I think I may have figured out the problem based on John's comment. This is from Nick Desmith's web page.
- For 50mA+ output, L1 should be DC rated at about 2A. The chosen inductor, an EPCOS B82479, fits this bill and works very well in this circuit. If you use a shielded inductor, you will get less RFI, but the efficiency will also drop by about 3%. Small surface mount, high current, shielded inductors are not common! You could try a Sumida CDRH127-101 which works ok even though it starts to saturate at about 1.7A. - For lower output currents up to 25mA, you can use a 1A inductor and change Rsense to 0R100 Ω. A good inductor here would be a Sumida CDRH125-101. Select an inductor with a low DC resistance as straight resistive losses will effect efficiency and lead to heating. Sure enough my board is using the 1A Sumida. Also I notice my friend used a different vishay FET, I assume to get the 300v (I only need 200) Nick specified this one. http://www.vishay.com/docs/91039/91039.pdf My friend is using this one. http://www.vishay.com/docs/91043/sihf720.pdf I notice that the RDS on value is higher on the 720. 1.8ohms instead of .28 ohms. Could this be contributing to the problem? Last question. From reading Nicks webpage: Switcher rate was approximately 62kHz This doesn't seem to be a problem but I was wondering since this is an audio application if there would be a lot involved in moving this up to 100 -120K or even more and still get 200v/50ma out. I really appreciate all your help. Bob On Thursday, December 25, 2014 1:00:23 PM UTC-5, jrehwin wrote: > > > I have been playing with an smps a friend built which is supposed to be > identical to Nick Desmith's board > > > > http://desmith.net/NMdS/Electronics/NixiePSU.html > > > > with the exception that its a through hole board rather than smd. Im an > audio guy btw not a nixie builder. With a 12ax7a at 300v/1ma it works > well. Noise free, stable, sounds great. But when I tried it with a circuit > that drew 20ma of current, both the voltage and current output collapsed to > 78v/4ma. I tried changing the resistor that sets the output voltage to > 200v and got similar results. Im using a 9v 3amp wall wart supply. I know > its impossible to really diagnose anything without a circuit which is the > next thing Im going to come up with, but I thought it couldn't hurt to post > this and see if there is anything obvious those of you much more familiar > with these supplies than me, might suggest as possibly the problem or what > to look for. > > Does the switching FET get hot? If so, you probably need a lower > resistance one. What's the DC current rating on the inductor? > > The efficiency and effectiveness of these single-inductor boost supplies > starts to really fall off at large voltage ratios (like you'd have with > 300V out). You might consider going to 24V or more on the input side (with > appropriate modifications to correctly power the control chip), using a > transformer, or adding a voltage doubler. > > - John > > -- 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 neonixie-l+unsubscr...@googlegroups.com. To post to this group, send an email to neonixie-l@googlegroups.com. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/134d1b03-d345-408c-9f12-6454f0a800a1%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
