3. The heatsink will need to dissipate even more energy because of the greater voltage drop
across the regulators.
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.
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.
It is essential that everything you build, that has both live mains inside and a metal case or
panels, has a safety earth fitted. UK legislation says that any metal panelling should be
adequately insulated, ie. double insulated, or connected to earth. Since making a double
insulated case is not practical you should ensure that any exposed external metal parts be
properly earthed.
The case should be bonded to earth using an M4 screw, toothed washer, washer, solder tag
and a securing nut (or two) bolted through the case and then via a thick piece of wire back to
the earth tang of the IEC power inlet. Remember that all parts of the case must be earthed too.
Painted metal parts of the case must be dealt with so that they too are earthed. This may
involve using secondary bonding points or scraping back the paint at the appropriate point.
The RPSU board should be securely mounted (using all four mounting holes) onto the earthed
casing using appropriate screws and toothed washers. R5 on the power supply board should
be a wire link. This should ensure that the power supply ground (0V) is tied to earth.
You will also need to provide earthing to any exposed transformer core. This does not apply
to toroidal types but EI types should have their metal frame earthed.
It is possible that by earthing the case and local ground you may introduce earth loops when
you connect your mixer to the sockets of your project. The outcome of this is audible
humming at 50/60Hz and its harmonics. It is produced by earth currents travelling down the
screen of the connecting cable(s). This can be avoided by careful studio wiring and/or by using
balanced audio lines to pipe signals to and from the module and mixing desk. Most mixing
desks and sound cards will have balanced outputs and inputs.
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