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RE: update

I agree with cpotl. Like anything else, filament transformers have winding resistance. They're rated for output voltage at full load. Your measurement is essentially correct.

About the lighting transformer, yours has a toroidal output transformer. Based on the photo, the output winding has 9 turns, which means approximately 3.89V pk-pk/turn (35V pk-pk total). So, if you remove three turns (11.67V), the output should decrease to 23.33V pk-pk, or (for a perfect square wave) 11.67V RMS. Because your output wave isn't as square as we would like, I would deduct perhaps 10-15% from that, which leaves 9.92-10.5V RMS. Under load, it will probably decrease somewhat, but in any event, this should be safe for the 813.

When you remove the turns, try to spread out the remaining turns so they're fairly evenly spaced around the core. That will aid primary-secondary power transfer.

Can you provide a link to the units you purchased? That oscillator transformer is also different than the ones I've been buying. I suspect it's a lower permeability, which would account for the higher switching frequency. Also on this point, if you wouldn't mind, could you confirm the measurement regarding frequency that you made earlier? Don't need a photo, just confirmation. Thanks!



Edits: 12/04/16
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