[Amps] power supply transformer

[Tom W8JI]

> What are the transformer design differences between, for 
> instance, a
> transformer designed to convert 480vac to 120vac and is 
> labeled a
> control transformer, to a plate transformer designed to 
> convert 120vac
> to 480vac and used with rectifiers to provide plate 
> voltage to an amplifier?

Transformers used in capacitor input supplies require 
special design care for good or optimum performance.
Capacitor input supplies draw current from the power mains 
and power transformer only over a very short period of each 
cycle.

The power factor, because of this very high ratio of peak 
current to average current, is very poor. As a matter of 
fact the better you build a capacitor input supply the worse 
the power factor gets!

Because of the very high peak current standard wire gauge 
tables for current don't work. To prevent peak clipping and 
excessive loss of peak voltage and heating under load, the 
wire SIZE has to be increased far beyond what you expect for 
a conventional load with low or modest power factor. The 
transformer designer has to pack as much copper as possible 
in the transformer window, use the fewest turns possible 
(high flux density), and make other changes that reduce 
transformer ESR.

Control transformers work fine as transformers, but they 
have little or no design consideration given to voltage 
regulation or peak current. The windings are generally sized 
to the minimum wire size for the load current and cores are 
operated well below saturation (which means the windings 
have unnecessary length). There also can be voltage 
breakdown issues when used with some rectifier systems, 
because no consideration is given to dc bias on windings.

This doesn't mean they won't work, but it does mean they 
often produce poor regulation and heat more than a 
transformer designed for the specific application of high 
power factor capacitor input supplies. It also means if you 
lift the secondary and run a doubler, you might have 
premature transformer failure from insulation deterioration.

As an example I have some 25 pound 2100VAC to 120/240 VAC 
hypersil control transformers here. They are about the 
physical size of an AL-1200 transformer and use the same 
core. Secondary dc resistance is 50 ohms. The AL1200 
transformer, about the same voltage range,  is only about 
ten ohms. The control transformer would run hotter and have 
much poorer voltage regulation.

Regulation is much less of a problem in a choke input supply 
because power factor is lower, but heat can still be a 
problem since we tend to abuse transformer ratings and the 
copper losses are high.

73 Tom