Transformer question (only slightly OT)

[Tony Duell]

> 
> > [...paralleling two primary windings of a transformer...]
> 
> Just make *very* sure you get the phasing right; swap the ends of one
> of the primary windings and you have a close approximation to a dead
> short across the incoming power feed.  Something will blow; if you're
> lucky it'll be your mains fuse/breaker rather than the transformer.

A useful trick is to connect a 100W mains light bulb in series with the 
input to the transformer when first applying power (and with the 
secondaries of the transformer unconnected). Such a bulb will easily 
handle the magnetising current of a small transformer, anf won't even 
glow (or drop much voltage), so you can check the output voltages, etc. 
But if you've got something wired up backwards so you're effectively 
putting a dead short across the mains, the bulb will limit the current to 
something reasonable (and tell you of the problem by lighting up).

-tony