Ohm’s law can be used to determine the resistor voltage for any component in a circuit. Simple calculations, also based on Ohm’s law, allow the resistor voltage over resistors in series or parallel to be calculated as well. Resistors are common components in electrical circuits, and they are manufactured in a wide variety of forms. Most are marked using a standard banded scale which can be easily read to determine the resistance, and, hence, the voltage for a known current, that can be expected at a given resistor.
Ohm’s law states that the electromagnetic force, or voltage, at any point in a circuit can be determined by multiplying the current by the resistance. Thus, voltage (V) = current (I) x resistance (R). When resistors are combined in series, one after another, this formula can be used to determine the voltage over any given resistor in the series. Alternatively, the resistance of all the resistors in the series can be added, and Ohm’s law used to calculate the resistor voltage over the entire series.
Resistors in parallel will have different levels of current, as electrical current will prefer the path of lesser resistance, and more current will flow through weaker resistors. The conductance — the reciprocal of resistance — of each resistor in a parallel configuration can be added together to determine the total conductivity of the array of resistors. Thus, 1/R(total) = 1/R(1) + 1/R(2), until all resistors are accounted for, and resistor voltage can be determined using Ohm’s law.
Resistors are made from a variety of different substances and are available in a many different shapes and sizes. Axial resistors are shaped like cylinders with wires protruding from each end to facilitate their mounting on a board. They were the most common variety of resistor throughout most of the 20th century. Resistors of other shapes and sizes are in common use, particularly for very small electronic devices, where axial resistors might be impractical.
The resistance value of a standard axial resistor is normally marked on the resistor using a series of colored stripes. The four or five stripes indicate the nominal resistance value of the resistor and the manufacturing tolerance — how widely the resistor might vary from that nominal value. These bands are black, brown, red, orange, yellow, green, blue, violet, green and white, corresponding to the digits zero to nine for the first two stripes or first three in five-band resistors. The remaining bands indicate a power of ten multiplier and the variance.
When evaluating a banded resistor, the resistance value can be determined from these marking bands. Once the resistance value has been determined, the flow of current over that resistor can be measured. Finally, the resistor voltage can then be computed using Ohm’s law.