At 10:14 PM 10/9/97 -0700, you wrote:
>The instructions for the Crane electronic ignition suggest that what's
>important is the not the size of the loop but how many times it is looped
>around. However, I have not seen more than one loop...
>
>Ulix
I wasn't going to touch this but....
The wire loop in the tach pick up is actually the primary winding of a
current transformer. As the current in the wire is discharged with each
ignition cycle the magnetic field around the wire collapses and, in so
doing, induces a small pulse of current in the secondary winding: the ring
thing on the back of the tach. For every pass the wire makes through the
loop on the tach the field generated will be that much stronger. Two passes
= twice the current etc.Therefore the number of passes (or loops of wire) is
critical. The actual size of the loop is irrelevant since at some point the
wire must pass through the core and its at this point that the collapsing
field is sensed by the pickup. The magnitude of the field will remain
unchanged but the frequency is directly proportional to engine speed. The
tach converts this frequency into mechanical motion to move the tach needle.
(Lecture mode OFF)
Ross MacPherson
1947 MG-TC 3528
1966 MGB-GT
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