Off topic,, to Gar....

[Scott Schaaf]

garfield at pilgrimhouse.com wrote:
> 
> On Sat, 27 Jun 1998 07:26:16 -0700, goflo at pacbell.net wrote:
> 
> >The data sheet is'nt forthcoming about the device internals, and the
> >statement above does'nt say it's ok, just that someone somewhere
> >is threatening to do it. What's you're take on this?
> 
> Nah, that's not a threat, they're saying you CAN parallel them. These
> gizmo's don't contain any magic, they just have a darlington or somesuch
> internal; it's just integration, not mystery. The only problem with
> paralleling bipolars is when you have a runaway or overcurrent condition
> (namely, parallelled driver bipolars, but no individual current limit on
> each driver); cuz they have a pos temp coefficient (ie, the conductance
> goes UP as they heat up), so if you overload a pair of them, the one
> that's overloaded (overheating) begins to take more and more of the
> current they were intending to SHARE, resulting in a galloping toast
> situation for BOTH of them (cuz of course once the first one fries and
> opens, the second one will immediately be overloaded also). This is WHY
> (really THE single biggie reason) power mosfets have swept the scene,
> cuz they are just the opposite, negative temp coefficient on
> conductance, so they automagically load share if you put a bunch of them
> in parallel. If one of them's outta balance on carrying it's part of the
> total current, it warms a tad, and it's current drops and others in the
> parallel pick up the slack.
> 
> The point the data sheet is making (cuz remember, they're intending to
> sell these as SOLENOID drivers in general), is that since there is
> current limiting and probably reasonably tight match on the "peak trip
> point", you can put them in parallel if you're drive requirements of
> YOUR solenoid don't match these norms of fuel injector current levels
> (namely the port injection peakNhold's vrs. the TBI peakNhold's).
> 
> For injectors tho, I don't see the point. The 453 has the right current
> levels for the TBI injectors, and the 452 is just fine for the normal
> P&H port injectors. And you really don't wanna use a 453 where it isn't
> needed, cuz the hold current will be higher than the injector solenoid
> is rated for, and you'll overheat it possibly during high rpm/duty cycle
> times.
> 
> The reason why SOLENOID drivers are kinda specially designed, is that,
> even more so than a coil you drive into saturation, and it's inductance
> decreases non-linearly as it's magnetic field saturates, solenoids act
> like this on *steroids*, which is one of the reasons they kick like such
> muthas for such small coil sizes. Cuz, when you activate a solenoid, the
> CORE of the magnetic part of the solenoid is pulled inside the solenoid
> winding's field and the inductance REALLY takes a nose dive fast. Cuz
> not only is the core beginning to saturate, but you've suddenly got a
> LOT more core material in the coil's field. So JUST the opposite happens
> when you open the winding in a solenoid; not only does the field
> collapse around the winding and cause back-emf, like any inductor, but
> as the current falls, core is released & pulled out by the solenoid's
> spring, the INDUCTANCE of the coil zooms way up! And since the voltage
> across an inductor is the product of the L (inductance) and the rate of
> change of I (current), BOTH of these factors are working to boost the
> back-emf. Here's another quote from the 453/453 data sheet: if you
> invert what they're remarking about, you'll see what I'm getting at;
> 
> "The inductance of the injectors are much larger at low current,
> decreasing due to armature movement and core saturation to the
> values above at rated current."
> 
> Hence, when you turn OFF the solenoid winding, the current drops, the
> armature moves out, and the core unsaturates, all leading to rising
> inductance in the coil, and higher transients than with a normal fixed
> inductor.
> 
> Gar


THANK You,  you have just explained WHY some of the pinball
manufacturers have started using Mosfet drivers on the coil drivers.
(Neg co. )  And to think,,, I cussed the heck out of 'em for using such
high priced drivers!!

					Thanks Again. (REALLY)
						Scott...