DIY_EFI Digest V5 #883

[Carter Shore]

On Thu, 15 Nov 2001 08:26:18 Dave Bannister wrote:

> I'm trying to modify my EEC computer on my mustang 
> to drive low impedance injectors..
> ...
> Having trouble selecting a transistor.  I'm not sure
> what type to select.

Low impedance injectors are typically driven by a peak
and hold strategy: A slug of current to get the
injector open very quickly, followed by a lower
maintaining current to keep it open for the desired
length of time. The 'slug' pulse is short, and usually
of constant duration.

Cherry Semiconductor used to make a chip for this
specific purpose, but AFAIK, it's out of production
now. You can maybe scrounge around and find some.

But it's not too hard to cobble up a circuit to do the
same thing:
Two drivers in parallel to the injector, each
delivering a different current. One at the 
'maintaining' current, the other at a current such
that the sum of the two equals the desired 'slug'
current. The 'slug' current pulse width can be created
with a simple RC - gate between the input and the slug
driver. The 'maintaining' driver is connected directly
to the input. Both drivers get triggered on by the
initial edge of the control pulse from the ECM. The
'slug' driver turns off after it's time elapses,
leaving the 'maintaining' driver on until the control
signal turns off.

So, maybe an extra IC, a few RC components, some catch
diodes, off you go.

Carter Shore

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