On Saturday, Jan 17, 2004, at 13:48 US/Pacific, Anthony Toft wrote:

> With Mr Lathrop's comments in mind, I have reworked the schematic as
> best I know how, and the image and web page along with it.
>
Much better.  You comment that the resistor's base resistor is
sized to match the current requirements of the solenoid (presumably
taking into account the transistor beta.)  In general, that's a
mistake; when using a switching transistor, you want to drive it
well into saturation, and let the solenoid itself limit the current.)
You definatly DON'T want a transistor to be operating in it's linear
region, cause it's not really a "power" transistor.  So you want the
base current to be much higher than Ic/beta; I'd go ahead and set it
so that you're getting near the PIC's Maximum output current (20mA?),
which puts Rbase at about 200ohms.  With 20mA of base current, you only
need a gain of 60 or so to get into saturation, which is well within
the specs of the transistor you picked.  With the 1.1k resistor you
have, you need a gain of over 300, which starting to get into the
range where you have to be careful AND lucky to get the right
transistor.

A lot of designing transistor circuits seems to be correcting for
the fact that a transistor's gain is NOT one of their tightly
controller parameters...

BillW

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