Re: [EE] amplifier for variable reluctance sensor

At 10.06 2012.02.09, you wrote:
>> I need to design a device that interfaces to the speed sensor of heavy
>> trucks. These are variable reluctance sensors and will remain connected
>> to the vehicle's ECU, my circuit will piggyback. These are two-wire,
>> differential signals full of common-mode noise and vary in amplitude
>> greatly with frequency and also position as the tailshaft bearing wears
>> in the vehicle.
>
>I had a similar but totally different [tm] application whose solution
>may be your solution.
>
>I had a variable speed alternator (exercise machine load) whose out
>put was a low voltage sinusoid at low speeds but which soon chose to
>flat topped trapezoids clamped to the mean load voltage. As this
>voltage varied depending on speed and net absolute load desired it
>would have been modestly challenging to speed sense from. Now add 10
>kHz PWM that takes a fixed resistor and PWM's it across the rectified
>alternator output to change the effective resistance seen by the
>alternator. Yee ha.
>
>After trying all sorts of things I was offended by the simplicity of
>what proved to be a superb solution.
>Stops to find a circuit from about 10 years  ago ...
>.... Found ...
>Hmmm. May 18th 2006. More recent than I would have thought.
>
>2 x BC337 or whatever
>Differential long tailed pair.
>100k collector resistor per transistor.
>Join emitters with 1k to ground,
>Input drive to each base via a 100k (one per base  ).

Like this, but with the base resistors?

http://upload.wikimedia.org/wikipedia/commons/1/15/Difference_amplifier.png

>Two drive inputs connect across an alternator winding.

The 2 wires from the sensor go to the bases (through 100k resistors), but
there must be a third leg to ground.. to reference the two signals?

>Diagram shows base to ground resistors per transistor but notes that
>they are O/C.

O/C = ?

>That's it !!!
>Probably Vcc = 5V. (Murphy says the value is on the sheet I didn't
>load BUT probably 5V).

-V (in the pic I linked above) is ground, or needs negative supply?

Cheers,
Mario

>Alternator is 4 phase custom built. Max output > 200 VAC.
>
>Speed output taken from collector of your choice.
>
>This is "offensive" in that (at least) it does not explicitly deal
>with the 10 kHz PWM or the sometimes very low voltage and max voltages
>of say 300 V peak  are applied to transistor bases directly with no
>attempt to limit voltage or current (apart from the obvious ways in
>which the transistors do this. Ibase max= 3 mA worst case but usually
>much less than that.
>
>The "magic" lies in the differential pair.
>It performs as a superb comparator.
>
>Noise immunity  MAY be aided by the driving of base junctions deeply
>into saturation. Ib > to >> Ic in most cases.
>
>As I recall, along the way I used 3 or 4 pole Bessel low pass filters
>using 1 or 2  emitter followers to  provide a low cost filter. This
>was extremely effective at removing PWM but not needed for the long
>tailed pair solution.
>
>
>
>    Russell McMahon
>-- 
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