Injector/Solenoid Driver Factoids

[garfield at pilgrimhouse.com]

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