AC Mains Isolation Transformer
An AC mains isolation transformer is an important piece of test equipment for both the experimenter and the security conscience individual. For the experimenter, connecting the equipment you are working on through an isolation transformer will "isolate" the unit from the incoming AC power line, should the hot and neutral lines be reversed for whatever reason. Anyone who has ever felt an electric shock while moving an old TV set will be very familar with this. This particular isolation transformer will also include a switchable output current limit. This is a handy addition when checking the operation of those old radios or unknown voltage transformers you find cheap at hamfests.
For the security conscience individuals out there, an AC mains isolation transformer can help to isolate your security compound from the "real world." The audio from hidden surveillance devices routinely rides atop the AC power lines coming into everyone's home. This method of transmission is called "carrier current," and has been used for many years to extend the range of baby monitors and those "no-wire" extension telephones. Modern carrier current surveillance devices used by federal intelligence agencies use spread spectrum modulation to hide the signal even further into the background noise and voltage spikes which are unavoidable in the AC power distribution system. Using an isolation transformer, and a good AC line filter, will help to remove any unwanted signals piggybacked on your incoming AC power lines.
From the National Technical Investigators Association (NTIA):
1) Advanced Carrier Current Monitoring System
Carrier current transmission is often used for audio monitoring within a facility where it is impractical to install special wiring. In the past, however, this approach has been limited to one or at the best a few voice conversations being carried over the AC power line at the same time.
Using advanced spread spectrum technology, it is now possible to transmit up to 200 voice conversations simultaneously on a single AC line. The same line can also carry coded commands for activating or de-activating selected microphones, video cameras, tape recorders, lighting, doors, and other functions.
Entire facilities such as prisons, hotels, and office buildings can now be pre-wired, with the assurance that specific areas can later be monitored on command.
The main component of the AC mains isolation transformer is, of course, the isolation transformer itself. The good news is that these are available for free at ham radio swapfests. The bad news is that you may need to look around a bit to find one, but you shouldn't pay too much for one. For this isolation transformer, I managed to find a fairly heavy-duty AC voltage transformer which appears to have both high-current capacity and a "130 VAC" secondary winding (close enough to 120 VAC). By tapping this 130 VAC output secondary, we essentially have a 1-to-1 ratio voltage transformer that can handle several amps of continuous output current. This will be perfect for powering a small computer system in a "secure" location. It would probably be a good idea to add a large fan to the transfomer to prevent it from overheating or over saturating if you do use it continuously, though.
Construction Notes & Pictures
Needed parts overview. A large ammo box will house the transformer along with an incoming AC line filter and circuit breaker. A shatter-proof lightbulb, holder, and octagon box are also needed, along with the assorted support hardware.
The lightbulb will act as an output current limit and can be switched in and out of transformer's secondary. A 15 amp circuit breaker will protect the transformer's primary and a 5 amp fuse will protect the secondary.
The final isolated 120 VAC output will be from a standard, single-gang AC outlet.
An external Earth ground post and neon light indicator are also added, but are not required.
The transformer used. No idea who made it or what its specifications are, but it's heavy and has a 130 VAC secondary winding.
Because of the transformer's large inductance, they only pass low frequencies. This makes them natural bandpass filters for cleaning up AC line voltage.
Mount the transformer in the ammo box as so. The primary connection passes through a resettable 15 amp circuit breaker and a grounded AC line filter. Be sure the transformer has good mechanical support to prevent it from "humming" while in operation.
The transformer's secondary passes through a 5 amp fuse and then onto a SPDT switch with a "center off" position. One switch position sends the output directly to the AC outlet, the other switch position selects a series-connected 150 watt lightbulb. This lightbulb limits the output current to around one amp in case of a short circuit.
Completed overview. Since the lightbulb is just a temporary current limit and is not meant to be left in the circuit, you should not have to worry about it overheating and melting any nearby wiring.
Overview, alternate view.
Outside case overview. A short AC power cord with three alligator clips connects to the equipment under test. A small hole in the front panel is used to monitor the internal lightbulb. The green Earth ground post is on the lower-left.