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Resistance

Overview​

  • Resistance is measured in ohms, often denoted as Ω.
  • Resistors are electronic components made of materials that restrict the flow of electricity.
  • Ohm's law states that the resistance provides a proportional reduction of current flow and is codified as:
    • Voltage = Amperage * Resistance or V = I*R
    • Amperage = Voltage / Resistance or I = V/R
    • Resistance = Voltage / Amperage or R = V/I
    • Where I is amps, R is ohms, and V is voltage.

Power Rating and Calculations​

  • Resistors have a power rating which specifies the amount of power (wattage) they can safely handle.
  • In addition to the standard definition of a watt (volts * amps), power can be calculated when only volts or amps and resistance is known:
    • When amperage (current) and resistance is known: P = I^2 * R
    • When voltage (force) and resistance is known: P = V^2 / R

Calculating Resistance in Resistor Circuits​

Resistors can be connected in parallel (side-by-side) or series (end-to-end) arrangements. The following reference guides explain the calculation in their respective configuration:

Resistors as Components​

  • Axial resistors are color coded to aid in identifying them. See this chart for reference.
  • Resistors have a tolerance which specifies a range that their actual resistance falls into.
  • Resistors come in a set of values that covers the range of possible values, adjusted for tolerance, with only small overlap.

Pull-Up and Pull-Down Resistors​

  • A pull-up resistor provides a default HIGH value at an input in a circuit by connecting the input to a voltage source and allowing a small amount of current to flow.
  • A pull-down resistor is the opposite of a pull-up, and provides a default value of LOW at an input in a circuit.
  • Pull-up and pull-down resistors are one of the most common tools in circuit design.
  • Pull-ups and pull-downs with smaller resistance values are known as strong pull-ups and pull-downs, and act quicker than weak pull-ups and pull-downs, which have a larger resistance value.