The purpose of the
ignition circuit is to accurately
and reliably switch on large currents across an air gap, such as may
be found in an electric engine nozzle or an electric welder. Completing
a circuit across an air gap can be accomplished by jumping a spark
between the two electrodes, which creates a low resistance path for
current through the ionized channel that we see as a spark. This is
most easily accomplished by applying a voltage across the gap that
exceeds the gap's breakdown potential. From this brief discussion we
can extract two basic requirements of the ignition circuit - that it be
able to impose a large voltage across a spark gap and that the gap is
then able to carrier large currents, perhaps tens of thousands of amps.
This, unfortunately, is not as easy as it sounds - but lets think this
through and see what we can come up with.
(To skip the theoretical
stuff and get right to the construction details, click here.)
To start with, we have
some power source and a spark gap connected together as shown below.
Fig. 1: Source
Supply Connected to Air Gap
Just to get
started, let's say that the
gap is 1cm long.
This would require approximately 30kV be applied across the gap in
order to get the air in the gap to breakdown and spark. This will
require some other voltage source (we don't expect to need nearly 30kV
from the main source), as shown in figure 2.
Fig. 2: Source and High
Voltage Connected in Parallel
we encounter our
first problem, namely that in applying the voltage required to create a
spark between the two electrodes of the gap we've also applied a very
large voltage directly across our source. Even though we haven't said
anything about the design or construction of our source, it's a pretty
safe bet that this is going to cause damage to the supply, if not
completely destroy it. Using ideal components this would be a trivial
problem; we could just place a diode between the main source and the
Fig. 3: Source and
Voltage Decoupled by Diode
that seems like a
reasonable solution, finding a diode that can both withstand 30kV
reverse bias and survive while carrying several hundreds or thousands
of amps is for all practical purposes impossible. Another solution
would be to use a high voltage AC source instead of a DC
source and then replace the diode with an inductor to block the high AC
Fig. 4: Source and
Voltage Decoupled by Inductor
certainly solves the
reverse breakdown requirement problem with the diode in the previous
design, but again, an actual inductor with a large enough
inductance to block the high AC voltage and a large enough core to keep
from saturating at several thousand amps would be completely
impractical. The big problem with all of these designs so far is that
they attempt to put the high voltage in parallel with the source, and
decoupling the source from the high voltage is very difficult. A more
practical configuration places the main source and the high voltage
in series. If we stick with the AC high voltage source, we can even
make it easily controllable by using a transformer.
Fig. 5: Source and
High Voltage Connected in Series
arrangement allows a
high voltage to be applied to the gap without simultaneously imposing
that voltage across the source as well. If we make transformer T an air
core transformer, then there is no saturation issue or weight issue
associated with a large iron core. So, if we put a switch between the
high voltage AC source and the transformer, we can control when a spark
is created across the gap and consequently when the source is able to
drive current through the now ionized air gap. Huzzah!
high voltage AC
source is somewhat more complicated than simply
using a high voltage transformer. Here I used a circuit which is very
similar to one found in Tesla Coils. Its purpose is to generate a very
high frequency high AC voltage which can be used to drive the air core
6: High Frequency Starter Circuit
this circuit's operation. The neon sign transformer's output voltage
oscillate at 60Hz, from 0V to approximately 21kV. The spark gap, which
measures 0.285" across, will breakdown and create a short circuit at
just about 21kV (the very top of the sine wave.) Prior to the gap
sparking, the capacitor will have charged to approximately 21kV. At the
moment when the gap sparks the neon transformer is removed from the
circuit and the capacitor and inductor formed by the transformer
resonate at a high frequency. This high frequency high
will be transfered to the output of the circuit by the air core
transformer. The resulting waveforms are shown
below in figure 7.
Fig. 7: High Frequency
This is the method
that I used to construct my ignition circuit, and is
very similar to the Miller HF-15 welding arc starter. I originally
found the schematics for this device here.
OK, now let's look at the details and
construction of this circuit.
So, the most
practical topology for the ignition circuit places an air
core transformer between the source and the air gap, which is in turn
connected to a high voltage AC source as shown in figure 5.
|Ignition Circuit Construction
The air core transformer
is simply two wires wrapped around a PVC tube.
I used 20 turns for both windings; the high current winding is made of
00 AWG copper cable and the high voltage winding is made of 18 AWG 30kV
Fig. 8: Air Core
the 00 AWG cable
that I used is very stiff I drilled two holes approximately 1 cable
diameter apart and zip-tied the cable down on one end while winding it.
The high voltage winding was wound so that it sat on top of and between
each turn of the high current winding. This is important because it
maximizes the coupling between the two coils, which is already weak due
to the lack of an iron core.
Fig. 9: High
Voltage Windings and
I go on to explain
how I built this circuit I would like to make it understood that the
voltages in this circuit as well as it's output are high enough to kill
you instantly. No second chances, no trips to the hospital, just dead.
It should go without saying that if you're not 100% sure how to work
with circuits like these you shouldn't go anywhere near them.
|Warning - Electrocution Hazard
Voltage AC Source
used a neon
transformer, two 590pF 30kV capacitors, a custom made spark gap, and
high voltage wire to construct this circuit. The first thing I obtained
was a neon sign transformer from eBay (I get a lot of my parts from
there.) It's a big, heavy, ugly Magnetek 15kV 30mA neon transformer, as
shown below. It cost about $90 not including shipping charges, which
were significant on account of it's weight. The small 30kV 590pF knob
capacitors also came from eBay, as well as the high voltage wire.
Fig. 10: Ignition
The spark gap was
somewhat more difficult to obtain. In the end I wound
up making it myself out of 4 pieces of aluminum bar stock, 4 tool steel
electrodes, a non-conductive Delrin block, and assorted hardware. The
electrode spacings are adjustable via a set screw on the top of each
block, and the total gap was set to about 0.285". The large aluminum
blocks also serve as heatsinks, since prolonged operation can generate
a significant amount of heat. It isn't obvious in the photo, but the
Delrin has started to warp slightly from the heat.
Fig. 11: Custom Spark Gap
mounted to a sheet of 3/8" Lexan as shown below.
Fig. 12: Complete
Ignition Circuit Assembly
the rear you can see the neon sign transformer, which connects
directly to the spark gap. The spark gap is then connected in series
via the high voltage winding on the transformer, the doorknob
capacitors, and a 1 Ohm 10 Watt resistor. The extra resistor doesn't
really effect the operation of the LC tank circuit much, but it does
limit the current that flows in the high voltage winding during the
discharge through the high current winding.
[ Back to MPD
[ Back to Main
visitors since September 2007
Comments? Suggestions? E-Mail me at MyElectricEngine@gmail.com
Copyright 2007-2010 by Matthew Krolak - All Rights
Don't copy my stuff without asking first.