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What do you want from an (isolation) transformer besides it having the minimum voltage/current/power(/frequency?) specs that the to be isolated device needs? Does it matter whether the transformer is being used to perform precise measurements, to prevent for getting in contact with live circuitry or something else?

The reason why I'm asking is that for a 230V isolation transformers that can handle at least 2A current, I've seen isolation transformers starting from €60 for a bare transformers to well over €200 for units that are in some kind of enclosure and have medical grade specifications. While my primary motivation for an isolation transformer is working on power supplies it would be nice if it is not a single purpose tool.

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    \$\begingroup\$ It pretty much is a single-purpose tool. \$\endgroup\$ – Ignacio Vazquez-Abrams Jul 18 '18 at 10:10
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    \$\begingroup\$ @IgnacioVazquez-Abrams , it is a single-purpose tool as in you need a different kind of isolation transformer for precise measurements and power supply work? Or an isolation transformer sole purpose is to isolate regardless of the reason why you want to isolate. \$\endgroup\$ – Henk Jul 18 '18 at 10:20
  • \$\begingroup\$ There are several good reasons why you want to isolate, but in the end its purpose is strictly isolation. \$\endgroup\$ – Ignacio Vazquez-Abrams Jul 18 '18 at 10:22
  • \$\begingroup\$ At least two coils? \$\endgroup\$ – Voltage Spike Jul 18 '18 at 20:16
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You are asking about an application specific device so you will most likely find a wide range of options for the isolation transformer. Some are related to safety and some are related to performance as Neil_UK has said. I cannot say which items are the most important for your application, however in general:

  1. When selecting transformers you should consider environment temperature to maintain the life of the device. So you may see temperature ratings. You should consider whether you are using your device in some kind of extreme conditions such as a hot work space or outdoors.

  2. Continuous current and instantaneous current are not the same thing and typically the device has ratings for both.

  3. Do you want it to have winding taps so you can change output voltage for future applications?

  4. Consider any fusing recommendations the device may specify for magnetising current.

  5. Efficiency ratings - How much power are you willing to supply to produce a specific output with losses included?

  6. Fault current - Many of the medical grade isolated transformers have fault limiting devices included which is part of what makes up the enclosures. Fault current in a transformer should always be avoided because amperage levels are usually much higher than then the continuous current ratings. Sometimes the instantaneous current rating will will be greater than a proposed system fault, however this is going to be system and application specific.

  7. Electrical Noise - Typically an isolation transformer is used to reduce common mode or differential mode electrical system noise. This is mainly for audio/video application. The medical grade isolation transformers will typically have these capabilities and also additional filters inside the enclosure.

  8. Voltage Regulation - Is it acceptable for your application to have your output supply voltage fluctuate?

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'Isolation' means different things to different people, there are different grades of it.

The differences come in two areas. Safety, and performance.

Safety. You do not want the isolation between primary and secondary to break down normally, but how much abnormal are you willing to tolerate? 1500V is a typical level for industrial and mains, though you might want to specify 2500V, or 5000V, or some other higher figure, if you felt the cost of a breakdown, perhaps the patient's life in the medical case, was very high.

Performance. Ideally, an isolation transformer will no connection between primary and secondary. Imagine a motor driving a generator on the end of a 3m long plastic shaft, that would be very isolated. In practice, there is some capacitance between all of the primary and its wires, and the secondary and its wires. You can model the effect of that as an AC current source at mains and its harmonics between primary and secondary. For an industrial application, that might not even be specified. For a measurement or medical application, it might be specified to be below a maximum measured in the uA or even nA.

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