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. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/59fc1313-e4f7-4aed-af46-7ba949c5348fn%40googlegroups.com.
