Transformers are not ideal, so even with zero load, they consume inductive current. Most of the energy gets returned to the AC line, but some of it gets dissipated as heat (winding resistance, and hysteresis). This is why unloaded wall transformers still get slightly warm.
You can measure the magnetizing current with multimeter. --------------------------------------------------------------------------------------------------------------------------------------- Be warned that large transformers will have a startup surge current; I have an isolation transformer in my workshop that can supply 20A/120VAC. It has dual primaries, so I can run it from 120V or 240V. If I try to run it from 120V, even with no load, it almost always pops the 20A breaker for my workbench. So, I use 240V and even then I can hear the wires in the conduit vibrate for an instant when it's energized. If you are worried about blowing the fuse in your multimeter when measuring the magnetizing current, it's good practice to use a variac to increase the line voltage starting from 0 volts into your transformer. On Monday, August 23, 2021 at 8:49:01 AM UTC-7 Paul Andrews wrote: > Thanks Martin, > > I'm OK with using a transformer. I would prefer something off the shelf, > but perhaps I should just wind my own anyway. Seems like a useful skill to > have. > > As with a lot of these things, I find that I have a lot of questions. For > example, this statement: > > "The transformer is 1:1, center-tapped. Since the output is to be 300 mA > max, the magnetizing current at the input should be no more than 30 mA. I > will try 10 mA" > > First of all, how does he get a 10:1 ratio of output current to 'maximum > magnetizing current'. Second, what is 'magnetizing current', does he just > mean current through the primary? > > Then he introduces terms in equations without defining them - they are > probably obvious to the initiated (which I am not unfortunately). For > example, Xl in L = Xl/2/PI/F and Al in N = (L*1e6/Al)^.5. Then having > calculated the required number of turns, he just ignores that and goes with > something much larger. > > On Friday, August 20, 2021 at 5:51:17 PM UTC-4 Dekatron42 wrote: > >> There is a nice article in NutsVolts magazine: >> http://www.nutsvolts.com/media-files/Forum-Articles/QA_201110.pdf where >> a powerful driver is shown, needs a small transformer but seems to be well >> designed with equations for calculations of the transformer. >> >> /Martin >> >> On Friday, 20 August 2021 at 21:37:46 UTC+2 Hannah Mishin wrote: >> >>> Theres a section on filament driving in my blog here: >>> >>> https://hannahmishin.com/blog/2017/4/30/russian-tri-color-vfd-indicator-clock >>> >>> *Hannah Mishin* >>> * She/her/hers* >>> <http://hannahmishin.com/> >>> >>> *hannahmishin.com* <http://hannahmishin.com/> >>> >>> >>> >>> On Fri, Aug 20, 2021 at 1:48 PM Toby Thain <to...@telegraphics.com.au> >>> wrote: >>> >>>> On 2021-08-20 12:58 p.m., Paul Andrews wrote: >>>> > I'm designing a clock for DT-1704 tubes and am stumbling at the >>>> filament >>>> > drive stage. They want 1.6V. I want to power it from 5VDC. ... >>>> > >>>> >>>> FWIW I've used LM2575 1A adjustable step down for DC filament voltages >>>> in that range. >>>> >>>> --Toby >>>> >>>> >>>> > ... >>>> > >>>> > So I would appreciate any suggestions for what direction I should take >>>> > here. I have too many options and no clear criteria. >>>> > >>>> > -- >>>> > 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 neonixie-l+...@googlegroups.com >>>> > <mailto:neonixie-l+...@googlegroups.com>. >>>> > To view this discussion on the web, visit >>>> > >>>> https://groups.google.com/d/msgid/neonixie-l/860d4d78-c807-415e-a740-b576a4b5d18cn%40googlegroups.com >>>> > < >>>> https://groups.google.com/d/msgid/neonixie-l/860d4d78-c807-415e-a740-b576a4b5d18cn%40googlegroups.com?utm_medium=email&utm_source=footer >>>> >. >>>> >>>> -- >>>> 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 neonixie-l+...@googlegroups.com. >>>> To view this discussion on the web, visit >>>> https://groups.google.com/d/msgid/neonixie-l/ce44816f-120e-abae-060f-fe062d24f836%40telegraphics.com.au >>>> . >>>> >>> -- 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 neonixie-l+unsubscr...@googlegroups.com. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/1cc9775d-de54-4b3b-8960-384aedb1f71dn%40googlegroups.com.
