# Low Voltage to High Voltage/Low Current

• posted

Hi All,

I want to do some experiment with electronic filed. So, I am trying to convert a low voltage (like 3v coming from 2 AAA batteries) to very high voltage like +1 KV with very low current. Also I need to make this circuit as small as possible. Does anybody have any idea how can I do this?

Cheers,

-Arash

• posted

Yes--use a small transformer in "flyback" mode, driven by a small oscillator. You can make a self-oscillating circuit with a single transistor which will do the job. If the transformer has fairly high turns ratio, it's quite possible to get 1000 volts and more from such a simple circuit. You will find such a circuit in the "electronic fly swatters" sold by places like Harbor Freight. See their item number

40122. It's difficult to build such a circuit as cheaply as you can buy it when the fly swatters are on sale! It runs as designed from two "D" cells, but it is possible to run it from 2 AAA cells, at least for short periods. Mine generates about 4kV. Be careful: high voltage can sting!

Cheers, Tom

• posted

Hi Tom,

Thank you very much for your advice. I checked the transformer in flyback mode. it seems that they are working in much higher freq that I need. I need to work with frequency range from 2 to 20 Hz! I guess I can fix this using a capacitor, but I was wonder if you could suggest something more appropriate for this range of frequency. Don't worry, I am trying to electrocute myself! Again, Thank you very much :)

Cheers,

-Arash

• posted

OPSSSSSSS!!!

Don't worry, I have no intention to electrocute myself! Last message missed a NOT!

o
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• posted

o
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The working frequency is set by the inductance of the transformer; you could "fix" the problem by using a bigger transformer (not a bigger capacitor) but something that would work between 2Hz and 20Hz would be impractically large and expensive.

What you almost certainly need to do is let the flyback transfomrer run at the frequency that suits it, but turn off the oscillation whenever you don't need the 1kV output; it will probably take a number of fly-back events to shift enough charge into you output capacitance to charge it up to 1kV.

-- Bill Sloman, Nijmegen

• posted

o
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Ah... Your original post said +1kV, so I assumed you want DC output. If you want high voltage, low frequency AC, then you can use a transformer with a high turns ratio. At high frequency, a high turns ratio is a problem, but it is much less of a problem at low frequency. Instead, the problem is that you need a lot of turns to get to a high impedance. If you do not have a high impedance, then the current on the primary side will be too much for AAA cells. As Bill Sloman suggested, such a transformer to work at 2Hz will likely be larger than you want. To put a number on it, 1mA at 1000V is one megohm. To get a megohm of inductive reactance at 2Hz, you need about

80,000 henries of inductance. That's going to be a LOT of turns (tens of thousands) of very fine wire on any practical core I know about.

Bill's suggestion of pulsing a high frequency supply is one way to do it. You can rectify the pulsed output, and you'll get a single- polarity output. Do you need the output to be sinusoidal? Do you need the output to have no DC component? You can remove the DC output by using a capacitor, but then you will have to waste power by adding a load resistor on the "input" side of the capacitor. Otherwise, the capacitor just charges up to the supply DC voltage and nothing more happens.

Cheers, Tom

• posted

An easier way to ask all these is "what is the application?"

I can only imagine something like powering a small xenon flashtube with the 2 to 20 Hz being a strobe frequency.

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