed on resistors. On cylindrical resistors, resistance is marked with three or more color bands. This application allows the user to decipher these markings by selecting colors.
In resistors with three bands, the first two bands represent significant digits, while the third band represents the multiplier. The error in manufacturing for such resistors can result in resistance being up to 20% higher or lower than the marked value. In resistors with four markings, the first three represent the same as in three-band resistors, and the fourth marking represents tolerance to this error. In resistors with five markings, the first three bands represent significant digits, while the fourth and fifth bands represent the multiplier and tolerance, respectively. Finally, in resistors with six markings, the sixth band indicates the change in resistance with temperature, expressed in millionths of Ohms per degree Celsius (parts per million).
A resistor is always read starting from the marking closest to one of its terminals. If the markings are symmetrically arranged, silver or gold bands, which are not used to represent digits, can help determine the correct orientation. Resistors are non-polarized components and can be placed in a circuit in any orientation.