Injectors: P&H and SAT - Part 3

[Garfield Willis]

OK, well this is in the form of a pre-emptive strike: I know from the
recent past :) that this will be one of the next questions to come up...

?1A		Can I drive P&H injectors with SAT drivers
?1B:	Can I drive SAT injectors with P&H drivers.

1A: Drive P&H injectors with SAT drivers?

Not usually (forget the dropping resistors, pulleeze). What you'd have
in this scenario is a driver that's turning full on in anticipation that
the impedance of the injector (a SAT injector) is going to limit the
current thru the driver to around 1A. But a P&H injector has taken it's
place, and it's HUNGRY. Remember the "final current values" we saw with
even one P&H TPI injector, IF there wasn't any current limiting? 5A! You
put a P&H injector on a SAT driver, and it's going to warm up
considerably. Even may give up the magic smoke.

Ploy 1A': Put two P&H TPI injectors in series in an attempt to have cake
etc., and you probly won't get enough pull-in current built up fast
enough to operate reliably at high pulse rates. You'd have 2.4ohms +
2.4ohms = 4.8ohms in series, which would give you a final saturation
value of 12V/4.8ohms or 2.5A. Enough perhaps to pull the P&H injector in
eventually (read lower rpm), but you'd be HOLDING 2.5A thru them (the
P&H injectors), and they're not made for that much continuous "hold"
current. So your injector windings would be stressed along with your SAT
driver. Either way, a lose-lose proposition.

1B: Drive SAT injectors with P&H drivers?

Yes & No, then maybe: P&H drivers will act just like SAT drivers if you
load them so lightly that their peak current limit isn't reached. Until
that happens they can't "fold-back" the current to the hold current
limit. So what can happen, and usually the data sheets for P&H drivers
warn you about this, is that somebody gets the wise idea to put a
*bunch* (say 4) of SAT injectors in parallel and use a P&H driver. They
know these injectors can't take a lower hold currrent, so they just plan
on having few enough SAT injectors that the P&H driver never reaches the
peak current limit, and then fold's back. OK, so what have you got now?
A driver that's continuously running at slightly below it's peak current
limit, which is NEVER sposed to happen. Hence the Driver now gets
toasty.

If you look closely at the the LM1949 P&H predriver chip, you'll see
they've incorporated a safety measure to protect the driver if something
similar to this should happen. It's possible for the +BATT voltage to be
low enough during deep winter cold cranking, that the drivers can't get
the injectors up to peak current, and so they sit there at nearly peak
current and never go to hold mode. BAD for the final driver, bad for the
injector too; ya know what they recommend? They've built a timer device
into the predriver to allow you to limit the time during which the
driver is in it's "peaking" mode, and if the driver isn't able to raise
the injector up to the peak current limit and then foldback by this
time, it switches the driver to the hold current level anyway!

Wull, OK National, but while that may save the injectors and/or drivers,
it also might prevent the engine from starting! Crumbs, ain't worst case
the worst!?

Bottom line is, use the kind of driver and injector together, that were
designed to go together. And, if you're designing P&H driver
electronics, be sure to design in enough peak current margin that even
in worst-case cold-cranking conditions, you can still get the injectors
to Peak. Make sure that whatever low BATT voltage, at which you hobble
the injector drivers to protect themselves and the injectors, is LOW
enough that no sane person would expect either the car to start nor the
starter motor to turn over more than a few agonizing cranks.

The "Yes, maybe" part to the above question is if you're driving just
one or two SAT injectors with a P&H driver. Probly they're robust enough
to run at 1-2A continuous hold, 1A perhaps in the case of the TPI P&H
driver, who's known to be able to handle 0.5A hold easily, and 2A
perhaps in the case of a TBI-size P&H driver, which of course can handle
a hold current of more than 1A. Still, it's not a good idea to try this,
because you don't know how much margin you're operating with.

Bottom-most line, don't do it (1A or 1B). Neither is a very good plan.

Gar

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