I wanted to try to avoid having a AC line in my design if possible. I did have to break out my old ECE notes, and I designed a different filter. Turns out my original RC filter requires a much larger resistor to limit the primary side current, but that raises input resistance too high for my 40 ohm loads.
Using a different filter I had some more success: https://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCDsB0AcYCYCsBTAtLEAGaBOWuYALFrgGwDMFRSSkSZIuuISrWr6YYAUAG7hE4Mh24IQRTB3FgscudjiKpipP0HiKZTGJDJG08PIU4pSitGJgkuBGSQUkBSLgqqeAcw3DRQsLexsT28tHSESDkieAHdvSXAA+KweACdWBB0M9J0AjhIeMDIWBBJwLIiylRx5WWY6+oaWMGhSAixato6zfzIiESJuJCxIWoRIEDQcYiwKSHbIMgQqWDkRiamSWfnF5bkqRWYYiQV-cT6iBPFkgSIKcRK8u70sKuPwQMjoNViKwseZERBH5PP4SJ4IF5BLy3e6lGF6SBXIESXqXbJo5IAYxRF0QmHOGL0lghxlJ8gQ6HG1RGuBGiGYFFsGUIyKQWVOrAWGKOdEYePR-OSsV5lRAIkyUiOHPiDwkktiun0elKSqFyukDHV3OFXI54u1Yshes1HLV+v59j5WTVsplpUFPAANjjRWyJR8WsZMJMWgwiCMEORbrBqBQhMlnb9yqjBYpjCVFNMtmAFksKCssJQgmkRfzc7ljqlDe7zQX8mlLaLK6bCwB7QIQURIay0ViKC6GDgUHj1tiN56SNwIL5xvt6Nvd3vvcZgRwpthsKbjxstSDwZA8IA However the problem is the output voltage is somewhat determined by the load. So if a filament burns out it can cause a cascade failure on the other filaments as the voltage rises. I guess monitoring the AC line would be possible but this complicates my design a bit, and I'd need a monitor that could do negative currents/voltages if my bias is not high enough. If such an IC exists I'd be interested, but I don't really have access to a FPGA if that's what it takes. It also seems it's easy to hit resonance with particular filter LC values, boosting the voltage higher which feels a bit dangerous for the filaments. Maybe just not having the filter would be the safest approach here. However I'm not at all experienced in filter design especially when there is a low impedance load involved, so maybe there's a better approach that I'm not using. On Saturday, 15 April 2023 at 02:03:39 UTC-4 gregebert wrote: > I use a mains-powered 2.5VAC center-tapped transformer for my NIMO clock, > and apply a DC bias to the center-tap. Basically the same as your approach, > but not running at 100kHz. The DC bias comes from a DAC (with an OP-amp > buffer), so I can use software to control the offset, rather than a pot. No > need for LC filters, etc. > > The transformer is rated at 3amps, even though 6 NIMO tubes only require > 1.2amps max. Also, each tube has it's own fuse and series resistor to > reduce the inrush current. The software-controlled DAC is also necessary so > I can have the FPGA monitor analog signals to check the fuses, filaments, > and resistors, and that has to be timed correctly to the AC line and the > offset has to be different for making these measurements than for regular > operation. It was probably overkill, but I enjoyed the design challenge. > > On Friday, April 14, 2023 at 2:25:57 PM UTC-7 ZY wrote: > >> Hello. I'm working on a driver for my VFD filaments. Say I need a >> Vfilament of 1.6V, and a current of 40mA, such as with a small VFD. >> >> So far I have a square wave driver set up, basically for my two filament >> pins I can alternate the two pins between 1.6 and 0 with some H bridge at >> maybe 100KHz. If I connect the filament load to this, it's the equivalent >> of using a 3.2V Vpp square wave biased at 0v. See Sim Link 1. I put two 40 >> ohm resistors as a stand in for 2 VFD tubes filaments in parallel. >> >> However, I'd like to add some sort of smoothing and DC bias. I was >> thinking of using a small transformer, such as this one: >> https://www.mouser.com/datasheet/2/445/750315240-3096542.pdf >> Applying a 100KHz RC low pass filter to the output, and approximating the >> turn ratio at 1:1 (it's 1:1.1 in the datasheet but ignoring that for now), >> I have a simulation below as Sim Link 2. >> >> The problem here is I seem to be wasting a huge amount of power just >> driving the transformer on the primary size. The plot on the very far left >> shows I'm delivering 2.2A to the transformer. I guess I could find a larger >> transformer with a higher inductance, or increase my switching frequency, >> but I'm not sure if I'm on the right design path here. >> >> Also, it seems like some of my issues stem from my LC filter. If I remove >> that, the simulation shows more reasonable primary currents of 100mA. See >> Sim Link 2. I don't really remember my ECE courses from a long time ago, so >> I'm not sure what is going on here. >> >> Anyone have any suggestions? >> >> >> Sim Link 1: >> >> https://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCDsB0AcYCYCsBTAtLEAGaBOWuYALFrgGwDMFRSSkSZIuuISrWr6YYAUAG7hE4Mh24IQRTB3FgscudjiKpipP0HiKZTGJDJG08PIU4pSitGJgkuBGSQUkBSLgqqeAcw3DRQsLexsT28tHSESDkieAHdvSXAA+KweACdWBB0M9J0AjhIYiQV-cSIyIgTxZNjS8sRMJCy6oOqyiuy25LS6RibujsKChpzxPuLm1kgerKHwLKq2+IQSCSkC3X09ZY35pekGTdFcgt3ZzAjT8fOxk7H5q6yai53lxeWm5IB7QIhRJGtaViKcqGDgUHhfNg-PRYSRuBDQNg4LCQvSAsEQ8BQcCOMD0QE4CAyRRYSDwZA8IA >> >> Sim Link 2: >> >> https://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCDsB0AcYCYCsBTAtLEAGaBOWuYALFrgGwDMFRSSkSZIuuISrWr6YYAUAG7hE4Mh24IQRTB3FgscudjiKpipP0HiKZTGJDJG08PIU4pSitGJgkuBGSQUkBSLgqqeAcw3DRQsLexsT28tHSESDkieAHdvSXAA+KweACdWBB0M9J0AjhIeMDIWBBJwLIiylRx5WWY6+oaWMGhSAixato7MNGbEWAYsCgRZMGosLOaseiIie2ItSARSGRbp2aR5skXhokVmGIkFf3FZ3eOggSIhvVKr8SWqw-BAyOg1WIrCvOuvoI+fkQSa4PIJeO43b73RYvA6nSrZeHJAAOIBEmUwaL0DBAbiiXkx+lRWEwCGhUQ+ZDOASQE1yqUOonKlMR4B4AGMJMy+pyqeJXsZ2oxmrAtHZ4IMiIRILpqu16XQhVkFQk+YcDjScuJledkrFtUqJlldaxIIqMcTEQdzvESnkpFahITbViDAdnU7Sjr2RCJCTPUbFMYIDgiIgMuQyKT8JJyIG5QB7QLB8AbWhsNgWVWqig8RNsZNLSRuBBvRQQCBanG51jPSDgRxgeg1nAV54thjIaujNuHfDNyw41lAA >> >> Sim Link 3, no LC Filter: >> >> https://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCDsB0AcYCYCsBTAtLEAGaBOWuYALFrgGwDMFRSSkSZIuuISrWr6YYAUAG7hE4Mh24IQRTB3FgscudjiKpipP0HiKZTGJDJG08PIU4pSitGJgkuBGSQUkBSLgqqeAcw3DRQsLexsT28tHSESDkieAHdvSXAA+KweACdWBB0M9J0AjhIeMDIWBBJwLIiylRx5WWY6+oaWMGhSAixato6zRGsiOyJILApEXEgyCDRq2AQKWG1ISAQ7OhE2SZbp2fnF5bGhxWYYiQV-cSIyIgTxZIEiCnESvPu9LCrj8EDI6DVYisKnmQiIK-Z7-CTPBCvIJeO4PUqwvSLT5Hc6XRCYJBZdFBAAOIBEmUwBL0DBAbiiXmJ+nxUIQSKivwuV2yzOSaRWlVYY1ZxyOmJy4g5p2BXMY2P5nOSsSFWWJ2KlzPijwkUiOumpyupCs1pOVwuSAGM9PDMHqsl9jBApmQRAhRhQsJJRrgsNcWu0USdykz5TwAPaBK3gJDWWisRSXQwcCj+8NByGSNxsHAyPThmMBtgQSDgRxgejhnAQVM4MjIWNgNxBkj4QuWMngHhAA >> > -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. 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