Hmmmm... a transformer short circuit current, which is the maximum current that it can supply, is not the design or specified one, which in this case would be 1.2A and which is the current it can supply indefinitely while maintaining all of the design parameters. The ideal transformer has, of course, infinite output current, but as the real transformers coils are made of copper, impure, there is its resistance that limits the current output (and of course heats the transformer). I'm dismissing losses due to the iron core as we are talking of the transition start up, which is likable to a short circuit.
One can calculate the short circuit current as the transformer secondary design voltage divided by the secondary total DC resistance, that would be the secondary DC resistance plus the primary resistance reflected on the secondary (the primary DC resistance divide by the transformer ratio). In this case, the transformer resistance is Rsec + Rpri * Vsec/Vpri, and the short circuit current would be Vsec/(Rsec + Rpri *Vsec/Vpri). The tube startup current without limiters of any kind (assuming there is only one being fed by the transformer) would be then Vsec/(Rsec + Rpri*Vsec/Vpri+Rfilament) So, the transformer actually supplies several times the specified current at startup, given the very low cold resistance of the filament. This accounts for the "flash" one sees in some receiving/transmitting tubes. The moral is: one needs to put some limiting element when feeding delicate tubes. Gastón On Monday, October 22, 2018 at 2:58:54 PM UTC-3, jrehwin wrote: > > > I need help with how much current a regular mains transformer would need > for the center tapped filament winding for six NIMOs. > > > > The filament for a NIMO is specified at 1.1V AC or DC +/-0.15V at some > 0.2A. The cold resistance of the filament is some 2.1 Ohm. > > > > Do I need to specify some overhead for the heater transformer to be able > to power six NIMOs or do I just need 1.2A current capability (6*0.2A)? > > 1.2A should be sufficient. The cold filaments will attempt to draw more, > but the transformer will be unable to supply it, so the voltage will drop > initially, which is what you want: this will give a gentler warmup than > smacking them with full voltage would. It is, of course, possible to add > more circuitry for an even softer start, but a transformer sized for the > equilibrium load is pretty simple and works well. > > - 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 [email protected]. To post to this group, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/9693d331-ea95-498d-b011-85ec4e19ecd5%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
