This is a high voltage power supply that produces between 400 and 1000 Volts DC. I must say before anything about this project : If you choose to build this project BE CAREFULL ! Although the current output is low, high voltages can be dangerous. The capacitors can hold a charge for quite a while. I discovered this the hard way by picking up the circuit board 10 minutes after it was powered down. After giving this warning I assume no responsibility for any harm or damage done by this circuit. The circuit can be used safetly if the user applies common sense and responsibility. The filter capacitor and the capacitors in the voltage multiplier can be safetly discharged through a 1/4 Watt, 10K resistor between the HV output and ground.
The application I developed this circuit for was powering a Gieger-Mueller tube for a "Gieger" counter. Other applications would include low current vacuum tubes (thermionic valves). Battery life is about 14 hours continuous operation using a standard 9 Volt alkaline battery.
Here's how it works -
The oscillator uses a transistor, capacitor and a transformer. The oscillator design I used is based on blocking oscillators I have seen used in Xenon flash-tube power supplies in cameras. The transformer is an audio output transformer run in reverse (output being used as input). I used a common audio output transformer purchased from Radio Shack in USA (part # 273-1380). The transformer output in this circuit is about 80 Volts. These transformers are not designed to deal with high voltages but they do - and they are inexpensive. The output of the transformer is connected to a four stage voltage multiplier. The rectifiers (1N4935) are high voltage, fast recovery diodes. The 1N4004 will work in the voltage multiplier but fast recovery diodes yield a higher voltage output. The capacitors are 200V, polyester tubular type.
On the circuit board, the connector labeled "Ref" is only to measure the voltage reference. The circuit does not need an external voltage reference. The connectors labeled "Pos" and "Neg" is the 9 Volt power and "HV" is, of course, the high voltage output.
In the picture the oscillator transistor is socketed. This is because I was experimenting with various small transistors. Any common NPN transistor with a beta over 200 should work well. However, some transistors work better than others so I encourage experimentation. The 2N4142 and 2N2222 gave good results. I tried a Zetex ZTX857 which worked well also.
The output of the transformer is then connected to the voltage multiplier. The center tap of the transformer is grounded through a 47K resistor. This resistor stabilizes the circuit and prevents it from "running away" (despite the regulator), which eventualy destroys the oscillator transistor. I don't understand why this works but I put the 47K resistor in the circuit intuitively thinking it might give a ground reference for the oscillator and help stabilize it.
The output of the voltage multiplier is then filtered by a 1500 Volt, .01 uf capacitor. The feedback for the regulator is connect to a CA3160 op-amp through a 22M resistor. An adjustable voltage divider is made with the 50K pot for high voltage adjustment. The CA3160 is operating as a comparator modulating the base of the oscillator transistor. I chose the CA3160 because it operates well from a single polarity power supply and it has very low input current sensitivity, which is good to have with a 22M resistor feeding the input. I used a LM385Z, 1.2V voltage reference in the regulator circuit. The circuit does not need a precision reference but I have a few of them on-hand so I used it. A plain Zener diode reference would probably work just as well.
The schematic -
Send questions and comments to WireHead@GalacticElectronics.com
(c) Jon Qualey, August 2006
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