Subaru TBI

[John Dammeyer]

>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

----------------------------------------------------------------------------
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