Figure 21 shows a classical diode- capacitor ladder for voltage multiplication: Capacitors stack voltages on top of each other, so the circuit operation is quite self- explanatory. The fuse prevents any mains connected equipment from becoming a possible fire hazard. Increasing the number of stages too much is ultimately pointless, since the available output current will be greatly reduced.
Being measly with the voltage ratings of high- tension diodes or capacitors is both poor design practice and false economy: In the prototype, capacitors are rated at three times the RMS mains voltage, diodes at five times the RMS mains voltage. If your mains voltage is about 110 V, you could double the value of the capacitors.
There is nothing sensational about the conventional inverter in figure 22, either: This option is provided to increase the output current. Performance will be enhanced considerably by using complimentary output devices driven from a symmetric power supply, so the design is not optimal by any stretch of the imagination.
An inverter is neatly split into oscillator, current amplifier, step- up (auto) transformer, and rectifier/ smoother sections. All transistors are small- signal, excluding T4, which is a power device mounted on a suitable heatsink. The step- up transformer can be obtained from the ignition coil of a car intended for scrap.
Again, it cannot be stressed too much that a generous margin must be allowed for the voltage ratings of high- voltage diodes and capacitors: The three HV capacitors in series impress this crucial fact upon the constructor. The HT diode is a 45kV type, though the spike- suppressor diode on the low- voltage winding is a non- critical 50 V device. Placement of the load at the collector of the power transistor also affords voltage gain (which is not really necessary in this instance, as the oscillator swings almost to the power supply rails.)
If the unit is a laser power supply, a 'balast' resistor is connected in series with the HV, to limit the output current (typically to about 5 mA) - or the laser could be damaged permanently.
Resistor values are best calculated after the supply voltage has been decided (for a start, 12 Volts.) What is the frequency of the oscillator? Literature suggests values from a few Hertz (!) to few tens of KHz- you might as well investigate the region near the resonances of the windings. The two diagrams produce a positive HV. If a negative voltage is needed (as in ionisers, and so on), simply reverse the polarity of all HT diodes. Solder a pin to the HV node for more efficient ion generation.
Many a trap awaits the unwary in HT projects: The board must operate within a plastic box; the lid must be impossible to open without removing several screws, in accordance with mains- powered equipment practice. If the unit is a negative- ion generator, ventilation is via holes on the case: They will be small enough to keep out children fingers. The case must clearly state there is a dangerous high voltage inside.
Any contact with any HV node will provide a nasty shock- at best! Skin burns could be caused even by the capacitor in the electronic flashgun, if accidentally shorted by a finger. In fact, it is not even necessary to touch the device. It is perfectly capable of attacking in its vicinity, too! (While tampering with a 4 kV ioniser, my best friend was shocked to find out that much.)
Do not attempt to measure the HV, unless assisted by a person experienced in precisely that task.
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