Quote:
Originally Posted by Herald1360
I don't know- a proper isolating transformer with HT and LT windings would need to be about 50VA rated whereas a heater only auto needs only to be about 20VA rated for a typical 4 or 5 + R set.
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Yes!
And if the heaters are in series, then the autotransformer can be rated even less - such as the Bush DAC90a. Heater volts 116.6V at 0.1A. Operating from 240V (so losing 123.4V), the 100mA appears as 48.58mA flowing in the 116.6V winding and 51.42mA in the 123.4V winding. So the autotransformer is the size of a 5.66VA rated double-wound transformer, so can be tiny.
There is one aspect which hasn't been considered with respect to safety - and that's failure! Transformers often fail with a shorted turn or turns, getting hot, smoking, catching fire even if wax impregnated (which they often are), without drawing enough current to blow a fuse.
Dropper resistors, on the other hand, when they get old, just go open so they are fail-safe.
If you want to leave a radio running unattended, make sure it's an AC/DC one!