The power-line isolation saga continues…

Ok, so the idea of adding an extra RCD device which trips on differences in current turns out to be a mistake. It adds no extra protection when used in the secondary winding of such a setup. I will leave it out.

Next step was to hook up the circuit as described in the above post. Soldered evertyhing in place, used heat-shrink tubing to isolate the loose ends, lots of hot-glue to fix things, and then:

POP, said the 5A “slow” fuse – and the next one too!

That’s not just a molten piece of metal, BTW, that’s a vaporized coating on the left. I suspect that the current was substantially over 5 amps. But no ill effect on the rest of the house, no lights dimming, and no other blown fuses (the main ones downstairs are all 16A).

Uh oh, something is wrong. Well, at least I now know the fuse works!

Here’s the plan again:

Ah, wait, not quite! … I have a transformer with dual secondary windings:

What I did was wire them up in parallel:

And that’s where I went wrong. Each winding generates an AC voltage. There are two ways to put them in parallel. Connect them right, and you get twice the power – connect them in reverse, and one will generate an AC voltage which is exactly opposite in phase w.r.t. the other winding. The result is not just a short circuit, but two power sources actively counter-acting each other:

Hence the blown fuses. I did not expect a 300 VA transformer to blow a 5A fuse, which implies its power draw was over 1100 watt – but that’s exactly what happened.

The solution is simple, once you step back and think about it. Instead of connecting the windings in parallel, I need to connect them in series:

(and the same on the other up-converting transformer too, of course)

Here’s the key: it’s still possible too hook them up the wrong way. But doing so will simply lead to no ouput, i.e. zero volts, instead of the desired double voltage output. But no blown fuse.

Once I made this change (which required a lot of undoing), I found out that the transformer secundaries were indeed wound the other way than their wires had initially led me to believe.

Problem solved!

I now have a working isolation setup which turns a 230.3V input voltage into a 230.9V output voltage under no load. An 80 watt test load worked fine.

P.S. I also gained more respect for AC “power”: such circuit mistakes are a different ball game from fiddling around with an AA Power board and getting all sorts of microcontroller- and sensor hookups messed up!