wrote:

The RMS voltage is halved - the voltage profile stays the same for one
half-cycle (hence same RMS for this portion) and is zero for the next
half-cycle, so the average over a full cycle is halved (ignoring diode
voltage drop). Nonetheless, the confusion on this point accidentally
gets the correct answer that the half-cycle version of 240v is
effectively equivalent to 170v in terms of power, but that's because for
a resistive loud the power consumption goes as v^2 (ignoring any
temperature dependence of heater resistance)

Sorry (a passive aggressive sorry) but if you halved the RMS voltage
then you'd get a quarter of the power. This is intuitively nonsense
because the load is getting the same power for half the time. So it is
not surprising that the actual calculation shows that the RMS voltage of
a half wave rectified sine wave is actually 0.707 of the RMS voltage of
a full sine wave. The RMS voltage is not linearly related to the mean
voltage.

--

Roger Hayter