Nixie Tube Power Supply

A flexible power supply for Nixie tubes. Input voltage can be from 3V to 12V. Output up to 200V. It uses commercially available transformers

Similar projects worth following
This project had a long gestation. Once I decided that I wanted to design and build my own Nixie clock hardware, I settled on a battery powered clock. Innocence is a wonderful thing. I had no idea how I would produce the roughly 180V needed to power it. My ideas have evolved over time, but that need for a power supply that would work from battery, never went away. There is at least one commercially available power supply that would work (google Taylor Edge HVPS), but ultimately I wanted to do this myself, partly because I wanted to be able to fit it in to a small space. Partly because, well, how hard can it be? The answer is pretty hard, at least for a novice like me.

Read the logs in reverse order if you are interested in the flow. Or any order you like. It is all good!

There are links to the KiCAD project in Github, there is a BOM with digikey part numbers in the files section (and in Github) and there is a link to the board at OSHPark in the links section.

I'm documenting this retro-actively, so I'll be adding more project logs over the next few days/weeks. The first few iterations of the power supply are done. I ended up with a couple of variations on the theme, and I am still tweaking them to get the best performance out of them. I'll add circuit diagrams, parts lists and OSHPark links in due course.

I hope that others will be able to produce their own variations of this and will share them with the public. For me, I need to move on to actually using then in some Nixie clocks!

What seemed to be apparent from the outset was that my design should use a transformer. Nearly all of the power supplies I have seen are boost converters. These are quite simple in principle - at least using physical principles, not engineering ones. They rely on a collapsing magnetic field in an inductor to generate a voltage, the term I was taught for this was 'back-EMF'. To get it to generate more volts out than you put in, you want the field to collapse faster than you build it up. The power stays the same (well, obviously less because of losses in the system). So volts up, current down.

A single inductor can typically give you in the order of a 10x increase in voltage. It seems obvious that one way to go higher is to use a transformer. i.e. let the transformer boost the voltage generated by the back-EMF. Apparently, these are called flyback transformers. You can, quite literally, replace the inductor in a boost converter design with a transformer. At least in principle. Maxim have a great tutorial on this exact idea. 'Great', I thought, 'My search is over!'. Except that the transformers it uses just aren't available. In fact, it turns out, the transformer was going to be the biggest problem. Closely followed by everything else. But seriously, the transformer is the biggest problem. It seems like you can find a suitable inductor for a 12V-180V boost converter in a Chrismtas cracker. Suitable transformers are as rare as hen's teeth. Most people just make their own. That was a step too far for me. I'm too lazy.


From the instructions that come with their model. This is for a different transformer than the one I used, so the values will be different.

JPEG Image - 40.20 kB - 11/11/2017 at 22:07



Bill of Materials - includes DigiKey part numbers.

Comma-Separated Values - 997.00 bytes - 11/10/2017 at 01:28


Boost converter chips.xlsx

Controller chips I examined for use in the project. See the logs for deciding factors.

sheet - 36.66 kB - 10/29/2017 at 18:09


  • Wrap-up

    Paul Andrews11/10/2017 at 01:49 0 comments

    You can get the board on OSHPark. The full KiCAD project and LTSpice model is in my Github repo. There is a full BOM with digikey part numbers in the files section of this project and in the Github repo.

    BTW, I tried reducing Rsense to 0.04Ω. The temperature of the transformer peaked at 160°F at an ambient of 70°. Still well below the absolute maximum of 257°F. I will produce a set of figures for this configuration too, as the supply can provide a lot more current with this lower value of Rsense

    And the MOSFET:

    A chart showing the maximum current for a given input and output voltage. I determined this from the readings that follow, by picking the maximum current I could measure before Vout started to be un-regulated. So another way of saying this is that it shows the maximum current for a regulated Vout.

    There might be thermal issues with running at these maximum currents. I couldn't really measure that because my load resistors need some serious heat sinks added first.

    In the readings below it is useful to note that for a given load and Vout, the output current remains constant regardless of the input voltage (until the output voltage becomes unregulated)

    Readings for a Vout limit of 200V:

    12V in
    Rload Vout Iout (mA) Vin Iin (mA) Eff
    19.31 199.8 10.40 11.96 237.90 0.73
    14.64 199.7 13.70 11.96 298.20 0.77
    10.00 199.4 20.06 11.95 414.00 0.81
    6.86 199.1 29.23 11.93 595.00 0.82
    4.65 188.4 40.00 11.88 765.00 0.83
    5V in
    Rload Vout Iout (mA) Vin Iin (mA) Eff
    19.31 199.3 10.38 4.80 600.00 0.72
    14.64 199.2 13.68 4.72 717.00 0.81
    10.00 177.2 17.82 4.63 865.00 0.79
    3.7V in
    Rload Vout Iout (mA) Vin Iin (mA) Eff
    19.31 199.0 9.30 3.67 865.00 0.58
    14.64 181.0 12.30 3.65 890.00 0.69
    10.00146.0 14.00 3.49 906.00 0.65

    Readings for a Vout limit of 177V:

    12V in
    Rload Vout Iout (mA) Vin Iin (mA) Eff
    19.31 177.8 9.26 11.97 195.50 0.70
    14.64 177.7 12.20 11.96 243.10 0.75
    10.00 177.6 17.86 11.96 345.50 0.77
    6.86 177.3 26.00 11.94 471.00 0.82
    4.65 177.0 38.23 11.91 678.00 0.84
    5V in
    Rload Vout Iout (mA) Vin Iin (mA) Eff
    19.31 177.2 9.22 4.84 502.00 0.67
    14.64 177.2 12.14 4.77 620.00 0.73
    10.00 177.0 17.76 4.63 870.00 0.78
    6.86 152.0 22.40 4.60 930.00 0.80
    3.7V in
    Rload Vout Iout (mA) Vin Iin (mA) Eff
    19.31 177.4 9.24 3.82 610.00 0.70
    14.64 177.0 12.17 3.64 900.00 0.66
    10.00 144.215.00 3.49 914.00 0.68

    Here is my new test load rig. An extra resistor and a smaller form factor! I really need to get some heatsinks though. Or even better, an actual test load!

    Here's a much more attractive test load being driven by a liitle LiPo battery:

  • Final Component Selection

    Paul Andrews11/08/2017 at 15:12 0 comments

      I got my thermal camera, so the results are in! It looks like the peak temperature I have managed to get for the transformer is 129°F. Here are some pictures. The first shows the temperature of the transformer. This is when the power supply is producing 20mA at 200V with 12V in:

      Next up, the controller:

      And next the MOSFET:

      Is that OK? Well the data sheet for the transformer says:

      It is recommended that the temperature of the part does not exceed +125°C under worst case conditions. 

      125°C is 257°F, so we are well under that. These measurements were taken with an ambient temperature of around 70°F. We could expect worse behavior in a closed case, but I think we may still have some headroom, so it could be that the current sense resistor value that I chose could be reduced some. I will try this with a 0.04Ω resistor and check the temperatures again.

      I will list the final component values in a moment, but for now here are some performance results:

      Rload (K) Vout Iout (mA) W Vin Iin (mA) W Eff
      20 199.6 10.40 2.08 11.96 238 2.85 0.73
      15 199.5 13.70 2.73 11.96 298 3.56 0.77
      10 199.2 20.00 3.98 11.94 415 4.96 0.80
      20 177.6 9.26 1.64 11.96 195 2.33 0.71
      15 177.5 12.18 2.16 11.96 243 2.91 0.74
      10 177.4 17.84 3.16 11.96 347 4.15 0.76
      Rload (K) Vout Iout (mA) W Vin Iin (mA) W Eff
      20 192.4 10.00 1.92 4.85 500 2.43 0.79
      15 172.6 11.84 2.04 4.84 502 2.43 0.84
      10 141.0 14.17 2.00 4.8 508 2.44 0.82
      20 177.0 9.22 1.63 4.85 500 2.43 0.67
      15 172.5 11.81 2.04 4.84 502 2.43 0.84
      10 141.0 14.18 2.00 4.8 508 2.44 0.82
      Rload (K) Vout Iout (mA) W Vin Iin (mA) W Eff
      20 174.0 8.80 1.53 3.6 373 1.34 1.14
      15 154.0 10.34 1.59 3.6 370 1.33 1.20
      10 130.0 12.80 1.66 3.6 364 1.31 1.27
      20 172.0 8.80 1.51 3.6 380 1.37 1.11
      15 153.0 10.30 1.58 3.6 373 1.34 1.17
      10 129.0 12.70 1.64 3.6 370 1.33 1.23

      For each input voltage, there are two sets of numbers. The first set is for a Vout limit of 200V. The second set is for a Vout limit of 177V. Here are some takeaways:

      1. My measurements for 3.7V are clearly wrong somewhere, because they give an efficiency of > 1!
      2. Ignoring that, we are hitting efficiencies of between 70% and 80%, which is pretty good!
      3. We can get 20mA out of it if we put 12V in. We could probably get more, but my test equipment (such as it is) can't go lower than a 10K load without starting a fire. In fact, the thermal camera shows that my 10K load resistor was hitting 400°F as it is! Anyone want to buy me some lab equipment? 20mA will power just about any 6 tube clock.
      4. On battery, we can get 8mA out, which is enough for 4 to 6 small tubes. Which is fine.
      5. On USB (aka 5V), we can get roughly 12mA out at a decent voltage. That is good for a lot of tube types. If I can decrease Rsense, so I can get more current flowing, we could improve these numbers.
      6. The voltage regulation starts to fail as load goes up and input voltage drops. Again, I know from experience that if I can drop Rsense, I can get a more stable output voltage

      So here is the final circuit diagram:

      1. The MOSFET is an ...
    Read more »

  • Adding controls

    Paul Andrews11/06/2017 at 15:05 0 comments

      For the final design, I also wanted to be able to provide a means to enable/disable the power supply and vary the output voltage.


      Enabling/disabling the power supply is necessary if we want to implement tube blanking:

      1. We can turn the nixie tubes on or off based on a schedule. This is typically done to extend the lifetime of the tubes.
      2. In a battery powered clock, we only want the tubes on when the user is actively looking at the clock, otherwise we will run out of battery pretty fast.

      Note that this is not used for dimming the display - we don't want to turn the entire supply on and off really fast. This is better done using direct control of the current flowing through the tubes themselves.

      The LM3478 already has an enable pin, so we can use that in our design.


      Varying the output voltage is handy for several reasons:

      1. At low input voltages, the power supply is more able to regulate itself if the output voltage is also lower.
      2. Other applications of the power supply might require a different output voltage.

      There are a couple of solutions to varying the output voltage, but the one that I went for is explained in an article by Simon Bramble. This solution provides several benefits:

      1. It enforces a limit on how high the voltage can go.
      2. It allows us to vary the voltage from zero to that limit.
      3. It allows us to control the voltage using PWM from an MCU.
      4. It doesn't require a potentiometer on the board itself.

      It has some drawbacks too:

      1. It requires more breadboard wiring to set the output voltage. An on-board potentiometer would provide a simpler solution.

      If the design is integrated directly with a clock - rather than being a separate power supply board - you could just set the voltage to whatever you need directly.

      Anyway, this is the basic resistor setup that I will be using. R1 and R2 are already part of the design. The new part is R3 to some voltage Vset. We also add Radj in series - R3 will be fixed: 

      So, directly from Simon Bramble: The current flowing through R1 plus the current flowing through R3+Radj must equal the current flowing through R2, because no current flows into Vfb. So we have (folding Radj into R3 for now):

      In our case Vfb is 1.26. If we substitute that in and re-arrange for Vout, we get:

      The first term is just the regular equation, without R3 and Vset. The second term shows the effect of including R3 and Vset. For now, let's assume that Vset and Radj are zero. It is clear that as R3 increases, the effect of the second term is lessened. At the limit, if R3 is infinite, the second term has no effect at all. As R3 decreases in size, the second term adds to the value of Vout. If R3 is is zero, we are in trouble, so we must choose some minimum value for R3.

      If we allow Vset to increase, we can see that the second term reduces the value of Vout. The lowest value for Vout is zero. I will arbirtrarily choose 5V to be the highest value of Vset, because it is a common logic level. So we want Vout to be 0V when Vset = 5V.

      Further, to make things simple, I will arbitrarily choose R2 to be 10K. This is what Simon did, and it as about...

    Read more »

  • Ringing

    Paul Andrews11/04/2017 at 23:50 0 comments

    It is time to address the ringing problem that we have seen in the simulation, and in the prototype. This analysis uses the AON6242 used in the original simulation.

    The problem is that when the MOSFET turns off an oscillation in the drain voltage is initiated caused by the leakage inductance of the primary transformer winding and the output capacitance - Coss - of the MOSFET. The issue with this is that it can cause the peak voltage on the drain of the MOSFET to exceed the the rating - the ringing voltage can often be twice the reflected voltage, which is kind of unnecessary.

    Here is a picture from the simulation showing the un-damped oscillations:

    Here is what it looks like with some damping:

    What you can see here is that the reflected voltage is around 40V. This is what the MOSFET should have to deal with. The peak voltage in this is down to 80V from 120V, which is an improvement, and the oscillations die out very quickly.

    There are a couple of solutions. One is to clamp the voltage with a diode. Another is to dampen the oscillations with an RC (Resistor-Capacitor) snubber. Another solution is to do both. I chose to damp the oscillations with a snubber, because we also don't want the circuit to be radiating EMI, so dampening the oscillations is a good thing to do anyway. I chose the topology in this application note. I started by setting the capacitance (Csnub) to 2xCoss. This halves the frequency of the ringing. The resistor value is set to:

    With LTSpice we can measure the ringing frequency as follows:

    So the frequency is 9MHz. From the datasheet for the AON6242, Coss is 540pF, so Csnub is around 1200pF. This gives an Rsnub of 15Ω.

    You can see that the frequency has approximately halved in this image:

    We can damp the ringing more by increasing the resistor value, but this will dissipate more power. According to LTSpice, the RC snubber dissipates this much power:

    About 0.4W. This is quite a lot, so I am left wondering if there is a better way to do this, or even if it is necessary at all.

    The whole circuit now looks like this:

    Here is what I measure with the actual prototype with no snubber (remember this uses an IRL640A, not an AON6242):

    Note that channel one has a 10x probe, so each square is actually 20V. We can see that the peak is only around 60V, so there is no real need for a snubber here, but we will continue just to test it out. Note that the frequency of the ringing is 1/50ns or 20MHz. Coss for the IRL640A is only 200pF, so we only need a Csnub of 400pF, and our Rsnub is now 20Ω. Here is what it looks like after applying the snubber:

    So yeah. It's damped. Seems a bit unnecessary with this prototype, but I will leave it in the circuit when I have some actual PCBs fabbed - I can always leave it unpopulated, but I can't add it after the fact.

    BTW, I re-ran the simluation with the IRL640A, and the frequency of the ringing is very similar: 17MHz.

  • Prototype

    Paul Andrews11/03/2017 at 02:19 2 comments

    After I had spent a month or so messing with LTSpice, I decided that I really needed to build something to try it out. From what I had read, trying to prototype this power supply using a breadboard would not be a good idea. Breadboards have too much additional resistance, capacitance and inductance that would just mess up the results. The next best thing, I decided, was to use perfboard - with this I would hopefully suffer less from all the drawbacks of breadboard.

    I started by designing an adapter for the transformer and having it fabbed at OSHPark. I also ordered a SOIC8 adapter from there for the LM3478. This makes it easier (i.e. possible) to incorporate them into perfboard. Then I ordered all of the parts I thought I would need from Digikey. For the key components such as the output capacitor, the current sense resistor and the loop compensation resistor and capacitor, I ordered the values from the simulation (or close to them) and a few either side. It turns out that this was really important - the performance of the physical circuit was very much affected by some of these values.

    This was the wiring diagram for the transformer:

    This is what it looked like all wired up:

    It looks a real mess in that picture, because that is what it looks like now, after it had been inserted, removed and re-wired several times. It still works though!

    This is what the finished prototype looked like (or looks like now!):

    There are some parts on the reverse side too. One important note here: It is obviously important to pay attention to the specs of the parts you order. Wattage on resistors. Voltage and ESR on capacitors. But one that is often overlooked is the voltage rating of the resistors. Typically this is around 100V, however the feedback resistor has pretty much the entire output voltage across it permanently, so that specific resistor is rated to handle it.

    An interesting one is the current sense resistor. I ordered some wire wound resistors, which was a mistake because they introduce inductance to the circuit. The non-wire wound resistors worked fine.

    I think I over-spec'd the diode!

    What followed was a comedy of errors. Firstly, I had the output stage connected the wrong way around. Remember this diagram?

    Those dots are important. I had them the wrong way around. I fixed it by swapping the inputs and presto, I was in business. However, I most definitely was not getting 186V, except with no load (actually 184.6V with no load. Near enough). BTW, this is what my test load looks like:

    Three high power resistors connected in series. I had considered building a test load, but I didn't want to get diverted into yet another project!

    I spent the next few weeks changing the values of various parts. One of the key ones turned out to be the frequency setting. I started with a resistor of 24k, which should have given me a frequency of about 600KHz. I gradually reduced it, and as I did so the performance kept improving. I ended up with a resistance of around 47k, which gives a frequency of about 350KHz. At that frequency, varying the other values didn't make much difference. However I noticed that the IRL640 was getting really hot. Apparently LTSpice doesn't model the thermal characteristics of the components. I could actually see the output voltage dropping as it warmed up ('warm up' is an understatement!).

    My reasoning was that the Rds(on) of 0.18Ω was just too high, so I searched for a replacement with lower Rds(on) and came up with the IRLI2910. This has an Rds(on) of just 0.026Ω. When I swapped this in to the circuit, it never got warm and my output voltages improved - less power was being dissipated...

    Read more »

  • Refining the Design

    Paul Andrews10/30/2017 at 00:23 0 comments

    I'm dealing only with the simulation here. I'll get to actual component selection when I discuss the prototype and the finished board.

    Input Voltage

    I chose 5V as a target input voltage. I could run the simulation at 12V, but that isn't my target input, that is just something I would like my power supply to be able to handle. I also want it to handle a LiPo battery, so I need to run the simulation at 3.7V too.

    However, the big problem with the first simulation is that there is no internal resistance specified for the voltage source. A real voltage source does have an internal resistance, and this causes the input voltage to drop as more current is drawn. I change the simulation to use an IR of 0.5 Ohms. This seemed to produce a realistic drop in the input voltage.

    Output Capacitor

    The output capacitor (C2) serves to smooth out the ups and downs of the output voltage by storing charge during the on phase of the diode and providing it when it is in the off phase. If the capacitance is too small, the output will have a ripple. If it is too large, it will take too much power to charge. 0.22uF is lower than most designs seem to recommend. I tried up to 1uF in the simulation. The only effect was to lengthen the ramp-up time, so I stuck with 0.22uF.

    Capacitors have a resistance called ESR (Equivalent Series Resistance). This affects how quickly it can discharge and, of course, dissipates power. The lower the better. My model used 0.025 Ohms.


    The transformer model provided by Wurth actually models things like saturation current, so I was intrigued as to how much I could affect the simulation by varying this resistor. I dropped it as low as 0.02 Ohms, which had no effect. I raised it 0.1 Ohms and this completely destroyed the circuit's ability to generate a high voltage. So I stuck with 0.05 Ohms!

    Input Capacitor

    This capacitor smooths the input voltage by charging while the MOSFET is off and providing power when it is on. Like the output capacitor, you want to have a low ESR so that as much power is delivered as fast as possible to the transformer. I left this value completely alone and defined an ESR of 0.074 Ohms.

    Ccomp and Rcomp

    And so we come to loop compensation. Do you know what Bode plots are? How about poles? Zeros? Right half planes? No? Me neither. Basically, what these values do is to control how quickly the chip responds to changes in load to keep the voltage constant. If it reacts too quickly the output voltage will jump up and down. If it reacts too slowly, the output voltage might never adjust to the load before the load changes again. I messed with these values. If I increase Ccomp (to say 220nF) the simulation overshoots the target, then drops, then overshoots. etc. If I decrease it, there is a ripple in the output. In other words, 22nF seems about right. Changing the resistance has no noticeable effect.

    If you are interested, this is all dealt with in note AN-1286. There is an online bode plot tool here or here. I did try and use the math to calculate the ideal values. I worked through the example in AN-1286 using excel and one of the online bode plot calculators, but my numbers didn't tally with theirs. I would be interested if someone could do this, though it doesn't really affect the actual circuit. I asked the folks over on Neonixie about this. They said 'Just build the prototype!'.

    The MOSFET

    Finally, the MOSFET. Nick de Smith has some recommendations:

    Select the FET for low RDSon, Qg and Coss

    So I did. I changed the FET to an IRL640A. Basically, again, we want the MOSFET to consume as little power as possible.

    However we need some other characteristics:

    • Vds needs to be able take the reflected voltage from the secondary. The IRL640A has a Vdsof 200V!
    • Vgs(th) needs to be significantly below the...
    Read more »

  • LTSpice

    Paul Andrews10/28/2017 at 03:34 0 comments

    Oh boy, where do you start with LTSpice? That’s precisely the problem I had. It seems like it is purpose built to model power supplies. Perhaps this is a biased point of view though, because that is where I discovered it. However it took me a long time to become comfortable using it - I found it’s interface to be particularly clunky. I’m pretty used to it now though. The one thing I still dislike strongly is how to add new components and sub-circuits to it. And we will need to do that to model my power supply. There is a tutorial here, a dedicated wiki here, and a yahoo group here.

    I spent months, literally, simulating my circuit in LTSpice, trying to become comfortable with what each component was doing in the circuit. This is not to say it was difficult, in fact my very first circuits pretty much worked from the start, but past experience had shown me that something that works fine in LTSpice can fail miserably in real life. So I wanted to get comfortable that I had produced the best simulation I could, i.e.:

    • Used real-world components rather than ideal components.
    • Explored the limits of what the circuit could do in terms of power sources, loads and component variations.

    The circuit I ended up using was pretty much lifted straight from the LM3478 datasheet but with a flyback transformer in place of the inductor. TI provide a ton of information about this IC, including how to use it in a flyback converter! You will find a ton of math in these papers. Especially, when it comes to flyback converters, just about every reference you will find is aimed at figuring out what the specification of the transformer should be. This is great if you are going to wind your own transformer. We aren't, we actually want to start with the transformer and work back. I went down the rat hole with these calculations. What I found, in the end, was that most of them don't really matter. The circuit pretty much works out of the box. LTSpice allows you to get close to the right values for the components. A few prototypes let you fine tune it.

    What is important is paying attention to the physical characteristics of the board. This was emphasized by Nick de Smith's design for a MAX1771 based Nixie power supply, and rammed home by looking at the datasheet for the LM3478 evaluation board (like I said, TI provide a ton of information about this IC).

    Anyway, I digress.

    The first thing I did was to download the spice models for the LM3478 and the Wurth Flex Transformers and set them up for LTSpice. Then just forged ahead and entered the circuit. It looks like this:

    • U1 is the LM3478.
    • U2 is the transformer.
    • D1 is the output diode. I wasn't picky, I just chose one from the LTSpice standard parts that has a breakdown voltage of around 350V.
    • R2 and R3 together set the output voltage. The LM3478 tries to keep the FB pin at 1.26 volts (typically). So the output here should be:

      Or 186V (typically).

    • Rload would be nixies in a clock. Using 10K here means we are draining 18.6mA. That would be 3mA each for a clock with six tubes.
    • V1 is the input voltage, which I have set to 5V.
    • Rfreq sets the frequency of the PWM signal being placed on the gate of the MOSFET.
    • That MOSFET (AON6242) was just selected from the standard set of MOSFETs that LTSpice knows about. It has a 60V Vds - i.e. it can withstand 60V across the drain-source. 60V should be OK, because we are keeping the output voltage below 200V. So as a rough first estimate, we would expect the maximum voltage on that side of the transformer to be at most 40V. This is because the turns ratio of the transformer is 1:5. So 200/5 is 40.
    • Ccomp and Rcomp control the loop compensation - I will get into that later. These values will do for now.
    • R4 is the current sense resistor: The LM3478 monitors this and will cut off the MOSFET if the current goes to high. We want the current to stay below the saturation current of the transformer. 0.05 Ohms will do for now.
    • The pieces...
    Read more »

  • Theory of Operation

    Paul Andrews10/28/2017 at 02:16 0 comments

      As mentioned earlier, the basic idea of a boost converter is to use the collapsing field in an inductor to generate a large voltage. To do this you need to be able to turn the current on and off. This is done using a transistor, usually a MOSFET.

      This is the typical diagram used to explain this:

      When the switch is closed, the current flows through the switch. When it is open, the field in the inductor collapses and generates an additional voltage in the same direction as that from the power source. The current can't flow through the switch, so it flows through the load. The capacitor provides current to the load while the switch is closed. The diode prevents this current from flowing back in to the inductor. After startup the diode blocks current flowing from the left side of the circuit while the switch is closed because the voltage on the right side is greater. This page gives one the best explanation of this I have seen.

      As stated earlier, a flyback converter essentially replaces the inductor with a transformer, like this:

      The rest is the same. This gives us two advantages:

      1. We get the voltage multiplying effect of the transformer (at the expense of current, conservation of energy and all that).
      2. The output is isolated from the input. With a boost converter, if the switch is left open the output voltage will be the same as the input voltage. Which is interesting, but not why we want to use one.

      Generally, you want the switch to be closed longer than it is open. The longer it is closed, the more energy is pumped in to the magnetic field of the inductor, so the more power will come back out in the form of voltage (give that the current is constant), once we open the switch. This ratio of on to off is called the duty cycle. An 80% duty cycle means that the switch is closed 80% of the time. This on-off mechanism is also called PWM (Pulse Width Modulation). It is a square(ish) wave. In most documentation about boost converters, they will talk about small duty cycles - say 60%. In Nixie power supplies, 90% is not uncommon because we want to boost the voltage a lot!

      In addition to this ratio, there is the period of the wave. After a certain amount of time, we will have pumped as much energy in to the inductor as it can take - the magnetic field won't get any stronger. There is no point pumping in any more. The larger the inductance, the more energy we can pump in. Unfortunately, large inductance and large physical size go hand in hand, and we want a small inductor. This effectively means that we need to switch the current faster so as to avoid saturating the inductor.

      So smaller inductor => higher frequency.

      If the frequency gets too high, circuits start behaving oddly, so we don't want to go too high. 600KHz seems like a recommended upper limit. Of course this also puts limits on how small the inductor can be.

      This is all relevant to our design - we will be adjusting all of these things to try and come up with the best solution we can, given the actual products available to us.

  • Switching Controller

    Paul Andrews10/27/2017 at 22:02 0 comments

    Apart from the transformer, I need to find a suitable switching controller. The key to a good power supply is that it produces a stable output. The keyword here is 'regulated'. To produce a regulated voltage, the power supply needs to be able to adjust to the load being placed on it (or, I guess, be so powerful that any load you put on it is negligible). There are quite a lot of tutorials on the internet for building Nixie power supplies, including some that use transformers (none that use an off-the-shelf transformer, hence this little project) and most of them use a switching controller. Here are some:

    • Coin cell-powered supply. I built this one. It isn't regulated, but it uses a transformer. I learn't a lot about batteries building this :)
    • A regular boost converter. This one has invaluable advice about the actual engineering of building something that works. It uses a regular inductor, not a transformer.
    • Obsolete Time Lite, right here on Hackaday. This actually uses a transformer. I was really excited! But I couldn't figure out which transformer, and the project seems to have been abandoned. My big takeaway from this was the specific controller it used.

    There are many more, and I'll provide a list from my bookmarks at the end, but my point in listing these is to give a sample that use different controllers. So, I created a spreadsheet of some of the controllers that included a list of specific features. This enabled me to focus on a few that had everything that I wanted. My key requirements were:

    • Needed to operate on an input voltage of 3V-12V.
    • Needed to be able to produce a high duty cycle, say 90%
    • Needed to be able to drive an external MOSFET - rather than having a built-in transistor.
    • Needed to have a way of enabling and disabling the output.
    • Needed to draw a small amount of current in disabled mode.

    I tried the MC34063 first, even though it didn't have a way to disable it - it is widely used and I could get an LTSpice model for it (modelling is a big part of this, as will become apparent).

    I tried the LM3478 next, as used in Obsolete Time Lite. This is as far as I got! This made it all the way into my finished supply.

    Next up would have been the MAX1771. Everyone swears by it. Many apparently swear at it! It is allegedly very finicky.

    After that would have been the LT1619. In many ways this seems like the ideal choice, I just never got this far - the LM3478 seems to work just fine. Plus I didn't want to be swayed by what the excellent Taylor Edge HVPS uses. Part of the reason for this project is to produce my own power supply.

    All of these companies sent me samples. Something I am very grateful for.

  • Transformers

    Paul Andrews10/27/2017 at 22:02 0 comments

    And so began the search for a transformer. I had thought that this would be easy. Specify the turns ratio, inductance of the primary and secondary and power handling capabilities and presto. Sadly, turns ratio seems to be about the last thing you can search on. Unlike inductors, which can generally be searched for like resistors and capacitors, transformers seem to be made with very specific applications in mind, and 'Nixie clock power supply' isn't one of them. When I did find transformers with the right kind of inductance they almost invariably turned out to have a 1:1 turns ratio - isolating transformers in other words. I vented on the nixie clock google group, which is full of exceedingly smart and tolerant people. The kind who would think nothing of winding a couple of transformers before breakfast. I strongly recommend it (the group, that is).

    So what was I looking for? This:

    • Turns ratio of about 1:10. From what I had read so far, this would be ideal.
    • Primary inductance of a few uH. Again, from what I had read.
    • Small size. Because.
    • High saturation current. This is important. If you try and force too much current through an inductor, it stops being an inductor. The net effect is that it acts as a resistor - usually quite a low-value resistor - effectively shorting your inputs. Plus high current capability helps to stop the transformer melting.
    • Low resistance. This lowers losses due to resistance (duh!).
    • High voltage isolation - you don't want 200V to break down the internal insulation.

    Of course, some of these are trade-offs: Small size, high power? Sure.

    In the end I scoured the Wurth Electronics and the Coilcraft web sites. Both companies produce a range of transformers that can be wired in multiple configurations - they contain six inductors wound on the same core. I found these on the Wurth Electronics web site. They seemed to be about right, and Wurth were kind enough to send me several samples of each of them. Wurth also have LTSpice models of their transformers - a major benefit that I will get into later.

View all 10 project logs

Enjoy this project?



roger_archibald wrote 11/03/2017 at 03:18 point

GM tubes draw virtually ZERO current when there is no count rate.  Current increases with count rate, but unless you put it in front of a strong source we're still only talking about a couple of uA actual load from the tube.  Depending on the PS topology you go with, a voltage doubler might be a valuable addition to the design.  The extra cap/diode consume a bit of PCB real estate but if it will afford you saving space elsewhere it might be worth it.  

  Are you sure? yes | no

Paul Andrews wrote 10/28/2017 at 02:35 point

As I understand it, Geiger Muller tubes don't need much current. Both Wurth and Coilcraft have small transformers with a large turns ratio. I used the 1:20 version at this page in one variation of my power supply: 1:20 is pretty close to 1:17 (though the devil is in the details with these things). My version that uses this transformer can power six Nixies from a single LiPo battery.

That range of transformers goes up to 1:100!

  Are you sure? yes | no

Bharbour wrote 10/29/2017 at 13:47 point

That looks pretty useful, thanks

  Are you sure? yes | no

Bharbour wrote 10/28/2017 at 02:21 point

I have been looking at a  power supply for a Geiger Muller tube. Same issues, higher voltage (about 400-450V). Someone was using Cold Cathode back light transformers for a similar power supply, and they look pretty good. about 17:1 turns ratio. Sadly, Digikey quit carrying the transformer that was still available, Mouser has a bunch in stock still. I have not built a supply yet though...

Good Luck, I am looking forward to see how this comes out.


  Are you sure? yes | no

Similar Projects

Does this project spark your interest?

Become a member to follow this project and never miss any updates