I notice that Tracy uses a separate ground for the injectors. He is
not fooling. If you connect the battery ground to the injector
ground it wont work. He should have mentioned that. It is an attempt
to keep the injector spikes out of the computer. IMHO a few chokes
here and there would also have worked without the uncertainties :)
Paul Lamar
Not necessarily -- putting the currents where you want them, where
they don't share common impedance paths, is a better approach.
"Chokes here and there" doesn't work very well unless you have a lot
of energy storage in the power supply so it never sees the spikes
anyway. Otherwise you will always be fighting the voltage spikes on
the computer power that follow the current spikes from the
injectors. Strongly recommend Henry Ott's book "Electromagnetic
Compatibility Engineering." It's mostly about meeting EMC
requirements but the issues are the same. His earlier version "Noise
Reduction Techniques in Electronic Systems" is just as good for this
problem.
David Josephson
True! Capacitors are needed. A 1000 Uf and a small choke on each
injector would work well I suspect. Worth a try. Of course the
computer is well grounded to the engine.
Even better is mount the switching transistor on the injector like a
smart coil.
Thanks for the book tip. I'll get it.
Paul Lamar
Maybe this discussion should be offline, but having wrestled this
particular demon for decades in the audio and instrumentation biz I
can assure you that "I suspect" approaches (even mine, or especially
mine) are often a waste of time. It is tedious engineering but it is
doable without too much exertion. Your number one question, for
every instant of time for a given injector pulse, is "where does the
current flow?" From battery + back to battery -, figure out the
resistance of each path and see how the current through an injector
gets there and back.
I am not sure that putting the switching transistor at the injector
would help. I would rather have the switching transistor where the
source impedance for power feeding it was lowest. Yes, the loop area
for radiated EMI would be reduced, but then you have another handful
of connections that are more subject to engine vibration. What is a
"small choke" and what function does it serve? Why 1000 uF and where
would you put it?
David Josephson
Smart coils have no problems with ground loops nor does tracy use a
separate ground for the smart coils. The switching transistor is in
the coil.
Paul Lamar
... I suspect the NSA is using hidden characters to identify certain
class's of email users. It would be great if everybody used plain
text. We get a wide variety of html messages. Especially from cell
phones, tablets and Apple products. There is no effective standards
in html. Every email editor program and cell phone manufacture wants
to be different.
David, Tracy uses separate grounds but the same 12 volt supply, He
has had a lot of trouble over the years with glitches from the
injectors despite the separate ground.
A built in transistor switch with a large capacitor and small choke
would suppress the spike on the 12 volt supply. The choke need not
carry the full 2 amps as the capacitor will handle the high peak
currents. The choke will block the out going spike. If I ever do an
EFI that is the approach I will take. The injectors used to be shock
and hold and now they are mostly just plain high current. It is when
they turn off that causes the spike. They are not designed for a
100% duty cycle.
Paul Lamar
If the problem is a voltage spike when the transistor turns off
rather than a current spike when it turns on (as I surmised from what
I read earlier) then of course it is a different problem. You need
to reduce the source impedance of the DC line that's shared between
the injectors and everything else. A big capacitor will help as you
suggest, as it will provide some of the energy for the inrush
current, and it might absorb some of the turn-off transient when the
transistor opens. BUT! In an airplane it's all about reliability and
I would be very cautious about putting chokes and electrolytic
capacitors in an engine compartment.
Be sure to actually look at those waveforms with a fast scope, they
could easily exceed the voltage rating of the capacitor, which then
provides another failure path. And, I would be wary of adding yet
another inductor in series, unless it was a known high reliability
part like some from Renco. A spool of fine wire terminated to
who-knows-what is not something you want in an engine compartment if
you can avoid it.
The whole point is to reduce the source impedance of the power buss,
so that the energy in the coil cannot cause a big rise in buss
voltage. With this approach, the current in the line at turn-off
spikes more, which creates a large instantaneous magnetic field, so
keeping that wire bundle away from the others, and preferably twisted
with its return current line, is important. You can reduce the this
with capacitors and chokes if you want to assume the risk. But you
can also do it with bigger wire, transistors with lower on
resistance, and a separate +12 line back to the battery for the
injector circuit. A fast zener across the injector to shunt the
voltage spike could also be useful, but be sure the instantaneous
peak current rating of the zener is not exceeded.
David Josephson
The injector driver I have for my system operates much the same way
as you are saying Tracys does with the separate ground. I tie the +
terminal of the injector to a power source, and then run the ground
side to the driver board. The ECU triggers an NPN transistor to fire
the injector by connecting the ground. This way none of the power and
spikes of opening and closing the injectors is actually run through
the ECU system. There is a series of flyback diodes and other
protections, but it sounds like Tracy has done it the same way that I
am.
My LED strobe system that I built for my wing tip lights is the same
way- 6W of LED flashes at 2 per second with no feedback.
Kevin Alderman
You have probably done all the right things anyway, but there are some
important assumptions that people who don't have electronics experience
need to review. "Ground" does not exist, it is just a concept. The
transistor turns on, the - terminal of the injector is then connected to
some conductive path that ultimately goes to the - terminal of the
battery so current can flow. What other circuits share that path? When
the injector turns on, there is a surge of current in that path. The
path has some resistance, so there is a voltage drop. That voltage drop
appears across every other circuit that shares the path. If you're smart
and run the emitters of the transistors on their own "ground" wire back
to the battery (or whatever you have determined is the lowest common
impedance point, like a lug on the engine block) then you can avoid this
problem.
David Josephson
I like that explanation David. Indicates why using the airplane
ground is good. You will play hell trying to get the whole
metal airframe to show a voltage drop :) On the other hand
fiberglass airplanes frequently have ground problems.
Paul Lamar