This is a high energy ignition coil driver circuit capable of running from AA batteries.
WARNING: This circuit contains voltages and
currents which can KILL if you are not careful. Charged capacitors will
SURPRISE YOU! They can hold a lethal charge for hours! If you don't know much
about working with line (and higher) voltages or if you aren't crazy (like me)
then DO NOT attempt to construct this circuit. I CANNOT BE RESPONSIBLE if you
electrocute yourself to death! That said, let's have some fun!
Description:
A camera flash or other suitable charging circuit charges a 4uF
capacitor to 380V. When the Fire switch is pressed, a small
current flows through the gate of the SCR to turn it on, by way of a 1k
current limiting resistor and a small capacitor for debounceing purposes. Once the
small capacitor becomes fully charged, sufficient current flow to the
gate stops, thus preventing the SCR from remaining in the on state if
the switch is held down. A 10MEG resistor is connected across
the capacitor to discharge it once the switch is released.
This prepares it for the next firing.
While the SCR is turned on, it connects the main capacitor to the
primary of the Ignition coil. The voltage across the primary
rises to 380V, thus storing the energy from the capacitor as a strong
magnetic field. Once the capacitor is discharged, the voltage
falls back to zero and this magnetic field begins collapseing.
During this collapse, a reverse polarity to the initial
charging current is formed, known as a flyback. This pulse is
returned to the capacitor and charges it back up via a 400V Fast diode
connected backwards across the SCR. This flyback pulse is
expected to be around 700V. While the SCR is in the on state,
this process continues until the energy is taken from the secondary in
the form of an arc (commonly referred to as the spark).
The pulsed nature of the charging circuit assures that there is always
time when no electricity is flowing, thus allowing the SCR to switch
off.
Notes:
This circuit comes into its own when used in vapor and gas deflagration
cannons (potato guns, for example). The hot spark will ignite
troublesome mixtures that a piezoelectric grill igniter won't budge.
It can also be used to replace a grill ignition system, and many
others. There are many applications, including just looking at the
long bright arc and listening to the sharp POP. Most of you probably
already know what you're going to do with it.
4uF is a good value for the main capacitor. Any more or any
less brings the circuit farther out of resonance. It's a good
idea to use nonpolars since there are polarity reversals and pulse
discharges. I use 4 microwave oven capacitors. They
are usually around 1uF apiece. Just connect them in parallel. There are plenty of wires
with
spade lugs on both ends in microwaves, so save them to connect the
capacitors. Then the whole assembly can be brought together
with a
large hose clamp or 2 smaller ones screwed into one another.
If you're
talented, you can use point to point wiring and get the coil, SCR, and
everything to be held together with the hose clamps in one mass.
A
carrying arrangement can even be devised using a plastic bucket.
For the ignition coil, ones from cars are usually the largest and the
best to use. If you're going the salvage route, make sure you
pick up a few to insure that you get at least one that isn't burnt out.
High school and college auto shops and scrapyards are good
places to find them. The primary terminals on the cylindrical
coils are labeled BAT and DEC, short for battery and distributor earth
connection. Newer coils have unlabeled primary terminals.
Since determining the internal wiring of the ignition coil is
difficult, don't worry about it. The few hundred extra volts
isn't really noticeable. One possibility though is, since most
of the GM coils use the same connector, if you can find one that's in a
car, find which terminal connects to the battery and note which battery
terminal it connects to. If it connects to the positive
battery post, then the other terminal is the common primary/secondary
connection. If it connects to the negative or chassis ground,
then it is that terminal.
To give you an idea of the origins of this circuit, a description of
the automobile circuit follows. In a traditional automobile
[low energy] ignition system and most cylindrical coils, the BAT
terminal connects to the +12V supply and the DEC terminal connects to a
special rotary switch inside the distributor that is driven by the cam
shaft. This switch connects the coil to the chassis ground
when a spark is needed. The high voltage terminal is
connected to the upper part of the distributor that is turning and
directing the pulses to the right spark plug at the right time.
In a high energy ignition system, there is a capacitor that
is discharged into the primary electronically, just as in the ignition
coil driver circuit above. This eliminates the high currents
on the rotary switch or even eliminates the switch altogether by using another
type of cam shaft position sensor. The HV rotary switch is
retained, however, or in some cases, the distributor is eliminated
completely by either firing all spark plugs simultaneously or by using
several separate ignition coils.
If you notice,
the primary and secondary are wired in series with respect to the HV output
terminals. This is to get an extra few hundred volts into the
spark (thanks Jochen).
Also, the primary is connected in reverse polarity to the way
it would be in a car. This has no effect on output, however.
It was only to facilitate a direct connection of the ground
to the rest of the circuit without having to go through the diode/SCR.
It could be run the other way too, but I like the elegance of
it.
For the charging circuit, as pictured above, a camera flash circuit is
preferred since it allows you to move the circuit around and use it in
places where an outlet is not available. These can be found
in disposable cameras or broken ones. All that needs to be
done is to remove the xenon tube and storage capacitor after
determining the polarity. The ignition coil driver circuit is
then connected in place of the storage capacitor.
If the circuit is gong to be used near an outlet and not
moved very often, then a rectifier diode and 100 ohm current limiting
resistor can be used instead. A bridge rectifier may also be
used if a pulse rate of over 60 times a second is needed, up to 120
times a second.
In case you're wondering, the 400V Fast diode makes a very big
difference in the arc produced. It makes the arc almost twice
as hot and twice as loud. It also saves the SCR from
potential failure so don't leave it out!
You can experiment with different sizes of the 3n3 cap and 10M
resistor. Those were just parts I had lying around and they
work. If the resistor is too small, the SCR will fire when
the button is pressed and
released. The sweet spot is somewhere between 1 and 10 MEG.
You may want to try different values for the capacitor to
vary the on time of the SCR for more or less oscillations.
The SCR won't fire if the gate resistor is greater than 1k.
Another thing which gives you greater flexibility is, instead of
soldering the switch into the circuit, put a headphone jack in its
place. Then you can have a switch that plugs right in, or you can
have it on a long cord so you can stand back from whatever it is you're
sparking at.
Parts Inventory:
Resistors:
1k 1/4 Watt
10M 1/4 Watt
Capacitors:
4uF, 700V Nonpolar
3n3, 700V Ceramic or Polyethylene
Others:
Camera flash charging circuit
400V 4A SCR
400V Fast diode
Tactile or other normally open pusbutton switch
Automotive ignition coil and spark plug wire
Optional:
400V rectifier, 100R 5 watt resistor, and a 240V transformer or other such AC power source
