The input voltage can range from about 5 to 24 V. Using a flyback from a MAC Plus computer which had its bad primary winding excised, an output of more than 20 KV is possible (though risky since the flyback is probably not rated for more than about 12 KV) from a 24 VDC, 2 A power supply. By adjusting the drive
frequency and duty cycle, a wide range of output voltages and currents may be obtained depending on your load.
With the addition of a high voltage filter capacitor (.08 uF, 12 KV), this becomes a nice little helium neon laser power supply which operates on 8 to 15 VDC depending on required tube current and ballast resistor. See the document: "Lasers: Safety, Drive, Info, Parts; Diode, HeNe, Ar/Kr Ion Lasers" for details.
The drive transformer is from a B/W computer monitor (actually a video display terminal) and has a turns ratio of 4:1 wound on a 5/16" square by 3/8" long nylon bobbin on a gapped ferrite double E core. The primary has 80 turns and the secondary has 20 turns, both of #30 wire. Make sure you get the polarity
correct: The base of the switching transistor should be driven when the driver turns on.
Where the flyback includes an internal rectifier and/or you are attempting to obtain the maximum output voltage of a specific polarity, the direction of drive matters as the largest pulse amplitude is generated when the switching transistor turns off. Since flyback transformers are not marked, you will
have to try both possible connections to the drive coil. Use the one that produces the higher output voltage for a given set of input conditions (drive and pulse rate/width).
Many variations on this basic circuit are certainly possible. However, one nice thing about running it at 24 VDC or less is that it is much more difficult to let the smoke out of the circuit! The 5 A power supply I was using shut down on several occasions due to overcurrent but the only time I blew the chopper transistor was by accidentally shorting the base to collector.
from http://www.repairfaq.org
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