Coil primary voltage (using the 10:1 attenuator)

The purpose of this test is to examine primary ignition voltage on an external coil type ignition system.

 

WARNING

This test involves measuring a potentially hazardous voltage.

Please ensure you follow manufacturers' safety instructions and working practices and ensure the rated voltage for all accessories you are using meets or exceeds the expected voltage.

To avoid possible damage to your scope, you may need to use an attenuator for this test.

Scopes with a 200 V range, such as PicoScope 4x25 models, do not need an attenuator for this test.

All other PicoScope Automotive models need an attenuator on the channel input. You can use either a 10:1 or a 20:1 attenuator provided that you adjust the PicoScope software accordingly. Select from the appropriate Channel Options menu:

  • Probe > x10 for a 10:1 attenuator
  • Probe > x20 for a 20:1 attenuator

 

How to perform the test

View connection guidance notes.

  1. Connect a test lead with a 10:1 attenuator to PicoScope Channel A.
  2. Connect the test lead to the coil switched earth circuit (often marked CB, T1 or -) and to earth.
  3. Start the engine and allow it to idle.
  4. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
  5. Start the scope to see live data.
  6. With your waveform on screen stop the scope.
  7. Stop the engine and turn the ignition off.
  8. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Example waveform

Waveform notes

This known good waveform has the following characteristics:

  • Without a path to ground, when the circuit is off, the switched earth terminal voltage is the same as the coil supply voltage, at or very close to battery voltage.
  • When the circuit is switched on by the supply of an earth path, the voltage decreases to 0 V.
  • The primary circuit remains on, at 0 V, for around 4 ms.
  • When the circuit is switched off, the primary voltage spikes, very briefly, around 400 V.
  • After it is switched off, the primary circuit mirrors the features within the secondary circuit.
  • There is a raised voltage, around 25 to 40 V, for about 2 ms after the voltage spike.
  • At the end of the raised voltage, there are 3 to 5 cycles of diminishing oscillation before it stabilises back at the coil supply voltage, at or very close to battery voltage.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select, Distributor ignition primary voltage

Further guidance

General principles

All (inductive) spark ignition systems use one or more ignition coils. The coils act as both an accumulator, to store energy, and a step-up transformer, to generate the high voltages necessary to produce an electrical spark within a combustion chamber.

An ignition coil consists of a primary coil and a secondary coil, wound around each other in close proximity. The secondary coil has a high ratio of windings to the primary coil. This arrangement creates conditions of high mutual inductance, meaning changes in the magnetic field in the primary coil will produce changes in voltage in the secondary coil.

The primary coil is connected within the primary circuit. When current flows in the primary circuit, energy builds within the coil’s magnetic field. If the current is quickly removed, the magnetic field rapidly collapses and induces a high voltage in the secondary coil. The high voltage is delivered to a spark plug via a secondary circuit.

The time the coil takes to reach its maximum magnetic field strength (its saturation time) depends on the peak primary circuit current, which, in turn, depends on the total primary circuit resistance and the primary coil's tendency to resist the build-up of current (its inductance).

The period during which the current flows within the primary circuit is known as the dwell period (or the dwell angle, if referenced to the angle of crankshaft rotation). The dwell period must be sufficiently long (at all engine speeds) to allow the primary coil to reach maximum magnetic field strength (i.e. to saturate).

The peak current and dwell period are Key Performance Indicators (KPIs) for primary circuit control. Please refer to manufacturer’s technical information to find the specifications for your vehicle.

Distributor ignition

Distributor based ignition systems use a single ignition coil.

The switching of the primary circuit can be controlled using one of two mechanisms:

  • mechanically, by a contact breaker driven by a rotating cam within the distributor.
  • using transistorised current switching, triggered by a timing reference signal.

Most mechanically triggered primary circuits require a ballast resistor to regulate the current flow, whereas a transistorised system is able to vary the current more freely.

A component rotating internally within the distributor, the rotor, directs the secondary voltages to each of the engine’s spark plugs, in their firing order, as it passes peripheral electrodes connected to the spark plug leads.

Primary voltage diagnostics

  • When the primary circuit is off, the switched earth voltage should be at, or very close to, battery voltage.
  • When the primary circuit is on, the switched earth voltage should be very close to 0 V. If not, there is likely to be a high resistance in the earth circuit (e.g. from a bad connection or a wiring issue).
  • The circuit on time represents the dwell period for the coil and indicates the time available for the circuit current to charge and energise the coil.
  • The voltage spike occurring when the circuit is switched off is known as the ping line and represents the stored energy that has been released from the primary coil. It should have a consistent voltage amplitude for every coil charging event and no less than around 250 V.
  • Waveform events occurring after the ping line mirror secondary ignition events (please see secondary ignition waveform descriptions for additional descriptions), such as:
    • The raised voltage after the ping line represents the spark line.
    • The coil oscillations at the end of the spark line represent the dissipation of energy within the coil secondary windings. Fewer than 3 x coil oscillations indicates insulation breakdown.

GT386

Disclaimer

Suitable accessories

  • 10:1 Attenuator

    £64.00

  • Multimeter Probes

    £6.00

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Guided test: Primary Voltage