I ran the temperature test with 120°C hot air aimed directly at the JFET. 
Conclusion - the higher the temperature of JFET, the lower the current - so 
it is self stopping rather than increasing current, which is good for tubes 
(slight undercurrenting shouldn't damage them like overcurrenting would). 
Because of nature of the circuit (10k resistor + JFET limiter + Z567M as 
load), lower current means higher voltage drop on JFET, so power 
dissipation increases a bit, because voltage drop on the resistor 
decreases. I'm looking for a transistor with similar parameters as BF256B, 
but with higher breakdown voltage (at least 50V), so I could run the test 
without additional resistor. If I find one and it will pass the tests, I 
think that it would be it - a small circuit limiting anode current, 
allowing to conserve power by using HV just above striking voltage, while 
still having current under control. Next step would be some kind of a 
constant current step up converter.

If this is too unstable due to differences in Vg curves between 
transistors, then only an active current sensing and driving circuit is 
needed. I have an idea to use an isolated 5V supply, which would allow to 
use some simple circuits near HV voltage - an opamp amplifying voltage on 
current sensing resistor and driving a transistor, all connected somehow to 
HV. I'm afraid it is only a loose idea - I am not willing to test it 
myself, for me the JFET circuit is good enough, if I ever decide to use 
current limiting instead of good old resistors.

W dniu sobota, 8 kwietnia 2017 14:52:13 UTC+2 użytkownik Tomasz Kowalczyk 
napisał:
>
> I'm afraid all active current limiters will change with temperature, just 
> some will be affected more and some less. But small changes (0,1mA or less) 
> won't be critical in a nixie clock, I think. So if a circuit will be stable 
> enough to provide desired current +-0,1mA in temperature range of 0°C to 
> about 70°C, with supply voltage changing 20V max - I'd call it good enough.
> About variations of Vgs curves - within one production batch of 
> transistors differences are usually minimal. So after ordering a batch of 
> transistors you have to check just one of them and you can safely assume 
> that all other will behave almost identically. Vbe differences in current 
> mirrors are much more critical, as the difference gets amplified.
>
> If those two factors are still too big, then you have to use a circuit 
> using a current sensing resistor, an amplifier of Rsense voltage drop and a 
> drive transistor. Multiply that by 15 segments and by number of tubes and 
> you end up with a large and costly circuit. I think that that level of 
> current stability is not needed here.
>
> I'll test J-FET current limitter with a hot air gun - how it behaves in 
> temperature range of 20-150°C, both with 5V and 30V dropping on it.
> If I find some time, I'll do the same with a very similar circuit using 
> LM317 (similar - both use only one resistor and one cheap active component).
>
> W dniu sobota, 8 kwietnia 2017 05:02:25 UTC+2 użytkownik gregebert napisał:
>>
>> My concern with current limiters that rely heavily upon the datasheet 
>> specs (Vgs  for Depletion-mode regulator; Vbe for current-mirror)  is that 
>> variations due to process & temperature will have significant impact on the 
>> actual current. Using a slightly more complex+costly design will mitigate 
>> this; well-worth it in my opinion when you consider the value of the tubes 
>> you are protecting. 
>>
>>
>>

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