[neonixie-l] Re: MMBTA92 & MMBTA42 anode switching circuit problem

[Tomasz Kowalczyk]

Nick, Martin, thanks for the read!

W dniu wtorek, 23 stycznia 2018 00:56:25 UTC+1 użytkownik gregebert napisał:
>
> There is 1 other thing that could be happening, and although it *wont* 
> cause ghosting, it *will* add confusion from the measured waveforms: If 
> the PNP driver turns-on momentarily, for a few usec, it will charge the 
> parasitic capacitance (scope probe AND nixie tube). If the duration is 
> short enough, the tube will not ionize so there is no path to discharge the 
> parasitic capacitance. From the waveforms, you can see the voltage level is 
> below the ionization voltage. You can see the classic RC-discharge 
> characteristic from the waveshape.
>
> I dont know how the drive-signals to the NPN pre-driver are generated, but 
> if they come from a shift-register, rather than a parallel output register, 
> it could explain what you are seeing on the scope (strange and unexpected 
> anode voltage when it should be off). Decode-glitches are a reality in 
> digital logic; whether or not they are a source of trouble depends upon how 
> the surrounding logic is designed. In this case, I think they are a source 
> of confusion, and not actual trouble.
>
> As far as the ghosting is concerned, how much blanking time are you using 
> ? I saw 1 mention of 100usec above. What happens when you use a much longer 
> blanking time, say 1msec (which should be gross overkill) ?
>
> I'm using direct connection to a STM32 for every anode and cathode, no 
shift-registers or other driver chips, just anode switches as above and 
simple 10k resistors + MMBTA42s for cathodes. I'm using direct port 
commands - clearing all anodes at once for blanking, then clearing whole 
port at once and setting an anode and a cathode within a short period. I 
might try reversing it to "set cathode and then anode".
I can see the RC discharge, too. It would make much sense if not the drops 
during blanking time - anodes are pulled pretty close to 0 during blanking, 
yet they get to higher voltage again later  - and they conduct, as ghosting 
happens, not only as these waveforms, but also as visible glow of other 
digits.

Blanking time is about 160us on later photos. I tried going higher, but 
even with 400us I'm seeing visible ghosting. For 1ms I'd had to tweak the 
timer, which I surely will do.

I have one test in mind - I recall having a fully differential probe at 
work. I'm going to measure anode resistor voltage drop - it will tell me 
exactly *when *the tubes are leaking (and hopefully I'll be able to 
calculate the current flowing). But that has to wait for a week or two, 
unfortunately. I'll perform all the tests suggested by you (BTW big thanks 
to every single one of you for helping) and port tweaking mentioned above 
then.
 

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[neonixie-l] Re: MMBTA92 & MMBTA42 anode switching circuit problem

[gregebert]

There is 1 other thing that could be happening, and although it *wont* 
cause ghosting, it *will* add confusion from the measured waveforms: If the 
PNP driver turns-on momentarily, for a few usec, it will charge the 
parasitic capacitance (scope probe AND nixie tube). If the duration is 
short enough, the tube will not ionize so there is no path to discharge the 
parasitic capacitance. From the waveforms, you can see the voltage level is 
below the ionization voltage. You can see the classic RC-discharge 
characteristic from the waveshape.

I dont know how the drive-signals to the NPN pre-driver are generated, but 
if they come from a shift-register, rather than a parallel output register, 
it could explain what you are seeing on the scope (strange and unexpected 
anode voltage when it should be off). Decode-glitches are a reality in 
digital logic; whether or not they are a source of trouble depends upon how 
the surrounding logic is designed. In this case, I think they are a source 
of confusion, and not actual trouble.

As far as the ghosting is concerned, how much blanking time are you using ? 
I saw 1 mention of 100usec above. What happens when you use a much longer 
blanking time, say 1msec (which should be gross overkill) ?


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[neonixie-l] Nixie driver chips

[andybiker]

I know there were several plasma driver chips used in the early 90s (I have 
some SN751518s used from a laptop neon display)
I have data on these and they are open drain - ideal nixie drivers but 
these chips have been obsolete for years.

Modern-ish plasma televisions use newer chips.
I recently scrapped a plasma tv and it uses SN755870 (I also have a board 
containing SN755881 from a different TV)
These seem to be absolutely made as nixie drivers.
They run off 5v (unlike the supertex devices), have push-pull outputs, any 
output could be 200v or ground.
This could be a universal driver for direct-drive (64 output pins) or 
multiplexed (pins can be set to HT and used as anode drivers)
Does anyone have any data on SN755870 and sn755881 ?
I can find a pinout and that's it.
These chips seen cheap and readily available new or from old TVs
Cheers,
Andrew

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[neonixie-l] OT: All Tube Color Organ

['threeneurons' via neonixie-l]

Not just being enamored with nixies, and dekatrons, but tubes in general. 
At least they're gas filled thyratrons. Not a display tube, but they can 
control some pretty bright light emitters (light bulbs), in the form of a 
color organ. Of course, I'm not the first to do this. There's a project 
article, on it from 1959:

https://threeneurons.files.wordpress.com/2017/11/radio-news_color-organ.pdf

But this circuit has some deficiencies, that possibly propagated into the 
solid state circuits, of the 60's and 70's:




The main problem, is that both thyratrons & thyristors (TRIACs & SCRs), are 
either ON or OFF. No linear section. That means brightness is adjusted by 
using some for of pulse width modulation (PWM). When using AC line voltage, 
the device [i]turn-ON[/i] is delayed (per cycle) relative to the desired 
brightness. Earlier in each cycle for brighter output. This is how your 
everyday household lamp dimmers work.

But for some reason, this never made it into color organ circuits. In those 
circuits, if the music is loud enough, it turns ON the light. If it ain't 
the light stays OFF. Pretty hap-hazard.

In recent years, I've made TRIAC versions of the color organ, that DO 
properly convert the audio level, to the corresponding phase angle, in each 
half AC cycle.

I've been wondering if proper control could have been done with tubes only. 
That means thyratrons. I've purchased several types, in the last few 
months. I've already had 2D21s and 2050s, in my stash, but bought beefer 
6012s, 6014s (aka C1K), and 5559s for some serious current switching. I 
just finished a "proof of concept" circuit, and here's the results:

Basic concept:




My thyratron page:
https://threeneurons.wordpress.com/miscellaneous-projects/thyratrons/

And a video of the working circuit:
https://youtu.be/z7YYyiGx9Gg

Enjoy.

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