Are Nixie Tubes cool again?

I’ve been playing/designing with Nixie tubes for some time now. I found Nixies very fascinating as the numbers inside them glow just magically.

There are many Nixie Clock designs available on the net. They are usually two types; AC main powered clock without MCUs, or low voltage DC powered with MCUs. I prefer low voltage variety because of the safety reasons, as I like exposed PCBAs.

All of the low voltage designs have some kind of high voltage (180V typical) generation circuit – and I noticed that all of the designs that I see use a pretty hefty MOSFET driven by a PWM controller IC. Somewhat complex and not so small. I kept thinking – there has to be a simpler solution.

I’m sure many people reading this are familiar with Joule Thief circuit. It’s a simple blocking oscillator based boost converter. I have done some work with two transistor variation of Joule Thief extensively, and thought I should be able to use that circuit for Nixie power supply.

Looking at the basic circuit I realize that the output voltage is limited by the breakdown voltage (Vceo) of the switching transistor. So I tried testing with high voltage capable transistors. The result was not so good – you can get the voltage, but could not deliver the current Nixies needed.

So I decided to add voltage doubler to the circuit, which looked promising. After many tries with different transistors and voltage doubler or tripler combinations I was able to come up with a supply that can deliver about 7mA of driving current into a medium sized Nixie. The circuit only uses two transistors, a not so big inductor and a few diodes and capacitors. It is much simpler and smaller than all of the Nixie power supply I have come across.

It’s not as strong (only 180V and 7mA driving current as opposed to 200+V with 10+mA) and voltage regulation is not so good. However it’s more than good enough for small to medium sized Nixie tubes. It can also work with input voltage as low as 2.4V when you don’t need much output current (i.e. miniature Nixies like IN-17).

I have designed a couple of Nixie clocks using this power supply. I will follow up with some descriptions of each stage of the designs.

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