There's a lot to this question of why tungsten halogen bulbs last longer - while the redeposition of tungsten on the hotter (i.e. thinner) parts of the filament is one cause, it's not the only one. In fact, with some halogen bulbs it doesn't happen at all; it depends which halogen you use - I...
It sounds as if the original was about as basic as you can get in chargers - two secondary windings with diodes to give a full-wave output. The problem with this is to get a reasonable charging current without ending up with too high an open circuit voltage, resulting in excessive 'trickle'...
Just in case anyone else has been searching the web for the book that Viktor mentioned - which sounds like a really worthile read - and not seeing worthwhile results, I've noticed (thanks to Amazon.com)that the author is actually Henry Petroski.
Mike
