Nixie Clocks – Early Designs
I got my first Nixie tubes in early 2016 and started experimenting. I didn’t know anything about then at the time, but quickly realized that they were pretty simple devices to use.
Only part that needed developing was the high voltage power supply. I did not want to use mains AC as the power source, and ideally wanted to use 5V DC so that the clock can be powered from USB.
After a bit of research I found that most of the high voltage power supply designs use boost converter driven by a PWM controller IC such as MC34063. A large MOSFET switching a good size inductor. Those designs looked a bit overkill to me, so I started designing my own from scratch.
Since I’m familiar with transistor based blocking oscillator circuit to boost voltage, I wanted to see if I can use similar circuit. The switching transistor has to withstand the output voltage of 180V so I picked some high voltage transistors and experimented. Turned out that typical high voltage transistors (C-E breakdown of more than 200V) were too wimpy for the purpose, and the simple two transistor circuit that I was using was not capable of very high duty cycle demanded by high input/output voltage ratio (I’d estimate close to 99%).
One way to reduce requirement for the boost converter is to add voltage multiplier at the output. I added a 3 stage Cockcroft–Walton multiplier to a circuit using pretty ordinary (inexpensive) transistors. This circuit was able to provide required voltage and up to around 3 to 4 mA of driving current to medium sized Nixie like IN-12.
While this power supply was not quite powerful enough for larger Nixie tubes, I went ahead and designed a clock circuit to get my feet wet.
First design was a 4 digit clock using ATMEGA328 – I wanted to make the software easy to develop, so I loaded Arduino boot-loader. I also wanted to use the clock as a multipurpose numerical display so I added a V-USB port.
This prototype had some stupid bugs, but the basic functions such as multiplexing worked. I made a revision of this prototype right after.
Here I experimented with a tapped inductor to effectively double the boost converter output voltage and do away with voltage doubler instead of tripler.
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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.
