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Power transistor wiring harness9/1/2023 This voltage spike is then reflected across the secondary winding of the coil in increased orders of magnitude. This rapid change in current through the inductive primary winding forces v switch to hundreds of volts (see Figure 6 below).įigure 6: Switch voltage and ignition coil primary winding current as a function of switch state (not to scale). Based on a triggering mechanism that is carefully aligned to the position of the engine's crankshaft, the ignition system opens the switch and forces i coil to zero almost instantaneously. This current is eventually limited by the primary winding's resistance and any ballast resistance in series. The voltage across the switch, v switch, is forced to zero as current through the primary winding, i coil, gradually increases. To generate a spark at the spark plug, the vehicle's ignition system closes the switch, connecting the primary winding of the ignition coil to ground for a period of time called the "dwell" period. The ignition coil can be seen in the upper left of the photo. Each terminal along the circumference of the distributor cap is connected to a spark plug by a length of heavily insulated wire called a plug wire.įigure 5: Photo showing distributor cap removed to expose rotor (both in blue). An electrical contact at the end of the rotor comes into close proximity with each of four, six or eight evenly spaced terminals around the circumference of the distributor cap, shown in Figure 5 below. Underneath the distributor cap, the rotor pivots about this terminal as a function of the engine's rotation. Meanwhile, the coil's secondary winding is connected to the center terminal of the vehicle's distributor cap. Figure 4 below shows a schematic representation of the ignition coil's arrangement.įigure 4: Schematic representation of ignition coil and associated circuitry. Most auto-makers phased out breaker points in favor of electronic ignition during the 1970s. With breaker points, a capacitor (or "condenser" as it's often called) is placed in parallel to mitigate arcing that can wear the breaker points prematurely. The coil's primary winding is selectively connected to ground through a mechanical switch (called "breaker points") or a power transistor. One terminal common to both windings is connected to the vehicle's battery, often times through a series "ballast" resistor that is bypassed while the vehicle is started so that power to the coil is roughly equivalent between starting and running. The ignition coil is an autotransformer separated into primary and secondary windings, all wrapped around an iron core. The majority of gasoline-powered four-stroke engines from the 20 th century rely upon an ignition coil to generate the spark responsible for igniting the combustion mixture in each cylinder. Photosįigure 2: Trigger box with cover removed.įigure 3: Trigger box installed inside engine bay. The trigger box's circuitry is housed in a simple aluminum enclosure from Hammond Manufacturing with holes and grommets added to support wires entering and exiting the box. A separate Weather Pack connector provides power to the box and connects to the dash-mounted unit by way of a custom-built wiring harness. Matching male and female factory connectors allow the trigger box to be installed directly in series with the truck's wiring harness without any modification. The self-contained trigger box mounts under the hood, and connects to the truck's factory ignition system by interrupting the connection to the ignition control module. The trigger box senses current through the primary winding of the truck's ignition coil, and delivers a logic-level signal to the dash-mounted unit that is then measured to determine RPM. Both the Digitach and Nixietach projects built for my 1979 Ford F100 pickup rely upon a separate device that I've called the "trigger box" to report revolutions per minute (RPM) of the engine.
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