The mains transformer's secondaries should be rated:
18-0, 18-0 (twin secondaries) or 18-0-18 (centre tapped)
80VA
This will give you a power supply that should be theoretically capable of just over 1A each rail
assuming your heatsink and smoothing capacitors are up to the job. In this case you should
make both R2 and R3 0R56 2W resistors.
Take note that the 5U high 3U wide front Master Panel design is insufficient to carry away the
heat safely when continuously supplying over 520mA per rail. A different method of mounting
the power devices must be found unless you limit the maximum current to less than 520mA.
The 4U 19” panel detailed earlier may well be adequate for a 1A supply but remember that the
secondary voltage will affect just how much heat needs to be dissipated. It is up to you to
verify that your chosen panel is up to the job of keeping those power devices cool. Remember
that air flow to the panel is essential so make sure any tests you do are representative of the
situation your heatsink will eventually be fitted to.
The transformer secondary voltage is suggested to be 18V. Slightly higher transformer
secondary voltages can also be tolerated although you need to consider three things:
1. You may need a physically larger transformer for the same output power.
2. The power supply's components, including the smoothing capacitors, should be rated at a
high enough working voltage to handle the increased voltage across them.
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.
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