Sunday, June 23, 2013

Villard Cascade Voltage Doubler

I recently watched an interesting EEVBlog video about Villard Cascade Voltage Doublers. I have had some neon tubes (NE-2) in my parts collection for a few years now. I have always wanted to drive them from a relatively low voltage DC supply. I decided to spend the afternoon/evening designing a circuit to drive the neon tube from a 12V supply.

The Glowing NE-2 Neon Tube

The first step was to design an oscillator. I don't own a waveform generator yet so I decided to build up a simple 555 timer based 50% duty cycle oscillator. I used values from the datasheet application note directly and had an oscillator running within minutes.

I attached an H-Bridge to the output and had an inverting waveform on my scope at 20Vpp. This is a good starting point for the voltage multiplier array.
Oscillator and H-Bridge
Next, I added a few stages of voltage multiplication. I used electrolytic capacitors to build up the array.
3 Stages of Voltage Multiplication, 58.5V!!
Organized Chaos
I added a few more stages to raise the output voltage.
5 Stages of Multiplication, 93.8V!
Once I reached 5 stages, the neon lamp was able to strike. Unfortunately it would not remain lit and would often flicker. This called for one more stage to increase the stability of the light output. The unloaded output was around 94V, but once the lamp was attached it would fall down to 74V.
First Glowing Tube
The next tweak I made was to change the 10uF capacitors for 100uF and upgrade the size of the H-Bridge transistors. The poor 2n3904 and 2n3906 cousins were overheating due to the load. I switched to some TIP31C and TIP32C transistors. They are overkill, but I had them on hand. I also finished adding the final stage of multiplication to get the unloaded voltage to approximately 120VDC.

I never captured a schematic of this, I just doodled a few notes on a piece of paper and started building.

Block Diagram
Finished Circuit
Artsy, Overexposed Picture. Lots of vibrant colors :]
I'm happy with the results of this project. The next step, if I can find the time, will be to assemble it on a piece of glass. I think the glowing neon tube on a glass backdrop would look nice.

Thanks for reading!