Technician Handbook ! 673 Electronic & Computer Controlled Systems Simple ECU Outputs The simplest way for an ECU to control a vehicle function is to turn a circuit on or off. A circuit can be ground-side switched or power-side switched. Transistors as Switches Electronic circuits use transistors for switching circuits on and off. A transistor is a solid-state electronic component having a base, collector and emitter. In the more commonly used NPN transistor, when sufficient voltage is applied to the base, current flows from the collector to the emitter.
One of the advantages of the transistor is that a low voltage at the base is able to control a large current flowing through the collector and emitter. In that respect, a transistor is similar to a relay. Some transistors also regulate current flow based on the amount of voltage applied to the base. Within the transistor’s limits, a higher base voltage results in a greater flow of current through the collector/emitter. This feature is used in amplifier circuits where the low voltage signal from a microphone regulates current flow in higher power speaker circuits. NOTE NPN transistors are used on ground-side controlled circuits. On power-side controlled circuits, PNP transistors are used.
(Refer to “NPN and PNP Transistors” in the Appendix.) Technical Training 13