30-03-2017, 02:50 PM
As Lawrence says, these are all AC voltage measurements, no polarity.
The only time polarity is important in a transformer is when combining two (or more) windings.
Take the centre tapped 250 - 0 - 250 HT winding, for example. As Lawrence says, you should measure ~500V across this winding but, in normal use, the 0V centre tap is the common and is grounded to provide two 250V windings to feed a full wave rectifier. However, if you start from this common point, the winding directions for the two halves of the winding now appear opposite: one will be clockwise and the other anti-clockwise, as viewed from the centre tap.
This means that the two 250V outputs will be in anti-phase, which is exactly what we want because the two diodes which comprise the full wave rectifier will conduct on alternate half cycles to produce a 100Hz ripple which is easier to get rid of that the 50Hz ripple which would result from using one winding to feed a single diode, aka a half wave rectifier.
The only time polarity is important in a transformer is when combining two (or more) windings.
Take the centre tapped 250 - 0 - 250 HT winding, for example. As Lawrence says, you should measure ~500V across this winding but, in normal use, the 0V centre tap is the common and is grounded to provide two 250V windings to feed a full wave rectifier. However, if you start from this common point, the winding directions for the two halves of the winding now appear opposite: one will be clockwise and the other anti-clockwise, as viewed from the centre tap.
This means that the two 250V outputs will be in anti-phase, which is exactly what we want because the two diodes which comprise the full wave rectifier will conduct on alternate half cycles to produce a 100Hz ripple which is easier to get rid of that the 50Hz ripple which would result from using one winding to feed a single diode, aka a half wave rectifier.
