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I've been working on this schematic to create a nixie clock. But I don't understand the need for recifying diode D5 in the schematic. Wouldn't a standard diode work? Does it have to do with the "back emf" produced by the inductor?

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D5 is part of a high frequency boost circuit formed around the MOSFET Q7 and the inductor L1. together they dump energy into capacitor C4 and they will use high switching frequencies possibly circa 100 kHz. Generic boost converter: -

enter image description here

Your boost converter featuring D5: -

enter image description here

This means that you must use a fast diode or the extended reverse recovery time will significantly reduce your high voltage rail and/or efficiency. Reverse recovery time means that when a diode goes from forward to reverse bias, current will flow against the grain for a short period of time and for a 1N4007 (for instance) this can be as long as 30 us. 30 us would severely disrupt the working of your boost regulator.

Fast diodes are in the low tens of nano seconds with a diode like the BAS16 being about 5 ns (for example). Nixie tubes are cool BTW!

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    \$\begingroup\$ Nixie tubes of any decent size are EXPENSIVE, too. Would be nice to see the new LED filaments (having 28 LEDs in a string requiring about 70 V to run them at 10mA) being incorporated in some new NIXIE replacement devices. Or some new manufacturer coming into the NIXIE marketplace due to the prices now being high enough to make some money at it. \$\endgroup\$ – jonk Jul 5 '17 at 19:41
  • \$\begingroup\$ @jonk: as soon as new ones are made they will no longer be cool. \$\endgroup\$ – dandavis Jul 5 '17 at 19:59
  • \$\begingroup\$ @dandavis Well, I have ZERO coolness genes. I DO however enjoy being able to read a clock at night from a distance and would very much appreciate building a nixie clock. But I have NO interest at all in having something that is there because it is expensive and it shows off that I have more money than they do. (I am in the top 1% of the US, but I also spend 400 hours every year volunteering to help others less fortunate and donate significant funds to help those groups, as well. So I enjoy nifty ideas that others can afford themselves. Nixies are not affordable by the hoi polloi.) \$\endgroup\$ – jonk Jul 5 '17 at 20:36
  • \$\begingroup\$ @jonk Have you considered one of those jumbo LED clocks? You can see them from across the room and they're a lot cheaper than a custom Nixie one. \$\endgroup\$ – Paul Uszak Jul 5 '17 at 21:04
  • \$\begingroup\$ @PaulUszak I have a couple of analog clocks with the bright hands that turn on when it is dark. Very efficient and they work fine. But I like Nixies too. And so do others. It just would be nice to have a new manufacturer making them and getting the prices back down so that regular folks could enjoy them without sacrificing rent money. \$\endgroup\$ – jonk Jul 5 '17 at 21:21

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