Do you like the idea of high-voltage electronic sparking devices, but don’t have the time or patience to learn the complex theory? Here’s a simple tutorial that can help you to make many high-voltage projects very easily and cheaply. Since the concept calls for a low-current source of high voltage dc, first of all let me give an introduction to dc voltage boosters/multipliers.
Traditional and still broadly used dc voltage boosters/multipliers are based on a simple two-part diode-capacitor rectifier network. To understand its basic operation we can start off by referring a basic circuit, as follows:
In the above half-wave rectifier dc power supply circuit (left side), AC230V is applied to the input and the circuit produces a positive output voltage of 325V, merely equals to the peak positive value of the transformer’s secondary voltage. In the next figure (right side) you can see the traditional two-part voltage multiplier circuit that gives a positive voltage output of 650V from an ac input of 230V provided by the secondary winding of the transformer. Note that, here the waveform’s peak output value equals the peak-to-peak value of the ac input voltage, and the final dc voltage output is equal to the peak-to-peak value (rather than the peak value) of the ac input voltage (actually sum of the peak-to-peak value and the common reference voltage).
Although we can improve this idea to get a higher output by raising the reference voltage level, there’s a serious drawback. When we interconnect a number of the basic circuit to get various values of output voltage, each section generates an individual output while output of the first section acts as the voltage reference of the second section, and so on. This introduces a hitch as we need costly capacitors with absolute minimum voltage rating match with the voltage across output and ground rails of the occupied section. That’s quite speculative and impractical!
An easy solution is to try it up with the cheerful “Cockcroft-Walton” voltage multiplier circuit (see next figure). Note that a 10-stage version of this model, if driven by a 230V ac input, will give a dc output more than 6000V (6KV), but the components used in each stage just require a minimum voltage rating less than 1000V (1KV).
A quick & simple way to build a high-voltage generator circuit in order to gain some experience is shown below. It’s based the component values on what I had available, and there is plenty of scope for further tryouts (the number of sections you use will set the final output voltage). However, ensure that the components can handle requisite voltage levels safely. Capacitors should ideally be 2KV ceramic types (1nF, 10nF, and 100nF are usual values), and each diode must see a voltage well below its suggested rating.
Top part of the figure is the circuit diagram of the Cockcroft-Walton (CW) voltage multiplier section, whereas bottom part depicts the ac voltage source circuit. In this section, a power mosfet (T1) is used to drive the step-up transformer (X1) but it needs a pulse input (~500Hz) from an external source. Any microcontroller or the venerable 555 timer chip can be configured to work as a pulse generator for driving the power mosfet. Give it a try…
The trick used here is to drive a step-up transformer from the output of a low-voltage oscillator (free-running square wave generator) which can provide the required high-voltage ac on its output winding. The resultant ac voltage can then easily be multiplied with the help of the Cockcroft-Walton voltage multiplier mechanism. Keep in mind, output impedance of the Cockcroft-Walton voltage multiplier is rather high and it can hence render only small output currents. Below you can see the pulse generator circuit used for my prototype test.
See a candid shot from my workbench while I was testing the breadboard prototype,
& build up of the voltage-doubler (with <1nF capacitors)
Warning: Although the experiments presented here have worked flawlessly in my lab, I cannot be held responsible for any wrongs that may occur to you or your instruments while following this article. Be prepared for dangerous consequences and emergencies!