It all depends on R11 and desired current. Most nixies have a minimum voltage drop of around 120V (although they will maintain 130 for most of the time, they will just continue to work with lower voltage), so if voltage drop on R11 is more than (supply voltage - 120V - LM317_max_voltage), then it is safe - the only case where the nixie doesn't have that voltage drop is when it is internally shorted. In this schematic, assuming that U15 is fully open, so voltage at R11 left side is just about 180V, everything will be okay - the LM317 is set for 8mA, so voltage drop over R11 would be around 34V and everythin would be safe. The problem is that if current is significantly lower that (nixie is heavily poisoned), then stuff can get bad: as far as I understand, R11 voltage drop has to be lower (Ohms law), so the voltage across LM317 will get higher, I think - it has to drop somewhere. Long story short, you are right - LM317 can fail in non-standard situation. That's why I would go for a two-transistor design using MMBTA42 (or MPSA42), which will withstand almost anything. The only problem is power dissipation - 5mA with 50 volts drop across current limiting circuit is 0,25W. With such constant current design one should lower HV voltage to minimum (just above striking point of all nixies), normally the significantly higher voltage (200V) is used to make choosing limiting resistor easier.
W dniu poniedziałek, 27 marca 2017 04:17:46 UTC+2 użytkownik gregebert napisał: > > I dont see anything fundamentally wrong; I just have paranoia about > running devices in an environment where this is potential for overvoltage. > The LM317 is rated for ~35V, and the HV supply is around 180V, so you are > relying on the voltage drop across the nixie tube. There are all kinds of > unexpected things that happen during power-up/down; maybe there are > scenarios that could damage the LM317. > > > > > -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/b7218ec0-7532-48d0-9220-074fc3febf69%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
