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|Time-lapse photo of
the Capacitive Discharge pulser driving a spark gap
consisting of a pair of rounded brass electrodes, separated
by 22 mm.
|Two coils of wire are
wound onto a "soft" (magnetically and mechanically) iron
core and consist of a primary
and secondary winding,
(connected as an autotransformer)
with roughly 1:100 turns ratio. The primary is
connected to a battery in series with a mechanical switch,
resulting in a primary current that rises from zero and
reaches a maximum value depending on the DC resistance of
the transformer primary, and at a rate (amps per second)
determined by the magnetizing inductance of the ignition
coil. When the switch is opened, the magnetic field
encompassing both the primary and secondary windings
collapses rapidly, inducing a high voltage in the secondary
winding (between the high voltage output terminal and
ground). In principle, the opening of this switch
causes the primary coil current to be interrupted
instantaneously. That would imply
an infinite back EMF generated across the switch, which
would cause an arc to form at the switch, dissipating most
of the energy stored within the magnetic field.
Instead, a capacitor is place across the switch. Then,
as the switch opens, primary current will continue to flow
through the capacitor, albeit rapidly decreasing at a
controlled rate, and inducing the desired high voltage at
the secondary, and simultaneously inducing a rapidly rising
voltage across the capacitor. Energy flows back and
forth resonantly between the magnetic field in the coil and
the electric field in the capacitor, with a period given
approximately by 2 pi x Sq. Rt. (LxC), the energy decaying
rapidly as energy is transferred to the secondary side spark
and to the wiring resistance. Therefore, a series of decaying
high-voltage pulses appears between the high voltage output
and the ground terminal.
|Here is the
assembled board for the capacitive discharge pulser.
Battery or other power supply connections are on the left,
and connections to the ignition coil are on the right.
Controls along the lower edge of the board include a power
switch, a multiple or single-shot fire switch and a rate
control. Various jumpers along the lower edge allow
the selection of different operating modes.
|Triggering||Multiple Mode, from pushbutton.||Multiple Mode, from pushbutton or external electronics.|
|Pulses||Fixed Rep Rate, 50 Hz||Single/Multiple pulses, wide rep. rate range, up to 300 Hz|
|Power Source||Best at 12 volts||Tolerates a range of supply voltages|
|Ignition Coil Type||Needs high primary inductance||Tolerates different coil styles|
||All parts contained on one
||Power source and ignition
coil separate from circuit board
|An appropriate coil
for the Capacitive Discharge pulser is Emgo part number
24-71536, offered by Oemcycle.com as their part number
or this vendor:
It appears to have a small ferrite rod core, instead of a closed, laminated soft iron U-I core.
Approximate measured parameters:
Primary resistance - 0.5 ohms
Secondary resistance - 4.9 kohm
Turns ratio: 1:100 (published spec; our measurements show that it is slightly higher)
Magnetizing inductance - 0.11 mH
Leakage inductance - 0.05 mH
While the low primary resistance might allow high currents, primary current in the Kettering pulser is limited more by the on-resistance of the MOSFET switch. So the lower inductance stores less energy, making this coil more useful for Capacitive Discharge uses, and impractical for the Kettering Pulser.
|A more conventional
ignition coil, the Emgo 24-71532, with Oemcycle.com part
number 2102-0029, with a laminated soft iron core, has been
tested on the Kettering and Capacitive Discharge pulsers.
Approximate measured parameters:
Primary resistance - 1.0 ohm
Secondary resistance - 6.4 kohm
Turns ratio - 1:75
Magnetizing inductance - 6.2 mH
Leakage inductance - 0.4 mH
The core in this transformer begins to saturate at an applied volt-seconds of 0.012 volt-seconds so will not store quite as much magnetic energy as would be expected for the measured inductance as the primary current rises above about 2 amps.
In the event that the parts from Oemcycle are not available, try these links:
http://www.oldbikebarn.com Kawasaki Ignition Coil #21121-1186
A very nice coil; most (but not all) of the frame, which is the common autotransformer
connection, is not so accessible to physical contact, thereby reducing the shock hazard. But it's rather expensive.
http://buffalosmallengine.com Briggs & Stratton Ignition Coil #1779
A fully-encapsulated coil at a moderate price, but without mounting holes.