Isolating Transformer

Isolating transformer (8K)An essential item for safety. For example, when working on switched mode power supplies and TV sets where you might need to safely connect test equipment to the "live" parts of the circuitry. 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...

As is often the case, this project was based on components rescued from skips! A studio at work was being refurbished and two 500VA isolation transformers, provided for the safety of performing artists with 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 5mm thick aluminium panels, and are joined by 4 aluminium extrusions. The top and bottom panels are thin plastic coated steel. There were no front or rear panels, so I cut down some panels of 3mm clear anodised aluminium (these were spare panels from another rack case system). These are bolted onto the extrusions using M3 bolts, which are perfectly adequate here.

Isolation transformers mounted in bay (23k)

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

Isolating transformer on the bench (45k)

This view shows the unit before mounting in the rack - you can see the rack ears that are clearly an addition. The lid is well ventilated, which is just as well because these transformers run quite warm. The slots are pressed upwards, which neatly prevents me from mounting something directly on top of the unit and blocking the air flow.

This picture shows the inside of the unit, viewed from above:

Top view of the unit (58k)

As you can see, it's really very straightforward. The two mains transformers are securely bolted to the side panels using countersunk M6 machine screws. Luckily, the side panels are relatively thick, and the extruded sections give the case a lot of strength.

Looking carefully at the corners, you might just be able to see the earthing cables that electrically bond the panels together - this is good practice from a safety point of view, especially when there are a lot of internal components and wires carrying mains voltages. As a principle, earth tags should be secured using a bolt that is not responsible for holding anything else. You won't see the two bolt heads on the front panels because they are cunningly hidden by the MK plug sockets!

Here is the view from underneath:

Bottom view (56k)

It's worth being as neat as possible... Along with copious cable ties, I've used P-clamps on each extrusion to hold the cables that run all the way from front to back - you can just about see them in these photos.

Let's take a closer look at the wiring:

Internal wiring at the rear of the unit (54k)

The mains enters via a fused IEC socket on the rear panel. Emerging from the insulating boot are a pair of 3-core cables and an earth wire that goes straight to the cahssis, and a separate, heavy-gauge earth cable that goes to the 4mm socket mounted on the front panel.

Internal wiring behind the front panel

The two 3-core mains leads from the mains inlet travel to the 2A thermal breakers on the front panel, and then back towards the rear of the case to connect to the primary windings of the transformers. Note how these leads were not cut completely through at the circuit breaker - look at the earth leads...

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


It might be a very simple project, but it 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 a bad weekends work...

Minor update - November 2014

I needed to mount something directly above this isolation transformer in my new bench - a mains distribution unit. This unit was relatively shallow, so isn't blocking much of the airflow, but it wasn't shallow enough to fit into the bay without colliding with the pressed ventilation slots on the lid. The solution was to produce a new lid from a sheet of perforated steel:

A new top panel for the isolating
     transformers (85k)

This sheet was recycled from an old 19" rack system, and it just took 5 minutes with the tin-snips to cut it to size. The rack area is reasonably well enclosed, so I'm not too worried about dust and debris finding its way into the enclosure. It's not as rigid as the original cover, but where it's been installed, this is not an issue. I've stored the original top cover in the same rack space so it can easily be re-fitted in the future.