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On Thu, 19 Jan 2017 16:25:20 -0600, Tim Wescott
wrote:

On Thu, 19 Jan 2017 11:44:40 -0800, etpm wrote:

I figure Clare will know the answer but maybe others will too. I
got my
hands on an old Acura fuel injector last night. It is apparently
the
high impedence type because it measures 14 ohms resistance and
draws
only .7 amps @ 12 volts. This means I don't need a fancy chip,
just a
555 timer circuit. So I put one together using a relay (because
that's
what I have on hand) instead of a mosfet to apply power to the
injector.
I put a snubber diode across the injector just like I did for the
relay
coil. All the injector circuits I have seen online use either a
pretty
high voltage zener, 75 volts, or a resistor across the injector.
From
what I have read the 75 volt zener seems to be the most common
and it
lets the injector close slower to prevent damage to the injector
from
slamming closed. I guess the circuits with a resistor in parallel
with
the injector do the same thing. I don't have any 75 volt zeners in
my
kit but do have lots of resistors in many different values and
watt
ratings. Does using just a resistor make sense? Maybe I misread
the
circuit.
Thanks,
Eric

The higher the voltage at the injector, the faster the magnetic
field
will collapse and the faster it'll close.

75V means that they're using transistors rated for 100V or so, which
means spendy. It also means that the magnetic field is collapsing
way
faster than it was generated. So they're at least tuning the
turn-off
time.
So can I just use a regular diode like I would with a relay but with
a
resistor in series with the diode if I want to have the injector
close
slower?
Eric

A resistor in series with the snubber diode makes the injector coil's
magnetic field collapse -faster- by dissipating the stored energy that
drives that current. Whatever current was flowing through the coil
when you switched off the power continues to flow through the diode,
whose purpose is to prevent that current from generating a high
voltage surge, like an ignition coil or a water hammer in plumbing.
The resistor absorbs the energy from the current faster than the diode
drop alone. I've seen a relay stay closed for 15 seconds with only a
diode drop impeding the current.

-jsw