[Boatanchors] Unused transformer windings.

[WA5CAB at cs.com]

Ken,

Your explanation of why it works (it adds turns to the primary and therefore 
reduces the primary to secondary turns ratio) is of course correct. You 
connect the primary Start to one side of the AC line, the primary Finish to the 
filament winding Start and the filament winding Finish to the other side of the AC 
line (or vice versa). So the two windings are in series aiding just as if 
they were one winding.

However, the connection technique itself, when connected to reduce the 
secondary voltages, is still properly called the Buck configuration, not the Boost. 
Even though the method of achieving the final results are different from that 
using a separate Buck-Boost transformer. There are transformers sold with a 
main primary winding and one or more LV primary windings where the desired 
secondary voltage is achieved by selective connection of the main and LV primary 
windings. Stancor used to sell a line of LV rectifier transformers designed like 
this. Those connections that reduce the secondary voltage below that using 
only the main primary winding are described as the Buck connections.

And incidentally, there are no significient efficiency penalties incurred in 
doing this (using a filament winding to reduce the secondary voltages). With 
more turns in the primary, R sub Primary is slightly higher. But both core (H &
E) losses and I Square R losses are less than they would be if the filament 
winding were being used normally (less current in the filament winding).


 
> --- Ken Hickman <n5cm at rtconline.com> wrote:
> 
> >  Hi Fellows,
> >
> >Don't ever short out unused filament or other transformer windings!
> >If you have replaced the vacuum rectifiers with solid state, use the
> >unused high voltage filament winding in "series aiding", NOT
> >BUCKING to lower the high voltage. 
> >The step-up step-down in transformers depends upon the "Turns
> >Ratio". Putting the filament winding in series adding will lower
> >the Turns Ratio resulting in lower output of the high voltage
> >winding.
> 

Robert Downs - Houston
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