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Old 21st April 2012, 10:24 AM   #11
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
Generally we find that the VA rating for a transformer is arrived at using internal temperature as the critical limit.

If the 160VA transformer is an 18+18Vac then the maximum continuous output current is 160 / [18+18] = 4.44Aac. Note AC current rating. This can only happen when the transformer is driving a resistor load.

That 4.44Aac applies to both secondaries individually, unless the specification tells you otherwise.

Some transformers are rated by regulation, rather than temperature. Here one specifies the VA and the regulation, but again that regulation and VA apply only when the load is a resistor.

The input VA will always be higher than the output VA. This is due to the fact that all transformers must have an efficiency <100%.

Assuming the transformer is 95% efficient at maximum power then the input VA will be 160/0.95 = 168.4VA.
The input current when the rated input voltage is 230Vac will be 732mAac
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