I see a +5 v supply going to the filament supply, but there is no mention of that being isolated like the 12V_isolated supply. I suspect there is missing isolation somewhere. The filament is biased near the cathode voltage, which is several hundred volts negative.
On Saturday, June 17, 2023 at 12:07:20 AM UTC-7 Max DN wrote: > Hello, > > Reposting with a new title for good order, as I didn't get a reply before, > probably because this post was comingoled in another post. > > I have built a Scope Clock based on David Forbes design. I hand wound the > transformer. > > All secondaries, including for the heater are on the same core, so the > voltage for the heather output depends on duty ratio, frequency and load on > the other secondaries. > > I noticed that even if I’m careful with the winding ratio, it’s difficult > to get an exact 6.3VAC RMS under load and the output voltage would also > depend on the overall load on the primary, being the regulated secondary > winding on the same core (SMPS regulated to +250VDC). > > So now I’m thinking to use a +6.3VDC power supply that I had designed on > the same board. I have rectified one of the secondary voltage to about > +12VDC, then I stepped it down to +6.3VDC regulated. The step-down power > supply works well this way. > > However when I connect the ‘IsolatedGround’ from the 6.3VDC power supply > to the Cathode PIN of the CRT (internally connected on this tube), the > power consumption increases and the focus pot gets hot very quickly, taking > the a full 600mA load. > > So, clearly something isn’t right in this last step, given the huge > voltage differential between CATHODE and IsolatedGround. > > My schematic attached. > > >> Any suggestions on how to connect the +6.3VDC isolated power supply to > the Cathode pin of the CRT welcome. I’m sure I’m missing something obvious. > > >> Also, any tips on what RMS voltage I should read (with and without > load) if I power the heater straight from the secondary. This works well > when connected to the Cathode pin of the CRT, I can even read 6.2VRMS but a > test light bulb of 6.2V 500mA got quickly very dark when receiving 5.7V > under 450mA load, not good. > > So, not testing it on the CRT again until I get this right. I manage to > get a green dot on the CRT, which is a good sign. But I need to get the > heater voltage right before I plug the CRT in again. > > Thank you > -- 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/09d2b69f-0a1a-48f2-a4c3-62ba697cde9bn%40googlegroups.com.
