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restricting the current to below 90% of maximum will keep the transformer somewhat cooler.
Hence the factor of two mentioned above is probably more realistic. Using a factor of two the
maximum current drawn from our 80VA transformer is now 1.11A from both rails.
In the wiring diagram shown I have included a suggested wiring method for connecting up a
mains transformer. Not all mains transformers are the same, some have additional windings,
others have tapped windings. I have simply used a single primary, double secondary type for
example only.
For the mains fuse you should use a 500mA anti-surge type. All wiring at mains potential
should be adequately insulated and protected from straying fingers.
There is no need to fit an AC standby switch since you will be fitting a proper mains power on
switch in series with the transformer primary coil. So you should link S1A to S1R, and S2S to
S2S, on the PCB.
Here I have used a SPST switch for mains switch, a standard IEC socket and a separate 20mm
fuseholder. This transformer has two 115V primary windings which for a 240V input need to be wired in
series. The three pole Wago block to the top right provides a safe way of connecting the primary
windings in the right fashion. Note the rubber boot around the fuseholder and shrouded Faston crimps on
the wiring to protect straying fingers from high voltages
Earthing
Remember it is up to you, the builder of the equipment, to make sure that your item is safe
and is built to the required safety standard in your country. These notes are only a guidance
and it is up to the reader to establish the exact obligations required in their own country.
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