Newbie's powering issues

[Zyraj]

Hello my fellow nixiers! This is my first post in this awesome group so before I ask my question I'll introduce myself, I'm Cris from Chile, IT Engineer with a little experience in electronics, not so much in the powering area, and I just started looking at nixies a few months ago. So I bought some tubes, did some research and finally picked this sketch for simplicity, then started testing.

Now I find myself with a technical problem, I'm putting together my power source with a design that uses a MPSA55 transistor, unfortunately this is impossible to find where I live (I even read somewhere that it was discontinued), so I tried with a MPSA92 with little success (got an output of around 5-8v with an input of 9v "lol"). 

If someone could guide me on how to replace this transistor I would be tremendously grateful. I attached an image of the design in question and the link from where I'm getting the instructions is this:

https://www.instructables.com/id/High-Voltage-Power-Supply-for-Nixie-and-Valve-Tube/

Also mi nixies are the B-5052, for which I need around 150v and I pretend to power 4 of those with this power supply.

Any advice is welcome!!

[Zyraj]

Forgot to mention that also the mosfet get's EXTREMELY hot, probably because of the mpsa92

[gregebert]

Welcome to the hobby. Be careful because it's very addictive.

Do you have access to an oscilloscope ? It's the fastest way to debug circuit problems.

I would remove L1 from the circuit and make sure it's properly oscillating. If you dont get it to oscillate properly, it wont work.

Next, I would jumper the connection between R17 and R19 to your +9V supply. By varying R19 you should see the duty-cycle if the waveform change.

If that's all good, remove the jumper and install L1. Now your supply is capable of generating high-voltage.

I dont recognize the driver circuit around the MOSFET, which includes Q15. It's a very strange topology (to me, at least).

[Paul Andrews]

Probably not what you want to hear, but I would try the design on this page: https://threeneurons.wordpress.com/nixie-power-supply/

[Zyraj]

Hey Greg!! Thanks for your advice! Sadly for now I don't have access to an oscilloscope, I guess I could build an rpi one so I will look into that later, will keep all this information at hand and try it out. I will post the results if I get any success!!

[Zyraj]

Hey Paul! thanks for the link mate, it actually seems like the way to go, the problem is to find the exact parts around here, I'm gonna give it a try, as long as I can get 150v from it I'm good :D

[GastonP]

Q15 is the usual active current sink that us used in so many mosfet switching circuits to drain the gate of electrons when the voltage at the input goes from positive to zero. The part is not critical and as Paul already said, Mike's pile o'poo is a good reference on that respect.

I'm sure a BC327 will be a good substitute for the MPSA55 for this application, and you will find plenty of them in Chile. The same is valid for the MC34063. It's a very common part and really worth the effort to substitute the 555.

[Zyraj]

Hey Gaston! Thanks for your input I really appreciate it, This week I'll try and get the parts you mention, been investigating and learning a lot about powering and Threeneuron's Pile o'Poo is definitely a great starting point. How do you feel about the MPS8598? I think I can easily get some of those tomorrow.

[GastonP]

The MPS8598 seems to be a good prospect, go ahead with it.

Now regarding to why use an MC 34063 instead of a 555, it's because it's specifically designed to be used as a switching power supply regulator, and us such it has short circuit protection and a wider range of control that a 555 doesn't simply because it was designed as a general purpose timer. The power supply controller is just an example of its flexibility but having better alternatives at hand (and probably cheaper), I'd go with the specific ones. Then also 34063 are common as dirt, so it's quite probable you will find them easily in Chile too. There are of course better alternatives but they are difficult to be found and need circuit layout knowledge or they will fail. You will find good and tested examples on Mike's site too.