> in my motorbike I have a reluctor pickup which measures 100 ohm.
> As I'm making my own CDI, I want to get the cleanest possible signal from it.
> Without any load, the stator interferes (induction) a lot.
>
> What is the best value for the load resistor for the reluctor pickup? I have
> thought it should be 100 ohm too, not because it's a transmission line :D but
> because that should give me the maximum power out of it, correct?
>
> However, the original CDI *seems* to have a 3K ohm resistor to load the pickup.

1.  2k7 to 3k3 :-)

2. Maximum power transfer is of interest only when maximising
transferred power is the key aim.
In this case it is almost certainly not what you are trying to
achieve. Probably maximum clean signal voltage is more important.

Quite apart from the above, maximum power transfer occurs when source
and load impedances match in a system where the source is linear and
not affected by loading. There is no guarantee that this system meets
those criteria.

Also, source impedance and source static resistance are not the same
and in your case may be substantially different, with the reactive
component of the inductive pickup 'potentially' (pun noted in passing)
being substantially or very substantially higher than its resistance.

What you want to use is what works best :-).
Time constant of an R + L circuit is L/R. Too low an R and you slug
the response to desired signal. Very low R and you get no filtering of
HF noise. If L/R is somewhere between 2 to 10 times smaller than the
shortest period of interest then chances are it will be about optimum.

Probably 3k is about right :-)

What you can then do if necessary is to add a post-filter to further
reduce noise. A 2 pole almost-anything or a 4 pole Bessel low -pass
filter will work wonders with noise. This can take as little as an
emitter follower per 2 poles. (You can get 3 poles with 1 stage and 5
poles with 2 stages for extra points but seldom needed).

Chances are that a single pole RC filter with an appropriate load seen
by the reluctor will do well enough. Looking at what the original
makers do along with the 3k load resistor is probably a fair guide of
what is needed.

Note that a CDI can easily provide lots of excitement (pun noted) for
the input circuit if care is not taken.



        Russell
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