Ignition coil
From HvWiki
Ignition coils are oil-filled iron-core transformers that rely on the inductive kick effect to produce high voltages in the range of 15-50kV, depending on the internal design and how they are driven. Most ignition coil circuits tend to produce a waveform on the secondary with a very high peak voltage, although the average power is relatively low. Powering ignition coils outside the context of cars is often achieved with a 555 timer circuit triggering a power transistor that switches 12-48V through the primary, at a frequency usually between 50Hz and 200Hz.
Pictured is the type of coil you're most likely to see. Typical primary resistance may be 1.5Ω, and the secondary 15kΩ. Intrinsically, the primary and secondary coils are tied together on one side so the output is never floating.
Construction/deconstruction
Built inside a steel can, ignition coils are surprisingly difficult to dissassemble without knowledge of how they're held together. At the base of the can is a star-shaped plastic standoff which holds the inductor in the center of the can:
- The laminated core is the innermost component, and can be removed a few strips at a time,
- Next, the secondary is composed of machine-wrapped layers each covered with wax paper. It is worth mentioning that ignition coils are not nearly as fiddly as flyback transformers; the secondary winding is significantly thicker and less prone to heat damage as it is oil cooled.
- Finally, the primary winding is formed outside the secondary on top of two or three layers of thick black mylar to insulate it.
This whole assembly is then connected to the cylindrical top terminal block, which drops down like the star standoff into the top of the primary. A large rubber ring seals this connection. The metal can is swaged over around the top edge permanantly. A small self-tapping screw and rubber washer are inserted into the 'hot'/EHT terminal after the cavity is filled with oil.
No amount of twisting or butchering will help access the inner parts of the coil. To get inside requires shearing or peeling the swaged ring off around the top.
Drivers
Light Dimmer Driver The classic and simplest ignition coil driver is the lamp dimmer driver. This circuit has three parts: an ignition coil, a dimmer switch, and a capacitor all in series. The dimmer switch takes wall current and breaks sine wave up. So instead of a nice smooth sine wave you get a something resembling square waves but with rounded tops. link Thissite has more information on light dimmers. The large dv/dt in primary induces an even larger dv/dt in the secondary and it makes a pretty spark. The capacitor can be any where from a few tens of a microfarad to 10+ microfarads. The capacitor is there to limit the current so the bigger the capacitor the hotter the sparks with be, but too big and the ignition coil can overheat. The capacitors should have a high voltage rating like 600 volts because the inductive kick from the ignition coil can be pretty high.
555 Driver This driver uses a 555 in astable vibrator mode to create a squarewave. The most common version tries to use the 555 to turn on a 2n3055 Transistor, but it can't fully turn on such a large transistor so EDY19 created sort of a darlington triplet that works. See the discusion of it here and the schematic is below. The Load would be the ignition coil.
There is also the BICDFH which is very similar, but uses a much better transistor and has zener Diode to clamp the voltage spike. See the discussion of it here
