Re: discrete comparator

kell wrote...
>
> A while ago I posted here with a question about current sensing
> in a circuit to limit the output of an antique generator.  Here's
> an idea for a comparator with hysteresis to cut off field current
> when generator output exceeds 10 amps.  36R is an estimate, and
> the value of the 470K hysteresis resistor is just a guess. 
>
> I haven't built this yet.  Also, I wonder if it would need a cap
> from the collector to base of the PNP transistor.
>
>  DC generator output
>   |
>   V
>   -       
>   |       
>   +--0R1--+----O B+
>   |       '-->|-->|---,
>    \e                 |
> PNP |--+--36R--+-->|--+
>    /c  |       |  10K |
>   |    |       \/\/\/\/
>   |    |           ^
>   |   470k         |
>   |    |          gnd
>   |    |   field
>   |    |     O
>   |    |     |
>   |    |   |--
>   |    +--||  N channel mos
>   |    |   |--
>   |  c/      |
>   +--| NPN   |
>      e\      |
>        |     |
>        '--+--+- gnd
>
> A voltage divider is connected to the base of the NPN transistor
> so it serves both current limiting and voltage regulation.

 I compressed your circuit slightly.  Your drawing is hard to
 understand, what's B+, which is in 0R1's current path, and
 what's "field 0", which is driven by the powerful MOSFET drain?
 Are we to assume a magnetic-field coil goes between these two?

 Your circuit is rather tortured.  The hysteresis part may be OK,
 but 0R1 with two diodes and pot plus 36R are troublesome.  When
 you get the right circuit there won't be any guessing.  One thing
 to keep in mind is diode voltage drops don't equal transistor
 drops (although they may match diode-connected transistor drops).

 Also, you mentioned a voltage divider "connected to the base of
 the NPN transistor," but we don't see it in your drawing.  Plus,
 it's not clear how a voltage divider on the NPN could be helpful.
 A resistor across the NPN base-emitter would establish a current
 in the PNP that might be helpful in setting the PNP Vbe voltage
 at some threshold current.  Perhaps that's what you had in mind.


-- 
 Thanks,
    - Win
.

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