Definition
A resistor connected between a signal line and the positive supply voltage in an electronic circuit, used to hold the line at a known high voltage when no other device is actively driving it. When a switch or output device pulls the line to ground, the resistor limits current flow while allowing the voltage on the line to drop to a low state.
Plain English
A small resistor that keeps an electrical signal at a steady 'on' level by default, so the circuit always knows what state the line is in. When something else takes over and forces the line 'off,' the resistor steps aside but limits how much current flows.
Context Anchor
Seen in aircraft electrical and avionics circuits, especially around switches, sensors, and digital signal inputs.
Derivation
Called 'pull-up' because it pulls the voltage on the line up toward the positive supply. The companion device, a 'pull-down' resistor, does the opposite by tying the line toward ground.
Why Pilots Care
Ensures stable signal levels in aircraft electronic systems, preventing false readings or intermittent failures in instruments and controls.
Analogy
Think of a spring that gently holds a switch in one position until something stronger moves it. The pull-up resistor gently holds the electrical signal high until another part of the circuit pulls it low.
Intuition Check
A pull-up resistor does not physically pull anything upward. It raises or holds an electrical signal at a high voltage unless another part of the circuit pulls that signal low.
Example Sentence 1
The technician traced the fault to a failed pull-up resistor that left the input line floating between high and low.
Example Sentence 2
Technicians check the pull-up resistor value when troubleshooting an intermittent warning light.