"John Phillips"
Nick Gorham


One you have missed out (though it could be covered by the last) is DC
on the mains, I have found that that can alter during the day, and could
have an effect, if only causing any torroid in the supply to saturate.

( snip interlude)

I imagine it will offset the transformer's magnetic cycle and reduce
its rating but I would need to ask an expert to be sure. I wonder if
there's a DC spec. for the UK mains?


** Having a " DC offset " on the AC supply implies the + and - peak
voltages of the supply waveform are not exactly the same. So, when tested
with an DC meter it shows a reading of a few volts.

Such differences are not uncommon and DO have the same effect on the primary
and core of a transformer as an actual DC voltage of the same value.
Toroidal core mains transformers are particularly sensitive to any DC
component and tend to magnetically saturate early on one half cycle of the
supply voltage and also make a humming noise.

I have been able to raise the off load primary current of a 300VA toroidal
from a few mA to near its full VA rating by simple operating a 1600 watt hot
air gun # at half power setting from the same power outlet.

At half setting, a diode in the gun causes load current to be drawn from
one half cycle and not the other and that does the trick.


( # Black and Decker, made in England)





........ Phil