Preface
The much I love tubes the more I prefer my amplifiers to be linear. So for over a decade I felt the urge to build a nixie clock - but I needed a little trigger to get the ball rolling.
Some years ago a friend of mine told me that he saw a video online and fell in love with nixies. He asked me whether I know about them - at a time when I was pregnant with the idea of building a nixie clock for quite a while. We talked to another friend of us who also loves the idea of some 'vintage, early cold war days'-energy wafting through his living room. As he is experienced in working with sheet metal we made a deal that he is going to design and build a nice stainless steel enclosure.
Some years ago a friend of mine told me that he saw a video online and fell in love with nixies. He asked me whether I know about them - at a time when I was pregnant with the idea of building a nixie clock for quite a while. We talked to another friend of us who also loves the idea of some 'vintage, early cold war days'-energy wafting through his living room. As he is experienced in working with sheet metal we made a deal that he is going to design and build a nice stainless steel enclosure.
Yes, this is the n-th nixie-clock project on the internet. However, it might be somehow interesting because it does not use parts made from unobtainium and uses a little trick in the HV-supply you might not have seen before.
Design Targets
- The clock should be supplied with 5V via a mini-USB jack (MOLEX) so it is possible to use one of these ubiquitous USB chargers.
- Each nixie 'lives' on a separate daughter-board. This makes it easy to replace a nixie or to build a clock with some other tube than the IN-14 we used.
- Four IN-14 nixies (hour, minute) only. But it shold be easy to derive a 6-nixie motherboard from the original design.
- No LEDs (we think that the nixies stand for themselves and must not be accompanied by some fancy semiconductor illumination).
- No Arduino (it's pretty easy to get a running AVR µC on a cusom PCB)
- No DCF77 - for the sake of simplicity
Electronics
Driving the nixies
It seems to be a common technique to drive the nixies using the 74141 chip or its soviet counterpart. And somehow it seems to be harder to get one of these chips than some nixies. I guess that this is why some people try to build a clock using this chip only once - which leads to anode multiplexing:
- Some people use HV-optocouplers for this purpose
- And some other people use transistors
My quesions is: if you are already fiddling around with some HV-BJTs why would you opt for wasting brightness by turning on each tube for only a fraction of a given time interval (in order to get the count of obsolete chips down) and not for avoiding obsolete parts in general and designing a proper nixie-driver using some more (cheap) BJTs?
The solution presented here uses one HV-BJT (e.g. BF820) in SOT23 per cathode. These transistors and a couple of resistors are directly placed on the nixie-daughterboard.
In addition there is a 10-bit latch per tube and all latches are driven from a single BCD-to-DEC (HEF4028) converter. So we need just 8 µC pins (4x latch enable and 4 BCD-lines) to drive four tubes. Everything is orchestrated by my favourite 8-bit µC: ATmega8 which is further equipped with a 32.768kHz XTAL and two microswitches to independently adjust hours and minutes.
About power
The HV supply runs directly from the 5V input whereas all the logic stuff runs from a regulated (MC...) 3V3 rail. Therefore every logic chip (ATMEGA8L, HEF4028, SN74LVC841) is specified for 3.3V operation.
HV Section
There are several options of how to convert a given DC voltage (5V) into a higher voltage (170V). The most well-known might be the boost converter and flyback converter. Both have some drawbacks:
- The biggest drawback of the boost converter is that it's duty-cycle is directly linked to the ration of input to output voltage. In this application we would end up well above 90%. Event if in the end it is no real deal-breaker it is definitely not convenient.
- While the flyback converter does not suffer from the duty-cycle issue (as the winding ratio can be adjusted to the voltage ratio) voltage stress in the components is higher as when either the transistor or diode blocks it 'sees' the reflected voltage from 'the other side' on top of the voltage from the 'own side'. Also the current stress for the components gets higher.
So I wanted to try something else. There is a less-known hybrid form of both converters, where the inductor is uses as a 'tapped' inductor - just as in an autotransformer. Only a part of the windings are used to store energy but the full amount of windings is used to discharge the inductor.
Here you can find a little (LTSpice) simulation showing the three topologies exemplarily doing the same job: stepping 10V / 2A up to 20V / 1A. Switching frequency is identical for all converters but component values are adjusted to achieve CrCM (transistor turns on again in the very moment the core is completely demagnetized) in each case.
Theoretically it would make sense to use thicker wire for the 'primary' or 'charging' winding - but that would have been a custom choke. Without that a fancy Coilcraft LPH8045 is going to do the job. The cool thing about the 1:1:1 winding scheme is that is allows for an effective 1:3 catio in the HV supply which brings the duty-cycle well below 50% which again allows to use a cheap and simple UCC3805 without the need for slope compensation or other nasty tricks. Almost. Because this chip needs 1V on the CS-pin which is a lot of voltage drop over a shunt resistor in the transistor's source path when you only have 5V available. So I decided to throw in an INA10A1 to compensate for that.
Finally you will find a simulation of the entire HV section here.
Schematics
Manufacturing Data
Here are links to everything necessary for creating a copy of the project:
- BOM
- Gerbers
- Assembly prints (2:1 / bottom mirrored / pdf)
- Schematic prints (pdf)
- 3D data
Issues
There are two known issues with the design that were not fixed in the manufacturing data:
- issue: unfortunately I messed up the pin mapping of the Coilcraft LPD inductor
- fix: if you do not assemble it as shown in the 3D screenshot above (the silk layer does not contain any mounting info anyway) but as in the photo shown below everything is correct
- issue: C15 needs way more capacitance than shown in the SCM / BOM (this is why the respective line of the BOM is highlighted)
- fix: as replacement it is possible (also see photo below) to solder 2x 47µF 16V X5R caps in parallel (e.g.: 1210YD476MAT2A)
Software
The software of the project is super simple. Yes, I'm aware that an ESP32 board is dirt cheap and might get the time via NTP an what not. But this really is not my piece of cake. So everything can be found in one file. Sorry, but the variable names are crap - "uos" means "units of seconds" and so on.
I used ATMEL-Studio 6 (or 7 I don't remember exactly) and compiled it into a little binary that I flashed onto the controller using this adapter.
I'm still in love with this mesmerizing effect:
Housing
Well, I guess that is a real shortcoming of the project. As I'm not a mechanical guy at all I cannot provide you with any details about the housing at all. It exists and it somehow fits. This is all I know. However, as I had not foreseen any means for fixation on the PCB itself I simply glued everything into the housing which doesn't have a bottom cover.
While the electronics can be built 'as is' and the software can be used 'as is' this topic certainly needs some skills and creativity...
Cheers,
P.
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