> Do we have any 555 experts on here?
> I am feeding the control voltage into pin 5
> and the duty cycle seems to be limited to 0% to 50%.
> Over 50% and the pulse width jumps to max.
> --
> Paul Lamar ...No rotor no motor.
Paul
Check your input signal carefully to ensure it is not going
more negative
than gnd. This can forward bias the substrate junctions.
Also input pulse
width at the upper threshold level must be shorter than the
output pulse.
regards, Robert
Thanks Robert. I did that. Did not help. I have some TI
TLC555IP CMOS 555 on order
from Digikey. In my experience CMOS are less prone to
glitches.
--------------------------------------------------------------------
Bingo!!! The 79 cent TI TLC555IP CMOS 555 is the hot tip.
A bonus for a CMOS chip is it's high output drive
current of
15 ma source and 150 ma sink. It is not your
grandfather's 555 or even
your dad's for that matter. That should be enough to
drive the injector FET
directly.
The control voltage is perfectly linear from 1 volt to 9
volts 1 ms to 9 ms.
which is exactly the range we need. Perfect!!!!!
Next step is getting it to work with Bob's op amp
and we will have a two buck 555 fuel injection system :)
Just about any 0 to 5 volt mass airflow sensor will work.
To be continued after OSH.
-----------------------------------------------------------
I was near Fry's after supper so I picked up a NTE 67
FET.
Worked like a charm. The stock Mazda yellow injector clicks
between 1 msec
and 9.5 msec which is about 95% duty cycle at 6000 RPM.
Of course I am not feeding it with fuel yet but that will
be the last bench test. That stock Mazda injector is
actually made
by Denso. Its pretty good. I don't know the flow yet of
course.
We will pee into a calibrated measuring cup to find out.
That NTE 67 cost about seven bucks at Fry's but I am
sure Digikey
or Mouser has them for less than half.
Not counting the injector total cost so far is under about
six bucks per injector. We still need the op amp and mass
airflow
sensor but only one for all four injectors. The op amp will
have to wait until after OSH.
Hard to beat this system for low cost.
An Ellison or Weber is about $1000.
Paul Lamar ...No rotor no motor.
Paul
You must include a network to absorb the inductive flyback energy
stored in the injector. I think a spike of 60-70V is visible on
the lower trace. The power device will be subject to high voltage
spikes and is certain to fail. A freewheel diode will protect against
this, but it is going to increase the time to close the injector.
An RC diode snubber network is a better approach.
regards, Robert
Those are built in to most FET's these days.
Not shown in my schematic.
I am so happy the way this is working. It is working better than perfect.
I'll need to do a bit of signal conditioning on the ragged pulse
from the crank angle sensor.
Paul Lamar ...No rotor no motor.
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