[Amps] boost transformer

[Manfred Mornhinweg]

John, Wayne, and all,

> The main issue is the excitation current of the transformer. If the
> transformer was designed (with enough turns and core cross section) for 50
> Hz and your running on 60 Hz then you have an automatic 20%  (60/50) safety
> margin for an increase on the primary voltage w/o worrying about core
> saturation.

This can be slightly misleading. Because the highest safe excitation for a given 
transformer's core is very often not a hard saturation limit, but instead is 
given by core loss. And the loss rises both with flux density and with 
frequency. So, a given transformer might use an iron alloy that reaches a 
certain degree of saturation at 1.7 tesla, but this same core might be safely 
driven to just 1.2 tesla at 50Hz, due to heating caused by loss. At 60Hz it 
would then work at 1.0 tesla, which is fine. In that case, running it at higher 
voltage, driving it to 1.2 tesla at 60Hz, would be fine from the point of view 
of saturation, but would cause excessive loss and heating in the core!

The actual situation depends on the specific characteristics of a given 
transformer. Some 50/60Hz transformers will be OK when driven to a 20% higher 
voltage at 60Hz, while others will not.

Of course, when feeding an entire power supply with overvoltage, additional 
questions arise regarding the voltage rating of the diodes and the filter caps, 
the power rating of the bleeder resistors, etc. My opinion is that when a 
designer built in some safety headroom, this should be kept as safety headroom, 
and not used up by deliberately rising the operating voltage. Unless the safety 
headroom in the design is much larger than necessary, of course, but that's
rarely the case.

So I very much agree that the best way to increase the output voltage is to add 
a small, moderate voltage power supply in series with the output.

Manfred

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