What is a ballast resistor, and where is it used in signaling lamp circuits?

What is a ballast resistor, and where is it used in signaling lamp circuits?

• A. A resistor that limits current to a lamp, preventing excessive current and extending lamp life.
• B. A device that increases lamp brightness.
• C. A fuse that protects the lamp.
• D. A switch that turns lamp on.

Answer: (A)

A ballast resistor is a resistor placed in series with a signaling lamp to limit the current flowing to the lamp. In signaling circuits, the supply voltage can vary and the lamp’s cold resistance is much lower than its operating (hot) resistance, which can cause a large inrush current when the lamp first lights. The ballast resistor drops part of the voltage as current increases, keeping the lamp current within a safe, predictable range. This protects the filament, reduces thermal stress, and helps maintain a steady brightness as conditions change, effectively extending lamp life. It does not increase brightness, act as a fuse, or serve as a switch.

A ballast resistor is a resistor placed in series with a signaling lamp to limit the current flowing to the lamp. In signaling circuits, the supply voltage can vary and the lamp’s cold resistance is much lower than its operating (hot) resistance, which can cause a large inrush current when the lamp first lights. The ballast resistor drops part of the voltage as current increases, keeping the lamp current within a safe, predictable range. This protects the filament, reduces thermal stress, and helps maintain a steady brightness as conditions change, effectively extending lamp life. It does not increase brightness, act as a fuse, or serve as a switch.