tdw wrote:
Regarding the halogen lamps undervoltage will not stop them working as
such but will greatly shorten lamp life.

I thought that too, as it would stop filament regeneration, but a
little research shows halogen lamps also have longer lives when
undervoltaged, as long as the voltage is above 35% of the nominal
voltage, or is periodically raised above that minimum. From a
manfacturer's website:

" Voltage: Reduced voltage produces a decrease in lumen output and an
increase lamp life."

" Dimming: Like all incandescents, tungsten halogen lamps can be
dimmed simply by lowering the voltage across the lamp. Full-range
dimming is relatively easy and low cost. The color temperature varies
over the dimming range, becoming warmer as lamps are dimmed. Dimming
generally extends lamp life, although the increased lamp life does not
follow standard incandescent lamp curves. As periodic high power
operation is required to raise a lamp's temperature up to the level
needed to activate the halogen cycle, continuous dimming below 35% is
not recommended."