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.
produced by the pick-up will be determined by several factors, these being :
--- the voltage produced will rise when cranking
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
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.
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.
A annular inductive type distributor pick-up.
Distributor Pick-up - Hall Effect Diagnostics.
This form of
trigger device is a simple digital on / off switch which is recognised and
processed by the ignition control module.
trigger has a rotating metal disc with openings; this passes between the
electromagnet and the semiconductor. The effect of a magnetic field that is
able to pass through one of the 'windows' will stop the flow of voltage. When
the 'window' is closed the flow is reinstated.
semiconductor has the ability to be a conductor or an insulator depending on
whether the semiconductor sees or is shielded from the magnetic field. This
magnetic field is switched on and off by the rotating disc that travels
between the two objects.
The effect of
a magnetic field that is able to pass through one of the 'windows' will stop
the flow of voltage. When the 'window' is closed the flow is reinstated. This
action will produce a digital square wave that is understood by the ECM or
amplifier and will not need the extra Schmitt trigger circuitry to convert the
analogue signal into a digital signal. Schmitt triggers are used with
permanent magnetic pick-up amplifiers.
may also be referred to as an Analogue to Digital (A to D) convertor.
because of its convenient output, is used in many other applications including
road speed sensors and speedo drives.
signal is observed or plotted against a primary waveform it can be seen that
the coil fires when the voltage from the Hall effect trigger drops to zero
volts. This occurs when the 'window' on the metallic rotating vane opens.
diagram of a Hall effect pick-up, the terminal marked 0 is the Hall voltage.