Introduction: Adjustable High DC Voltage Generator

This project is about building an adjustable high voltage generator (DC) from a low DC voltage supply. The voltage supply can range from 5V to over 25V (I use a 9V supply). Higher voltages can require heat dissipators attached to the transistors. Output voltages of 800V and more are possible (400V with 9V source). The generator should be connected to a high impedance, or the output voltage will drop.

ATTENTION: HIGH VOLTAGE IS DANGEROUS. Please be careful and do not touch the circuit elements while in operation.

Supplies

  • Perfboard
  • A DC power supply, I use a 9V power adapter
  • A 40 pin single row breakable male pin header, for input and output connections
  • Capacitors: 2x100 nF 63V, 3x15 nF 100V, 8x220 nF 400V, 220 nF, 4.7 nF
  • Inductors: 10 mH
  • Resistors: 2x1.2K, 1K, 12K, 33K, 82K, trimmer 10K
  • Diodes: 5V6 zener, 1N4148, 8xUF4007
  • Transistors: TIP31C, TIP32C, BC547C
  • NE555

Step 1: Basic Operation

The circuit is built around the properties of a series LC circuit. The circuit presents a resonant frequency at f= 1/(2*pi*√L̅C̅). At this frequency, the impedance of the circuit presents a minimum, and thus the current through the elements is maximum. More interestingly, the inductor and the capacitor exchange their energy at this frequency in a perfect manner, the sum of the stored energies in both elements is constant in time, and equal to 1/2LI^2 and to 1/2CVc^2. Therefore, if the series resistor is small, the intensity will be high, and so will be the stored energy in both elements. As the energy at the capacitor increases with the square of its voltage, if the stored energy is high, its voltage will also be high.

So at the resonant frequency, and with a small series resistor, we will get a higher voltage at the capacitor (and inductor, as their voltage is equal but opposite) than the input voltage (both sinusoid waves). This will happen if the series resistor is less than √L̅/̅C̅. Specifically, the voltage gain will be √L̅/̅C̅/R.

The proposed circuit uses a high value inductor and a small value capacitor, in order to maximize the voltage gain. With a 30 ohm series resistor (which includes the resistance of the inductor and the output impedance of the source), a voltage gain of 31 dB is achieved at the resonant frequency of 18.4 KHz. The frequency range in which there is voltage gain is small if the series resistor is small, as can be seen in the Bode plot above. By changing the input frequency around the resonant frequency, we will get a varying output voltage.

Instead of a sinusoid wave as input to the resonant LC circuit, a square wave can also be used. The voltage at the capacitor will still be a sinusoid, due to the strong filtering action. This is the principle used in the proposed circuit: a square wave is generated by a NE555 in astable oscillator configuration (50% duty cicle). This is the orange cable seen in the photo of the circuit. A trimmer allows for a variable output frequency, which in turn produces a varying sinusoid voltage at the capacitor as we separate from the resonant frequency.

The 5V square wave generated by the NE555 is fed into a couple of push-pull transistors, which scale the square wave to the voltage supply level and provide enough current to the voltage multiplier.

Step 2: The Voltage Multiplier

Following the previous step, we have a sinusoid wave with a relatively high voltage. The proposed circuit achieves a 120V peak to peak signal, from a 9V power supply. The next step is to multiply this voltage to get a high DC output. This could be done with a transformer, but they are bulky. Instead, the proposed circuit uses a Cockcroft-Walton voltage doubler. Starting with a sinusoid voltage of Vpp volts peak to peak, it generates a DC voltage of 2*Vpp. The proposed circuit uses two stages in cascade, to obtain a DC voltage of 4*Vpp. You can add more stages.

Step 3: Building the Circuit

The circuit can be built on a perfboard, following its schematic. You can start by placing and soldering the header input pins: one for ground, one for the voltage supply, one for the square wave input. Then you can place and solder the square wave generator (with the NE555), and attach a cable to its output. You can check that it correctly generates a square wave of 5V with a frequency which can be varied by the trimmer. At some position, it should generate the resonant frequency, about 18.4 KHz. If all is right, connect the output cable to the square wave input.

Then you can place and solder the push-pull stage and the LC circuit. Place a header pin at its output, so you can monitor it. You should get at this pin a sinusoid wave of varying amplitude (with the trimmer).

Then you can proceed with the voltage multiplier stages. Try to make good solder connections, avoiding sharp edges of solder. Make sure there is enough space between solder points. Place a header pin at the output of the first multiplier stage and another one at the output of the second one. These wil be the output connections of the circuit.

I suggest to attach a foam base or similar below the perfboard with silicone, to avoid touching the solder points.

You are ready. You should get a high DC voltage at the output, with a maximum when the trimmer is in a position that generates exactly the resonant frequency of the LC circuit.