Ignition Coil Symbol
Definition: The Ignition Coil symbol represents an automotive inductive transformer that steps up a 12 V DC battery supply to a high-tension (HT) spark voltage of 15–45 kV required to ignite the air-fuel mixture in a gasoline engine's combustion chamber, shown in automotive wiring diagrams as a three-terminal device with + (12V primary supply), − (trigger/switching) input, and HT (high-tension spark output) terminals.
Also known as: spark coil, CDI coil, high-tension coil, HT coil, coil pack, ignition transformer, induction coil.
What the Ignition Coil symbol means
The Ignition Coil symbol denotes an autotransformer or two-winding inductive device that stores energy in a magnetic core when the primary circuit conducts, then releases that energy as a high-voltage pulse when the primary circuit is interrupted by the ignition trigger (points, reluctor, or transistorised ignition module). In an automotive wiring diagram, the symbol identifies the device that bridges the low-voltage electrical system (12 V battery) and the high-voltage ignition system (up to 45 kV at the spark plug).
The three terminals on the ignition coil symbol correspond directly to the physical connections: the + terminal (sometimes labelled B+ or 15) connects to the ignition-switched 12 V supply; the − terminal (sometimes labelled CB or 1) connects to the contact-breaker points or transistor switch that interrupts primary current; and the HT terminal (sometimes labelled 4 or 'King Lead') feeds the high-voltage cable to the distributor cap or directly to the spark plug (in coil-on-plug designs).
How to identify the Ignition Coil symbol
The ignition coil symbol is drawn as a transformer icon — two coupled inductors (coils) sharing a common iron core — with three terminal leads: + (12V primary positive) and − (trigger/switch) emerging from the primary winding side, and HT (high tension) from the secondary winding side. The primary winding is typically drawn as a few turns and the secondary as more turns (indicating the step-up ratio of approximately 100:1). Some schematic representations show only the coil symbol as a rectangle with three labelled terminals.
Function in a circuit
When the primary circuit is closed, current flows from the battery through the primary winding, storing energy in the magnetic core (E = ½LI²). When the ignition module or contact-breaker points open the primary circuit, the stored magnetic energy collapses rapidly, inducing a high voltage in the secondary winding proportional to the turns ratio (typically 100:1 to 200:1). This produces a 15–45 kV pulse at the HT terminal, which travels via the HT lead to the spark plug where it ionises the gap and creates the arc that ignites the combustion mixture.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 represents transformer and inductor symbols under section 06 (transformers and inductors). The ignition coil is represented as a coupled-inductor transformer symbol with two windings on a shared core, per IEC 60617-06-07 (transformer with two windings). Automotive wiring diagrams may also follow IEC 62973 series (batteries for automotive applications) for the broader circuit context. |
|---|---|
| ANSI/IEEE 315 | ANSI/IEEE 315-1975 represents the ignition coil as a transformer symbol (two coupled coils on a ferromagnetic core, designated T). In automotive schematics, SAE J1858 (automotive wiring diagram symbols) uses a simplified coil symbol with primary and HT terminals labelled per SAE conventions. |
| Key difference | IEC and ANSI/IEEE use the same transformer symbol concept for ignition coils. The main standard difference is in terminal labelling: DIN 72552 (German/European automotive standard) labels the ignition coil terminals as 1 (primary negative / points), 15 (primary positive / ignition switch), and 4 (HT output). SAE/North American diagrams label them − (CB/breaker), + (battery/ignition), and HT. |
Terminals / pins
| Pin | Name |
|---|---|
| pos | + (12V) |
| neg | - (Trigger) |
| ht | HT (Spark) |
Typical values
Primary voltage: 12 V DC (12 V automotive system) or 6 V DC (older vehicles). Primary winding resistance: 0.5–3 Ω. Secondary winding resistance: 5–15 kΩ. Turns ratio: approximately 100:1 to 200:1. Peak secondary voltage: 15–45 kV. Primary inductance: 3–15 mH. Dwell angle: 50–70° (contact-breaker systems); duty cycle controlled electronically in CDI/DIS systems.
Where the Ignition Coil symbol is used
- Conventional distributor ignition system: single coil energised by contact-breaker points or transistorised ignition module; HT output routes via HT lead to distributor cap for distribution to each cylinder in firing order
- Distributor-less ignition system (DIS / wasted-spark): coil pack with one coil per pair of cylinders; each coil fires two spark plugs simultaneously — one on compression stroke, one on exhaust stroke
- Coil-on-plug (COP) ignition: individual ignition coil for each cylinder mounted directly on the spark plug, eliminating HT leads; controlled by the engine ECU for precise individual-cylinder timing
- Capacitor discharge ignition (CDI): automotive and small-engine ignition system where a capacitor stores energy and discharges through the coil to produce a faster, higher-energy spark; common in motorcycles, outboard motors, and small engines
- Small engines (lawn mowers, chainsaws, generators): magneto-driven ignition coil that generates primary and secondary HT voltage from a permanent magnet flywheel, requiring no battery
- Classic car restoration wiring: original points-based ignition system documentation for vintage vehicles using 6 V or 12 V positive-ground electrical systems
Example
In a classic car wiring diagram, the ignition coil symbol appears between the ignition switch output (+ terminal, 12 V) and the contact-breaker points (− terminal). When the ignition switch is on, 12 V energises the primary through a ballast resistor. The breaker points interrupt primary current at each firing event, generating a 25 kV HT pulse at the HT terminal. An HT lead connects the HT terminal to the centre of the distributor cap, which routes the pulse to each spark plug in firing order.
Key facts
- The ignition coil symbol has three terminals: + (12V primary supply, pin labelled pos), − (trigger/switching input, pin labelled neg/trigger), and HT (high-tension spark output, pin labelled ht).
- DIN 72552 automotive terminal numbering labels the ignition coil connections as terminal 15 (primary +), terminal 1 (primary − / breaker points), and terminal 4 (HT output).
- The secondary-to-primary turns ratio of a typical ignition coil is approximately 100:1, stepping 12 V up to 15–45 kV at the HT terminal.
- Primary winding resistance ranges from 0.5 Ω to 3 Ω; measuring this resistance is a basic diagnostic test — an open or high-resistance primary winding indicates a failed coil.
- Coil-on-plug (COP) ignition coils are individually controlled by the ECU, enabling individual-cylinder ignition timing adjustment and misfire detection via crankshaft position sensor feedback.
- An ignition coil is classified as an autotransformer or two-winding transformer in IEC 60617 and ANSI/IEEE 315; the designator T is used for transformers in circuit schematics.
- CDI (capacitor discharge ignition) coils operate differently from inductive coils: a capacitor charged to 200–400 V discharges rapidly into the primary, producing a very fast-rising secondary HT pulse of 30–50 kV with short spark duration.
- Modern COP coils incorporate a built-in ignitor transistor and signal conditioning, requiring only a 5 V logic trigger from the ECU rather than primary-current switching.
Diagrams that use this symbol
- ignition diagram
- ignition switch diagram
- ignition switch wiring diagram
- spark plug diagram
- motorcycle wiring diagram
- ignition coil wiring diagram
- 5 pole ignition switch wiring diagram
- ignition system diagram
Frequently asked questions
What does the ignition coil symbol mean in a wiring diagram?
The ignition coil symbol represents the transformer that steps up the 12 V DC vehicle battery voltage to the 15–45 kV high-tension spark voltage needed to fire the spark plugs. In a wiring diagram, it shows the three electrical connections: the + primary supply, the − trigger/switch input, and the HT high-tension spark output.
What does the ignition coil symbol look like?
The ignition coil symbol is drawn as a transformer with two coupled coil windings on a shared iron core — a few turns for the primary winding and more turns for the secondary. Three terminals extend from the symbol: + (12V supply) and − (trigger) from the primary side, and HT (high tension) from the secondary side.
What are the three terminals of an ignition coil?
The three ignition coil terminals are: + (or terminal 15 per DIN 72552) — the primary positive supply from the ignition switch; − (or terminal 1) — the primary switching input connected to the contact-breaker points or ignition module transistor; and HT (or terminal 4) — the high-tension output delivering the 15–45 kV spark pulse to the distributor or spark plug.
How does an ignition coil generate high voltage?
An ignition coil stores energy in its magnetic core when primary current flows. When the primary circuit is interrupted (by contact-breaker points or an ignition module transistor), the collapsing magnetic field induces a high-voltage pulse in the secondary winding. The turns ratio (approximately 100:1) multiplies the primary voltage to produce 15–45 kV at the HT terminal.
What is the difference between a CDI coil and an inductive ignition coil?
An inductive ignition coil charges via primary current flow and fires when current is interrupted; it stores energy magnetically and produces a long-duration spark. A CDI (capacitor discharge ignition) coil discharges a pre-charged capacitor (200–400 V) through the primary winding, producing a faster-rising, higher-voltage (30–50 kV) but shorter-duration spark. CDI systems are common in motorcycles, outboard motors, and small engines.
What standard applies to ignition coil wiring diagrams?
DIN 72552 (German automotive standard, also widely adopted internationally) defines terminal numbers for automotive electrical components including the ignition coil terminals (1, 15, 4). ANSI/IEEE 315-1975 uses the transformer symbol (designator T) for inductive devices. SAE J1858 covers automotive wiring diagram symbols for North American vehicles.
What resistance should an ignition coil have?
Primary winding resistance is typically 0.5–3 Ω (measured between the + and − terminals). Secondary winding resistance is typically 5–15 kΩ (measured between the − terminal and the HT output). Values outside these ranges indicate a faulty coil. Always consult the vehicle-specific service manual for exact specifications.
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