On Saturday, December 21, 2002, at 03:11  AM, Roman Black wrote:
> Dave Dribin wrote:
>> You mentioned using a cap to trigger the solenoid, using a buck
>> regulator to charge it.  What exactly is a buck regulator?
>>  Is there a particular setup you recommend?
>
> Yes! :o) I drew this in a real hurry so please
> forgive errors or sloppy parts values.

Wow, awesome.  I'm surprised (in myself) that I think I actually
understand the schematic.  My background is software w/ digital logic,
so all this analog stuff makes my brain hurt.  I've been pulling out
college textbooks and re-reading some basic principals, like how
transistors work. :)  That's the main reason I've been leaning towards
simpler.... I actually understand it.

Some questions about the circuit:

1) Why is Q1 PNP?  If it were NPN, then the PIC pin would be low for
off times.  Since this transistor is off unless charging, it will be
off most of the time.  It would seem that keeping the pin low would be
more power efficient.  Especially, when I put the PIC in sleep mode,
the pin can be set low, rather than high while sleeping.

2) Why is the sense pin necessary?  Once the time it takes to charge
the cap is found, it should not change, no?  Why not just charge it for
that amount of time (+ some error)?  This would also remove those two
resistors that drain the CS cap of some charge, too.

3) Why is Q2 not a Darlington transistor?  It still must handle the
full 300mA of the solenoid as if the solenoid was connected straight to
a +6V PSU.  Does the pulsing nature make the Darlington unnecessary?

4) What does the 1N5819 diode do?  It seems to be blocking a path that
is never taken.

And some questions in general:

1) How is this a buck regulator?  The output voltage (+6V) is the same
as the input, no?  In my newbie understanding of buck regulators, they
generally drop the voltage.

2) Why use an inductor at all?  Why not just have the transistor feed
right into the cap?  Just turn on Q1 until the CS cap is charged, then
dump it.  I assume the inductor is what makes this a buck regulator, so
there must be a good reason for it. :)

BTW, thanks to everyone who has helped out on the PIC list!  If you're
ever in Chicago, drop me a line and I'll buy ya a beer.

-Dave

> +6v (battery)
>  ---------------------*-------------------------------
>                       |
>                BC327  |        L1=470uH
>  ----,                E        ( <1ohm )
>     C|----R---------B    Q1
>      |   2k2          C
>      |                |
>     S|----R-----------|-----------------,
>      |   270 ohm      |                 |
>      |                |            A    |
>      |                *----L1------*----|---*-----,
> PIC  |                |            |    |   |     |
>      |     ,----------|------------*    |   -    SOL COIL
>      |     |          |            |    |   ^     |
>      |     R          |            |    |   |     |
>      |     |          |            CS   |   '-----*
>     V|-----*          -       470uF|    |         |
>      |     |          ^            |    |         C
>  ----'     R   1N5819 |            |    '-------B    Q2
>            |    scho  |            |              E  BC337
>            |          |            |              |
>  ----------*----------*------------*--------------*---
>  GND

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