DIY injector driver

[dn]

 Bohdan Bodnar writes:

> Shucks, a *real* do-it-yourselfer will *design* his/her own injector driver
> circuit!  I thought about this and said, "hmmm...  this IS interesting!"

This is exactly what I been cookin' up, but using an analog approach.
It is a simple matter to whip up a constant current source with an opamp
and mosfet transistor, with a current feedback resistor much as you suggested
in your digital approach.  In this manner you can set up a hold current at
whatever value you want.  Where I have run into a snag is how to force the
thing to just go into saturation for the initial time, say 1.5 mS, to slam
the injector open, without adding a bunch of timers and comparators.

I'd like to stick to analog circuitry for several reasons, including
reliability, no EMI, fewer parts, etc.  I figure a dual opamp and a few R's 
and C's should do it, say $3-$5 worth of parts and a square inch of PCB area.
Beats hell out of 10 chips and a uP any day.  Only penalty is a bit more 
power dissipation in the hold mode, due to operation in the linear region
of the driver transistor, nothing a little heatsinking won't cure.

Remember the KISS principle...

regards
dn

--
---------------------------------------------------------------------
 Darrell A. Norquay              Internet: dn at dlogtech.cuc.ab.ca     
 Datalog Technology Inc.         Bang: calgary!debug!dlogtech!darrell
 Calgary, Alberta, Canada        Voice: +1 (403) 243-2220            
                                 Fax:   +1 (403) 243-2872            
    @ +                                                                
     <                                                         
    __/    "Absolutum Obsoletum" - If it works, it's obsolete    
 --------------------------------------------------------------------