I was also wondering if i can add extra windings to the primary and eliminate the need for a secondary totally, like the attached picture. Sort of how Variacs have an output of 145 when 120 is supplied(overwinding).
Be very careful with this sort of arrangement. There is NO ISOLATION! And, because there is no isolation, it's far easier to receive a lethal dose of current [think ELECTROCUTION] -- especially if you don't know what you're doing.
Suggestions on how i can get 277 easily from either 120 or 240? Thanks!
When you say, 277V, are you talking AC or DC? Because if it's DC you're after, then consider that 240VAC is an RMS value, and if you full-wave rectify 240VAC, and filter it with an appropriate Capacitance, you'll wind up with:
(240VAC + 1.414) - ~1.4V = 338VDC
That's because you multiply an RMS value by the square-root of 2, to get the Peak voltage value. And in the case of what you fill get from a full wave rectifier, you gotta subtract the forward voltage drops of the diodes [two in series].
BUT, since "240VAC" is an approximate value, subtracting the forward drops merely gives you a "Nominal" voltage. The actual voltage will vary, depending on what comes down the mains lines.
AND AGAIN: DOING THIS WITHOUT AN ISOLATING TRANSFORMER [and, your Flea Market transformers are NOT isolating] MEANS THIS HIGH VOLTAGE IS NOT ISOLATED, AND THUS IS EVEN MORE DANGEROUS! And, that's not to say that doing it with an isolating transformer is not dangerous -- it's just a little less dangerous. You're just less likely to get a shock when you touch surrounding grounded points of conductivity, when the voltage is isolated from mains.
But, I'm unable to advise you on how to achieve what you want to achieve, because I'm just not versed enough on this topic [I'm more of a Digital guy] -- but I wanted to make sure you were aware of the dangers.