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Distributor Inductive Pick-up (Engine Running) Waveform Notes
This particular type of pick-up generates its own signal and therefore does not require a voltage supply to power it. Recognisable by its two electrical connections, the pick-up is used as a signal to trigger the ignition amplifier or Electronic Control Module (ECM). As the metal rotor spins, a magnetic field is altered which induces an Alternating Current (AC) voltage from the pick-up. This type of pick-up could be described as a small alternator because the output voltage rises as the metal rotor approaches the winding, sharply dropping through zero volts as the two components are aligned and producing a voltage in the opposite phase as the rotor passes. The waveform is known as a sinewave.
The voltage produced by the pick-up will be determined by several factors, these being:
- Engine speed - the voltage produced will rise from as low as 2 to 3 volts when cranking to over 50 volts, at higher engine speeds.
- The proximity of the metal rotor to the pick-up winding. An average air gap will be in the order of 8 to 14 thou, a larger air gap will reduce the strength of the magnetic field seen by the winding and the output voltage will be subsequently reduced.
- The strength of the magnetic field offered by the magnet. The strength of this magnetic field determines the effect it has as it ‘cuts’ through the windings and the output voltage will be reduced accordingly.
A difference between the positive and the negative voltages may also be apparent as the negative side of the sinewave is sometimes attenuate when connected to the amplifier circuit, but will produce perfect AC when disconnected and tested under cranking conditions.
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