Isolation Transformer:

An essential item for safety when working on switched mode power supplies and TV sets. I'd never really owned a decent unit, managing in the past by temporarily rigging a small transformer that I rescued from an old TV set - not exactly safe working practice!

Just for a change, this project was based on components rescued from a skip! A studio at work was being refurbished and two 500VA isolation transformers, provided for the benefit of performing artists with their electrical equipment, were no longer required. They were mounted in the studio furniture, so needed building into a suitable enclosure. They came complete with circuit breakers and standard 13A plug sockets.

I decided to build the two transformers into an old 3U rack case. This case didn't have rack mounting ears, but knowing how heavy the finished item would be, rack-mounting was an essential requirement. Modifying a recycled pair of rack ears involved a serious amount of filing to make them fit on the sides.

The sides of the case are made from 4mm thick aluminium panels, and are joined by 4 aluminium extrusions. Top and bottom panels are thin steel with a black plastic coating. There were no front or rear panels, so I cut down some panels of 3mm aluminium. These are bolted onto the extrusions using M3 bolts, which are perfectly adequate here.

As you can see, there are two identical outlets, along with 2 amp breakers and neon indicators for each half. There's also a pair of earth binding posts as the earth connections have been removed from the MK sockets in accordance with standard BBC practice.

This view shows the unit before mounting in the rack. You can see the rack ears that are clearly not an original feature of this case. The lid has vent slots, which is useful because the transformers run quite warm. The slots are pressed upwards, which neatly prevents me from mounting something directly in the bay on top of the unit and blocking the ventilation path.

These two internal views highlight the barrel distortion of my Fujifilm 4800. But you should be able to get a good idea of the internal layout - the transformers are bolted securely to the thick side panels using M6 bolts.

Looking carefully at the corners, you might just be able to see the earthing cables that connect the panels together - this is good practice from a safety point of view, although it's perhaps less necessary here because the panels aren't painted. Earth connections should be secured using a bolt that is not responsible for securing anything else. You won't see the two bolt heads on the front panels because they are cunningly hidden by the MK plug sockets!

Finally, here are two close-up images of the wiring. The schematic is too simple to bother drawing! The mains enters via the fused IEC socket on the rear panel. Emerging from the insulating boot are a pair of 3-core cables, an earth wire that goes straight to the chassis and a separate, heavy gauge earth cable that goes to the 4mm front panel sockets. The two 3-core mains leads travel to the 2A circuit breakers, and then onto the primary of the transformers. Note how they were not cut through at the circuit breaker - look at the earth lead...

The secondaries of each transformer are connected directly to the MK plug sockets via white 2-core mains cables

Conclusion:

It's a very simple project, but has made life much simpler and safer. I rely on it for fixing TV sets and switched-mode power supplies, but I also use it when working on kit that doesn't have terribly well insulated mains terminals internally. It was also invaluable when developing the control circuit for the Gainclones, as this runs directly from the mains. The only downside is that both transformers generate quite a lot of mechanical noise, and get quite warm, but as they are only switched on when necessary, this isn't a real problem.

So, not bad for just a weekend's work...