Since in last years nixies displays attracted my attention so much that I bought many of them, I finally managed to design a good power supply and my nixie clock is on the way... [UPDATE: done! read instructable here)
With this circuit you can test a nixie display, or power a multiplexed nixie clock. The potentiometer lets you change the output voltage to fit different types of displays (you can obtain from 100V to 300V DC). My prototype has been built on a breadboard, but I also made a pcb from the schematic so you can etch your own board and solder the components.
WARNING: this circuit raises the voltage to deadly 300V so you must avoid to touch contacts while working, I'm not kidding, please BE CAREFUL!
Step 1: The Schematic
To understand the way a circuit can raise voltage from 9V to about 180V I had to make some research in the web, because I'm not so skilled in electronics to design my own circuit. Anyway I merged some information from the most exhaustive sources, and my circuit tries to be simple and efficient, but most of you readers can probably add some revision or advice.
The two best sources, which are must-see, have been arduinix.com (device assembling instructable here) and Threeneuron's Pile o'Poo, and both their circuits are a bit different from mine, since from Arduinix I've taken the NE555 application, and from Threeneuron's the use of a pull-down transistor to improve the efficiency and reduce the mosfet heat. You can also read ian's instructable to better understand working principle and components details. Another source was this document from www.ledsales.com.au.
On Threeneuron's website you also can find informations about the calculations to find the right R36 anode resistance value to limit the current for each type of nixie display. Let's say that 15 Kohm will probably work for 90% of the displays.
Look the notes on the image for more informations about the circuit.
I also report the explanation of circuit functioning as -max- wrote in his comment:
An inductor is used to create high voltage kickback. This configuration is often found in boost converters, called that because the boost voltage. It works because one can not change the current flowing through an inductor spontaneously, inductors resist change in current. This means when the MOSFET turns on and passes a current through your inductor, the current flowing though it will steadily rise, "charging" it. It will eventually reach saturation where the magnetic flux can no longer increase and the inductor acts like a short. This is sort of like stretching a rubber band or spring. There is a point where you can no longer pull it tighter (otherwise it will break). Once this point is reached (or often just before it), the transistor turns off, and the inductor is allowed to "snap" back. Remember, the current absolutly cannot change instantaneously. so what happens is when the current is removed, the magnetic field in the inductor created collapses and induces a potential across the inductor. This voltage will slam up to infinity until a current flows between the 2 terminals.
However, in the real world, there is stray capacitance within the inductor as well as outside it, especially if a capacitor is connected. This creates a tank circuit, so the output will actually "ring," it is the equivalent to a bell, tuning fork, spring on a string instrument like guitar, etc. This can be seen on an oscilloscope with the probes connected across the inductor. You will see the ringing and even the initial voltage spike, known as a "transient voltage spike." The diode simply rectifiers this spike and transient so the voltage is DC, although quite a dirty one. (The voltage is all over the place as the inductor oscillates) the capacitor smoothes this out, offering a clean DC high voltage to the nixie tube.
Step 2: The Components
Some detail regard the components.
L1 is a fixed inductor 100 uH 1A, threeneuron's lists some similar models of it, and they could be a little different in dimensions, so my pcb has space for long and short components.
R17 and R18 should be 1% accuracy metal film resistors, to achieve a better voltage stability.
D1 has to be a ultra-fast 400V diode, as BAV21, UF4004, UF4007, MUR140, or MUR160 (thanks threeneuron's again).
Step 3: The Pcb
The pcb has been obtained in DipTrace from my schematic. Maybe you can design a better arrangement for components, I didn't etched this board because I'll incorporate the supply circuit in my nixie clock board. There is space for a big heatsink, also if probably you don't need it. In alternative you can lay down the mosfet. On the left you can see the long shape of the inductance alternative. On the lower right you find the Anode and Cathode contacts, where connect the display.
Attached to this step there are schematics, board and the ready-to-print pdf for toner transfer method.
Step 4: The Breadboard Prototype
Otherwise you obviously can insert components on a breadboard and test the circuit.
To build your breadboard prototype follow the schematic (not the board) and start adding the IC (the NE555 in this case), connect one pin at time, adding components when they're needed, then connect other components between them and to Vin and ground. Double check everything before connecting the circuit. Also check that resistors pins don't touch anything else.
WARNING (again): this circuit raises the voltage to deadly 300V so you must avoid to touch contacts while working, I'm not kidding, please BE CAREFUL!
I used a bigger pot to ease the voltage setting, and I left my circuit working with a nixie for some time (maybe half an hour) to test the heat dissipation, and neither mosfet or inductor were warm at all... so I didn't add the heatsink.
Step 5: The Glow in the Dark
Here you can see some different nixie displays, the red glow is very fascinating.
IN-1 are unpopular between hobbyists because they have a shorter life and they're not transparent on the side, but I like them, and once solved the life duration problem they will be great in a nixie clock. Ampoule displays as IN-16 or the very expensive IN-18 are nice, but I love more the top-view nixies as IN-4 and IN-12 (russian tubes had been produced further than american ones, so they're cheaper and easier to find). If you want a exhaustive description about many nixies see here.
Step 6: This Is Only the Beginning
So this is my contribution to anyone wants to build his own nixie clock, or needs a high-voltage power source to light his plasma devices (as in this impressive instructable, or also this other one) or some valve tubes.
Please comment this instructable and help me to improve the design. I'm waiting for some components to finish my clock, so that you will be able to see it very soon, anyway I know that some improvement is certainly needed and V2 of the power supply can born now...
[UPDATE: my clock is completed and published: https://www.instructables.com/id/simple-user-adjustable-DIY-Nixie-Clock/]