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 SRE330, 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 regarding the construction of mains powered items.
For powering the SRE330 then the mains transformer's secondaries should be rated:
Voltage: 18-0, 18-0 (dual secondary) or 18-0-18 (single tapped secondary)
Power: At least 25VA
This will give you a power supply that should be theoretically capable of providing just over
400mA to each rail assuming your heatsinking and smoothing capacitors are up to the job.
The transformer secondary voltage is suggested to be 18V. It may be possible to use a 20VA
transformer rated at 15V. Most transformers produce more than their stated voltage when drawing
less than their maximum current and I have found that 15V toroids always work well here in the
UK. The benefit of using a lower secondary voltage is cooler power devices. However, the
disadvantage is that you may be running your power supply very close to its lowest operating
voltage – particularly if your country's line voltage is less than the expected 230V (or 110V).
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 current.
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, ie. the case, 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.