AJH wrote:
On 09/04/2020 18:43, Paul wrote:
CDI Capacitor Discharge Ignition Circuit Demo
https://www.youtube.com/watch?v=0yK3Opq_i0M
Primary side V
| ___
| ___| |
| ___| |
| ___| | ___
| ___| \ ___|
| ___| | ___|
| __| \_|
+-------------------------------------------- Time
excitor charges C, ^
SCR dumps to coil |
spark on
Thanks Paul, this system is a bit later than that I think, I would have
to pull the flywheel to see, the magnets are on the inside of the
flywheel and it rotates round the excitor coil, Difference is the
exciter and sensor are all in one. I think the trailing edge current
from the discharge is what should cause the spark but because the
magnets are weak the current generated in the primary isn't high
enough to cause the voltage in the secondary to creat the spark.
Hence I was looking for something to switch the current in the coil
primary from a 12V battery off when it senses that trailing edge.
I was hoping someone would know of a simple circuit where the present
live feed from the CD was taken to ground by a suitable resistor and the
voltage across the resistor would cause a current to flow therough the
base of a transistor which would switch the current through the coil
off and reset.
To make a bipolar transistor work, you need to know
whether the pulse is negative going or positive going.
To use the pulse directly, would likely be ugly, and
involve a lot of "luck" regarding the incoming pulse shape.
The pulse shape probably does not make an ideal
"gate" for the job. The gating period might need
to be longer than the SCR firing period. The transistor
has to be held in the state we want, until the SCR is
finished.
For example, a monostable might produce a pulse of a
known length. And then the shape of the pulse
signal is rendered slightly less important. And
when using the monostable, we can set up the
circuit so that the monostable delivers the
right amplitude of signal to the transistor,
to make it saturate when on, and go open when off.
You can see examples here of the idea. The 555 isn't exactly
the friendliest chip, but the article shows how it
takes a short input pulse and makes it longer. There
are some other circuits that might have a bit fewer
pathologies. (I think I still have some 4538 chips here.)
You can see in one diagram, how they show the 555 used
with an NPN transistor, and in the other case, a PNP
transistor. The signal to the transistor base has
to be slightly different for those.
https://www.electronics-tutorials.ws...555_timer.html
It's possible the 555 runs between 5V and 15V. I'd have
to check the datasheet. This constrains how the
circuit could be kept simple. If the voltage the
excitor is working with, is well outside this range,
we might need to come up with another idea.
That's why I mentioned having an oscilloscope picture
of the thing when its running, to get some idea whether
the project is relatively easy, or absurdly hard.
If they kept the primary voltage low, they might be
able to use cheaper components to switch the primary.
But perhaps the turns ratio on the coil would have
to be extended, and it would take many many additional
feet of wire while making the transformer. If the
primary drive was say, 100V, then the turns ratio can be
a bit lower. If the primary was 100V to 1000V, maybe
I could use an IGBT to switch the exciter replacement source.
You probably have a much better feel for this circuit
than I do. I don't really know what to expect, and
I also don't expect the components in the thing
are well marked, so we can't get any hints from that.
If you found a 1000V SCR, we might assume one thing.
And if we found a 40V BJT, we might assume another.
If the capacitor only had a 25V WVDC, then the
555 circuit is looking more feasible. Maybe our
555 won't be destroyed by kickback from the
transformer :-)
When I made a 15kV supply once, using a couple 2N3055,
a flyback and a voltage doubler, the kickback from
the action on the primary, blew out the feedback
loop in a Hewlett Packard 10 amp bench supply.
They had used insulated gate MOSFETs (diff pair), with
only about 40V gate rating, and I guess the switching
transients got in there. The instrumentation guy
invited me into his little repair room, to show
me what I blew up, and how he was installing a new
one (the transistor he was installing is easily
damaged by ESD, and it has a shorting strap that
stays around the four legs of it, until it's soldered
into a circuit). I ran my high voltage supply off
two 6V lantern batteries from then on.
The best part of my 15kV supply ? Never getting
a shock from it.
Paul