Subaru TBI

[Mike (Perth, Western Australia)]

Of course v=Ldi/dt well I'll just put me right knee under left armpit
and turn red with embarrasment - forgot about the damn formula drilled
into me back during engineer :(

Tah,

Yes got it - assuming most are ideal, I'll still get me CRO out and
check them current peaks on me FET :)

Rgds

Mike


At 12:25 AM 8/03/2000 -0800, you wrote:
>>Subject: Re: Subaru TBI
>>
>>At 06:12 AM 7/03/2000 -0800, you wrote:
>>>>No resistor required. Injectors are normally driven by sinking
>>>>current through them; no "ballast" required. The duty cycle of
>>>>pulsing determines the flow.  No fancy timing requirements.  It's
>>>>just a cheap way of implementing an electronically-controlled
>>>>carburettor. ;-)
>>>
>>>Yikes!  or How to screw up an injector in one easy pulse.
>>
>>Yeah good point, when I first read the post I assumed he meant no fancy
>>gear off the end of the injector driver IC...
>>
>>>This is a PeakHold type injector.  13.8V / 1.6 Ohms is almost 9 Amps.
>>
>>Aren't all injectors in use in last 20 years or so equivalent,
>
>Nope!  Both types are available.  Drivers for saturated are less expensive.
>
>>
>>ie. Pullin current is far more then hold current.
>
>This is true but not imporatant for saturated.
>>
>>>Injectors normally
>>>don't take more than about 1A.  You have a choice on how to drive a PH
>>injector.  You can
>>>place a Ballast resistor in series to limit the current to about 1A.  This
>>has the effect
>>>of driving the injector hard until the current through the resistor is
>>high enough to
>>>reduce the voltage across the injector and therefore limit the current.
>>
>>Sorry can't see that - surely the ballast resistor reduces 'sharpness' of
>>the initial pull in by limiting it to 1A hence also slower to pull in ?
>
>
>I know.  It isn't intuitively obvious.  Look at inductor theory.  At time
zero when a step
>waveform is applied to a coil the voltage across the coil is equal to the
applied voltage
>and the current is 0 amps.  At infinity, the voltage across the ideal coil
is 0V and the
>current is infinity assuming the coil has no DC resistance component.
>
>So over time,  the current increases until the voltage dropped across the
coil is limited
>by the internal resistance.  Now here is the key.  The size of the voltage
applied across
>this coil determines how fast the current reaches maximum.  The faster the
current reaches
>the maximum value, the faster the magnetic field increases and the sooner
the injector
>opens.
>
>So the idea is to create a winding with a low inductance, low resistance
and oodles of
>magnetism (oodles is a technical term).  In reality what happens is that
the coil has a
>low resistance and the current builds up very quickly opening the injector
quickly but
>this happens at about 4V for a 4 Ohm coil.  (1 Amp).  However,  there was
12 volts across
>this coil and so the resistor is chosen to limit the current to 1A by
dropping the 8V
>across a power resistor.
>
>Now remember,  the resistor drops 0 volts at 0 amps,  and 8 volts at 1A so
you can see as
>the current builds up in the inductor the voltage across the inductor also
drops because
>the resistor starts resisting.  Now this turn on time will not be as fast
as an active
>circuit that Peaks the current at 4Amps for n microseconds and then Holds
it down to a
>steady 1 amp for the rest of the injection time and the name of the game
for accurate
>injection is to minimize the non-linear portion of the injector fuel flow
during turn-on
>and turn-off.
>
>Got it?
>>
>
>
>Cheers,
>
>John Dammeyer
>
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Rgds


Mike Massen

Ancient Sufi saying:
        "Should your God save you from adversity, choose another God"

Pictures of site installation at Mendulong near Sipitang, Sabah (Malaysia) for
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Vehicle modifications on GMH Turbo, twin tyres, possible 175Kw at wheels
Preliminary pictures at http://www.wantree.com.au/~erazmus/Twin_tyre_vehicle/

My editorial on twin-tyre opinion and good reference about tyres:-
http://www.geocities.com/MotorCity/2195/ttyreopinion.html

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