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I'm using the RAC02-GB (Datasheet) as an AC/DC converter and I'm confused about the protection circuit they included in the datasheet:

Protection Circuitry

I want to make sure the AC/DC converter has the proper external protections but I'm a bit baffled by the datasheet's directions. I'm assuming the component labeled 'F' is a fuse but I'm not sure what the component labeled 'U' is. The datasheet lists the parameter "input fuse" as "internal" does that mean the protection circuit is already included in the converter and I don't need any external circuitry?

If I do need external circuitry, how do I spec the fuse and/or the component labeled 'U'?

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The U is a varistor (=MOV). It protects the converter if there's input overvoltage. Such voltages can easily occur as peaks when the input is taken from the mains AC. For the same reason measuring equipment must stand much more than the nominal mains AC voltage. Read as quickly as possible something about CAT ratings of multimeters if the thing is never heard for you.

External MOV is recommended in the datasheet (see the notes). The fuse trips if the MOV gets shorted which can happen in serious cases.

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