(Hypothetical) Transformer isolation and breakdown voltage in step up mode - Page 1
Forum section: electronics / projects

author: IanB
author_rank: Super Contributor
timestamp: August 26, 2018, 11:51:57 pm
content: This is a bit of a thought experiment, but I'm curious about how to answer it.Let's say I take ten 120 V / 12 V step down transformers and wire them in series, primary to primary and secondary to secondary. The ten 12 V windings in series add up to 120 V, and the ten 120 V windings add up to 1200 V. Now suppose I apply 120 V to the inverted secondaries to generate 1200 V from the inverted primaries. I might exceed the insulation and isolation ratings for the transformers somewhere, but where exactly in the system might this occur?For each transformer individually, both primary and secondary windings are operating with the designed voltages. So there should be no problem with intra-winding breakdown. What about isolation between the primary and secondary windings? If the high voltage side is floating and not connected anywhere to the low voltage side, is there actually a voltage between primary and secondary? Or is there merely a potential voltage that could become active if the high voltage side is connected to something that causes it to be referenced to the low voltage side?I find on thinking about this that I am not sure where the answer lies.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 27, 2018, 02:09:53 am
content: Mains transformers are rated 2.5kV reinforced (at least; or should be, hopefully!), so you've got a bit of a ways to go, actually.Okay fine, so we use twenty, and they have 240V primaries. Great.BTW, series-series transformers aren't good. The voltages won't share evenly, due to differences in inductance and core nonlinearity. If done in practice, you may find some transformers getting hotter than others. It's better to have at least one winding (per transformer) wired in parallel, to enforce equal voltages, then connect as many windings in series as you like. (So great, we have a 12V to 4800V composite transformer.)("At least one", meaning, you can connect windings in parallel, as long as their voltages match very closely, of course. Connecting just one winding in parallel, like all the 12V's in parallel, is perfectly fine, regardless of their actual turns ratios; connecting more than one in parallel, like all the 12's and 240's to get a single 12:240V higher amperage composite, requires equal turns ratios and phasing.)If the transformers are identical, and there is no load on the high voltage side, then the capacitance from each winding to ground (ground being through the low voltage side, which is more or less "common" in this case) sets the voltage. A many-tapped secondary, with equal capacitors to ground from each tap, makes a balanced capacitive divider, where the center is at zero volts and the ends are at +/-2400V (in this case).The capacitance is relatively small (100s pF?), so we might be okay if we leave it alone, but if we forcibly ground one side, all bets are off -- it may arc over.Neon sign transformers are typically made in this way, for this reason: a centertapped secondary, with the tap grounded. Instead of, say, 15kV, only +/-7.5kV insulation is required.Tim