AEM 30-6905 Universal Programmable EMS-4 - Manual (Page 111)

Page 111 of 279 EMS-4 Install and Tuning Guide_Rev 1.6
Transistors serve many functions but in this case they are applied as Power Drivers. The input
circuit to the transistor from the ECU requires very little current flow to turn the transistor ON.
When the transistor is ON, a high current capacity path is created between the coil negative
terminal and ground. High current means more temperature to dissipate. A stand-alone ignition
driver module or a driver module built into a coil assembly is designed to efficiently dissipate this
heat. The more current flow, the more heat must be dissipated. The amount of current the
drivers must sink to ground during each charge and fire event is determined by the dwell time
settings. What is Dwell Time? The diagram below describes dwell time.
The process begins when the ECU sends a signal to the transistors turning on the current to the
ignition coils primary winding. This „charging' stores energy in the coils magnetic circuit. The
current is then switched off. As the magnetic field begins to collapse the coil tries to resist the
drop in current causing the voltage in the secondary winding to rise rapidly, this high voltage
breaks down the air/fuel mixture in the spark gap allowing a spark to pass, causing ignition of
the air/fuel mixture.