Quote:
Originally Posted by GMB
I have to say that I really do not like modern double insulated devices that often put you in capacitive contact with the mains. For me an isolation transformer should include in its remit isolation from capacitive coupling as much as possible - so having an earthed screen between the windings is essential.
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Agree, but it's easier said than done!
Suppose you wind the primary, and then put on a single turn of foil which you earth. And then start winding the 240V secondary. The first layer of secondary turns will be fairly close to the earthed screen - separated only by a couple of layers on insulation from the screen. Whereas the last layer of turns will be remote from the screen.
So, the capacitance from screen (earth) to the start of the secondary will be fairly significant, maybe 100pF. Whereas the capacitance from the finish will be much lower - probably around 10pF (to the laminations, which will be a couple of mm away, unless you have wound your bobbin really full).
When powered up, if you measure the voltage between the unconnected secondary and earth, you'll see around 22V on the start leadout and 218V on the finish leadout. And short-circuit current at the finish leadout will be around 7uA, still enough to give a tingle!