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David R Brooks[_2_]

On 20/01/2017 10:27 AM, wrote:
On Thu, 19 Jan 2017 16:03:23 -0800, wrote:

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
You could - but you really want it to close fast. A resistor could
limit the current so you can use a 1n4004 instead of a higher current
device.

If you will be installing multiple injectors & drivers, you can also
use fast diodes returned to a common "supervoltage" rail, which has a
capacitor & Zener down to either ground, or the DC+ rail. The
supervoltage rail may float at +75V (say), being charged by the
"flyback" pulses from the injectors. This idea comes from an old Epson
application report, directed at dot-matrix printers (not so different,
if lower power.)
If you do this, you will quickly see just how much energy is wasted in
heating that Zener. There are ways to reclaim that energy, & return it
to the main DC rail, effectively using it to help fire the next injector
in sequence. (Sorry I can't discuss the details: they were the property
of my employer.)