SpudFiles

Fnord, I have been tinkering around with a flyback based ingition for a little bit and have access to almost anything related to coil winding. I have been unsucessful in using the existing windings in the flyback so I removed the core to give winding my own a shot. Have you found any sort of data on coil resistences, wire sizes etc? Any help you could provide would be greatley appreciated!

If you wind for a regular ignition coil, the turns ratio is typically 100:1. That way an inductive kick in a Kettering system produces about 300 volts in the primary and 30KV in the secondary.

You can feed the primary with either 12 volts DC and interrupt the current with a condenser on the switch to generate the primary voltage from inductive kick, or use a capacitive discharge to dump a 300 volt charged cap into the primary

Typical turns are 200 turns in the primary and 20,000 turns in the secondary. Proper insulation will be needed to prevent arcing in the secondary. Use wire sizes that fit the space.

The difficulty many people have with flyback transformers is the primary is not designed for 12 volts DC like an ignition coil so the turns ratio is unknown and have many coils on the primary side as a power supply for other parts of the monitor or TV.

Often simply adding your own primary and using the exiting the secondary is a way to use those.

This page shows the number of turns to use for the primary to drive a typical flyback transformer with a transistor driver.
http://wiki.4hv.org/index.php/Flyback_transformer

Motor:

Depends on how much voltage you want to get out of it.

It takes about 11kv for a one centimeter arc, so 10-20kv is usually fine.

You'll probably want somewhere around of 10 primary turns of heavy wire(20gauge-ish). Flybacks are not too picky about this but may run hot if you use to few. Flybacks are not wound like a standard transformer. You'll have a voltage spike in there experience using too few will cause it to run hot.
Maybe 5 turns thin wire for the feedback coil. Again it's not too critical.

Now, I've also found (and been told by others) that the turn ratio does that will multiply your input voltage by around x4.
So if you're using 12v, you'll want to treat it like 48.

for 11kv:
11000 / 48 = 230

Take 230 * desired number of primary turns. The wire should be as thin as is practical for you to wind. I've also heard some people mention that it's best to use slightly fewer turns every time you start a new layer.


You'll need adaquate insulation between the coil layers. Teflon tape works well. Try arcing though some layers of it with a peizo ignitor(w/ small spark gap). When you can't get a spark through it you'll know about how many layers to use.
If you get really ambitious you can try submerging it in mineral oil and vacuuming out the bubbles for extra protection.

A proper chopper circuit to pulse the primary with a tuning capacitor can run the flyback transformer in it's original designed flyback mode.

A transistor is turned on for a short duration allowing primary current to build, and then when it is turned off, a flyback cap and diode produces the high voltage pulse in the primary.

From a Wikipedia article;

The primary winding of the LOPT is driven by a relatively low voltage sawtooth wave, which is ramped up (and sweeping the beam across the screen to draw a line) and then abruptly switched off (and causing the beam to quickly fly back from the right to the left of the display) by the horizontal output stage. This is a ramped and pulsed waveform that repeats at the horizontal (line) frequency of the display. The flyback (vertical portion of the sawtooth wave) is extremely useful to the flyback transformer: the faster a magnetic field collapses, the greater the induced voltage. Furthermore, the high frequency used permits the use of a much smaller transformer. In television sets, this high frequency is about 15 kilohertz (15.734 kHz for NTSC), and vibrations from the related circuitry can often be heard as a high-pitched whine. In modern computer displays the frequency can vary over a wide range, from about 30 kHz to 150 kHz.

This transformer is not driven with a voltage sawtooth wave. It is driven by a nearly square wave in voltage. The duty cycle is slightly less than 50%. The drive consists of the following cycles. The current is a sawtooth. The high voltage is during the sharp change on the edge of the sawtooth as there is a rapid change in current. Rate of current change and voltage are directly related.

1 Transistor ON. Drive the CRT beam from the center of the screen to the right side. The ramp in current is due to inductance. There is not an applied sawtooth voltage waveform. The transistor is simply switched on. The current ramps due to inductance from zero to the amount required to reach the end of the sweep, then it is turned off.

2 Transistor OFF. Flyback portion. Voltage in the primary spikes due to the sudden loss of current. The rate of collapse is controlled by the capacitor on the primary. Current collapses to zero and the scan returns the center of the display.

3 Transistor off, at the time the current reaches zero, the flyback capacitor is charged. It begins to discharge back into the coil reversing the current. This continues the retrace portion of the scan. Beam moves to the left edge of the screen as current peaks in the reverse direction. Steps 2 and 3 happen very fast and take little time.

4 Flyback diode conducts. Current in the coil decays to zero. Scan beam moves from the left edge to the center as sweep current returns to zero.

Cycle repeats.

A typical sweep circuit voltage waveform looks like this. The positive high voltage portion is when the transistor turns off. This is the retrace. The trace portion looks flat in relation to the retrace. Diode conducts after the pulse and continues mid trace, then the transistor turns on from mid trace until the turn off which starts the retrace pulse.


This is seldom checked with a scope as the probe will change the tuning and may cause failure.

More on the subject is in this PDF.
http://www.repairfaq.org/sam/reppic/horiz-tv.pdf

An alternative chopper (to a flyback or say a 555 based one) is to use a standard automotive relay wired as a "buzz coil". Add an automotive ignition cap and you've basically made the equivalent of a old style car distributor, one that doesn't need an engine turning a shaft to operate the on/off switch that pulses the coil.

You can get an automotive style 12V relay for a couple bucks at a car parts store. You just wire the relay's normally closed contact in series with the battery and the relays coil. The coil will cycle on and off and buzz (hence the name) at anything from a couple hundred to a couple thousand hertz depending on the relay and current source.

The relays normally open contact is wired in series with the battery and your coil's primary winding.