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Primary Induced Voltage Waveform Notes
The vertical lines within this trace measured using the ‘X’ cursor, can be seen at 200 volts, in fact it is know that this voltage does measure 250 volts but only a 10x adaptor for the oscilloscope was available at the time of taking this waveform. This is called the induced voltage and is produced by a process called magnetic inductance.
The principal is based around a magnetic field (or flux) being produced when the coil’s negative terminal earth circuit is completed by the amplifier. When the circuit is complete, a magnetic field is produced and builds up until the coil’s magnetic field is maximised. At the point of ignition, the coils earth circuit is removed and the magnetic field or flux collapses across the coil’s windings, which in turn induces a voltage of 150 - 350 volts.
The height of the induced voltage (sometimes referred to as the primary peak volts) will be determined by the following points:-
- The number of turns in the primary circuit.
- The strength of the magnetic flux, which is proportionate to the current in the primary circuit.
- The rate of collapse, which is determined by the speed of the switching of the earth path.
A lower than anticipated induced voltage will result in a low coil output. The High Tension (HT) output will be proportionate to the induced voltage.
Use the settings shown in the example waveform to pre-set the oscilloscope to measure the voltages.
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