A tiny movement can command a large result. Explore normally open and normally closed contacts, switch positions, current paths, voltage-drop testing, and real automotive faults.
A switch changes the conducting path through a circuit. It can interrupt a load, select between loads, or send a low-current request to a module. A switch does not create voltage; its contacts determine where voltage and current can go.
Change the switch and introduce faults. The lamp, contact position, circuit current, and test-point voltages update together. The reference circuit is battery positive → fuse → switch → lamp → ground.
“Normally” refers to the unactuated state. An NO switch is open at rest; an NC switch is closed at rest. A stuck-open switch ignores the control input.
One movable contact controls one circuit path: ON or OFF. This is the simplest form of the lamp circuit above. “Pole” means the number of separate common circuits; “throw” means the number of contact destinations.
A double-pole switch can control two separate paths together. Contact type (NO or NC) is a separate description from pole/throw count.
One common terminal connects to either of two output contacts. Select the destination for a fused feed. The center-off setting represents an ON-OFF-ON three-position switch, not every SPDT switch.
Black lead is at battery negative / chassis ground. Red-lead test points refer to the diagram above. Select a test point to see the present node voltage. These are idealized DC readings under the simulated circuit state.
Open switch, healthy feed: TP1 is near B+ and TP2 is near 0 V (the intact lamp provides a path to ground). Nearly full source voltage appears across the open switch.
Closed switch, healthy circuit: TP1 and TP2 are near B+. Voltage drop across the closed switch should be near 0 V; most supply voltage is across the lamp.
Open lamp ground: TP3 may rise near B+ with the switch closed, even though the lamp remains dark, because no current flows and there is no load voltage drop.
Eight questions. A passing score is 7/8. You can retry; previously completed lessons stay complete.