Using an Internal Mains transformer
Be afraid, be very afraid...
Some of you old hands will laugh about the level of hysteria that surrounds direct mains
connection to DIY projects. However, this fear is more to do with our litigious society than
the real danger to the builder. Even so, I have had more than my fair share of high voltage
shocks over the years and it is not something I would want any builder to have to experience.
It has been purely luck that has saved me in several of those cases.
So I will say again – do not attempt to build a mains transformer into your modular case, or
any other project, without realising that to do so carries a risk of death to either you, or to the
person who may inadvertently put their fingers into your half built construction. Furthermore,
it is up to you as the builder of such an item to make sure, that once built, the item is safe to
use and meets all current safety legislation.
This is not a complete set of instructions on how to fit a transformer into a piece of
electronic equipment. This information is offered only as a guide and should not be
considered as your only source of information. No further information, other than that
included here, will be provided by myself or Paul Darlow regarding the construction of
mains powered items.
The mains transformer's secondaries should be rated:
18-0, 18-0 (or 18-0-18)
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.
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