Re: model ignitions (was Re: DIY vacuum-pump? (for vacuum impregnating))

pentagrid@xxxxxxxxx wrote:

AC coupled high voltage probes are easy. If you shunt
the 1 Mohm scope input impedance with 10,000 pf and couple it to
the HV source with a few pf you have a coupling time constant of
10mS which should be low enough for your application.

That's exactly what I'm doing right now. Yesterday, while falling asleep, I
remembered having a book (1977) titled something like "Electronic
Motortesters". and I found a neat circuit including a suggestion for a
HV-probe. Easy to do. Some POM, some wire and a tube over it. Epsilon of
POM is known. So capacitance can be calculated and will be 4.7pF. Just
bought a 2.5% capacitor for the divider. I'll add a suppressor-diode for
security.


I assume that your high voltage winding is the first
winding on the cylindrical powder core so that the  primary is on
the outside and near ground potential.

No it ain't. I know the argument better cooling of the primary, but it also
would increase the length (bigger circumfence(SP?)). Next version will have
1.12mm wire (ca. 30% less resistance) and I get the layers full, so no
space lost. Wire already sitting here.

High voltage is more important than high current for
effective spark ignition. The RFI suppression series resistance
in spark plug leads is typically 10,000 ohms! This means that the
resistance of the high voltage winding is unimportant

OK, good hint/argument!

- for
maximum voltage in a given volume it should be wound with the
thinnest practicable wire - 0.05 or 0.1mm.

So I'll change that too. Will give me room for more winding. But I'd like to
reduce the size too in a next step. But then, I only can base a completely
new design on results I get with the current one.


Because you are layer winding the HV secondary the
interturn voltage is only a few volts

No, it's getting close to the 0.5..1kV where start of one layer meets end of
second layer. But the double insulated (TEX-E is triple and quite thick) is
good enough (with above 2kV and thus 4kV wire/wire).

However, in order to keep down the secondary self
capacitance, be generous with the interlayer insulation.

OK, thanks for that hint!

With single insulated thinner wire you've room for a
lot more secondary turns.  Your turns ratio is only 15:1 - the
turns ratio on a 12v car coil is about 60:1.

Winding ratio is 15:1 but voltage ratio is 16:1. I know of the much higher
ratios of other coils, but they don't get the dI/dt on the primary I have
(500V inductance voltage currently; will investigate in other IGBTs that
can accept higher UCE). Anyhow, I'll measure, check and experiment with
that too.

Your're working on an interesting project so keep us
in touch with the results - experiments that don't work are just
as interesting as those that do!

No failures until now. :-)


Thanks a lot for your input!
Nick
--
The lowcost-DRO:
<http://www.yadro.de>
.

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