> dc in mains: ill effects

Let me put this in perspective: If you have one nightlight running
continuously at 20W and put a diode in series with it, nothing will
happen.

If your 4kW electric heater has a burned out thermostat and you wisely
decide to wire a 20A diode across the dead thermostat so you can have it
run at 2.2kW overnight, then you may or may not cause a utility
transformer on a pole outside your house to catch fire after a long enough
time (and assuming some higher load cooperation from other loads, like the
washing machine, a fridge or two and a couple more heaters). If the
electricity company catches you doing this you might be the guy paying for
all the burned out utility transformers in the past centuries.

The technical explanation has to do with transformer saturation which can
cause overheating followed by thermal runaway or a parted wire or
insulation -> flash, boom.

Peter

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