Injectors: P&H and SAT - DIY Flowbench

[Andrew Hunter]

Great information Gar...thx.  Well things brings me to a point regarding
the project fuel injector flowbench.  I finished my bench some time ago and
it works fantastic.  I incorporated a flowmeter and strobe light into the
bench since I have been doing alot of injectors for customers.
I would like to wire it to run 4 injectors instead of 1, but I really don't
know if the circuitry can handle it?  I don't want to add a few more
connectors and fry the rheostats or injectors or worse.  Anyone with
experience in this flowbench have any suggestions/input?  I would greatly
appreciate it.

Andrew

> ------------------------------
> 
> Date: Tue, 23 May 2000 17:31:49 -0700
> From: garwillis at msn.com (Garfield Willis)
> Subject: Injectors: P&H and SAT - Part 3
> 
> 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

----------------------------------------------------------------------------
To unsubscribe from diy_efi, send "unsubscribe diy_efi" (without the quotes)
in the body of a message (not the subject) to majordomo at lists.diy-efi.org