On Wed, 1 May 2002, Alan B. Pearce wrote:

>>I'm not very fammilar with hall effect devices, but
>>I would imagine their output wouldn't incraes with
>>speed like a variable reluctance device would?
>
>That is what I figure as well, because they are sensitive to the intensity
>of the magnetic field, not the rate of change.

That's correct however in a linear sensor the slope mimics the shaft speed
and since it is connected to a coil (eventually) it changes its dU/dt and
hence the spark timing in conjunction with the capacitance in the circuit.
Think of it as a low Q RLC circuit with the spark across it. If the coil
voltage raises fast (high dU/dt) then the breakdown voltage is reached
early. Raise slowly and it comes late.  Similar ideas apply to magneto
'pointless' ignitions. Getting these to work right is a very time
consuming and messy lab job. Usually there is a hysteresis element in the
circuit that triggers the ignition proper. It can be a voltage controlled
latching device (thyristor + zener works f.ex.).

Peter

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