Re: Replacement transformer


"David Looser"

Transformers are designed to produce the nominal output voltage at full
current draw, when under-loaded the output voltage is higher, typically
10-15% higher.

** True only for small transformers - say under 80VA.

Above that, regulation steadily improves to around 4% at rated VA.


In addition an over large transformer is, well, over large; bigger,
heavier and more expensive than you need. Oh, and it will be less
efficient than one of the correct size.


** Ignoring core magnetising losses, transformer efficiency IMPROVES when
the load VA is made less than the rated VA.

Percent efficiency = 100 minus the ACTUAL percentage regulation - so a
transformer rated at 75VA and 15 % regulation will have 3.6 % regulation at
20 VA.

The above transformer is 85% efficient at full load and 96.4 % at the
reduced load.

BTW

A 75 VA toroidal will dissipate about 1 watt due to magnetising losses.


...... Phil


.

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