Isolating transformers, more commonly known as isolation transformers, are used to reject stray common mode and DC voltages, ground loop potentials, electromagnetic interference (EMI) and radio frequency interference (RFI) from being conducted through pure AC circuits. They are commonly used in AC-powered analytical equipment, medical devices, telecommunications equipment and audio systems. For example, an electrically noisy device such as a vacuum cleaner would be prevented from trickling its noisy interference back into the power grid to affect televisions or other monitoring equipment powered from the same local grid.
Isolation transformers have primary and secondary sets of windings and a winding core like other transformers, but they differ in that they maximize magnetic flux within a narrow band of design frequencies and minimize capacitance; the one winding set with interference or electrical noise has a shield to block the interference from being further transmitted through the AC circuit. Unlike step-up or step-down, isolation transformers are usually configured in a 1-to-1 ratio where voltage in equals voltage out.
Vacuum cleaners and power tools head the list of "noisy" electrical devices because they induce high levels of interference back into the power line they are connected to. This is why running a vacuum cleaner may cause interference on your television set, radios, sensitive electronics such as computers and analytical equipment, and communications systems. Isolation transformers are increasingly finding their way into home special line current circuits to keep this noise and interference limited to the circuits supplying the tools.
Large manufacturers also run separate circuits for noisy tools and sensitive electronics. Isolation transformers in the lines to the noisy devices minimize the length of wiring the interference may traverse. A further complication in long or expansive buildings is the possible large voltage potential between two electrical grounding grids on opposite ends of the building, because of inductance and leakage in what may be connected to these grids. This introduces noise, buzz, hum and pops into communications and computer networks, and unacceptable noise in audio systems.
Theaters may have large distances and separate power circuits between projection and audio systems. Isolation transformers on AC and audio lines prevent the exchange of noise between these systems.
Telecommunications and computer centers use isolation transformers to segregate potential interference issues in much the same way theaters do.
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