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Is there a design practice for having both a varistor and a class X or Y capacitor for safety and voltage transient protection on AC circuits?

At my work, a previous employee designed a simple AC to DC converter using an IRM-15-12. The design just includes a varistor at the input, and nothing else. Unfortunately, design documentation is lacking (there's no documentation for this PCB.)

There does not seem like there's anything wrong with the design. I am just curious if there should be a class X or a Y capacitor in addition to the MOV.

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    \$\begingroup\$ Why should there be? Is there anything in the module that requires it and, does the module meet your EMC requirements? \$\endgroup\$
    – Andy aka
    Sep 19 at 16:12
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The datasheet first page mentions the module has Class B EMI compliance without external components.

The block diagram in the datasheet says there is an EMC filter on input.

If you already pass the required EMI/EMC tests for the class of tests you need to pass, you don't need an extra filter.

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  • \$\begingroup\$ Okay, thanks for the answer! \$\endgroup\$
    – the-dstrp
    Sep 20 at 15:49
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Those capacitors are normally used for EMI filtering. If the design is not sensitive to external EMI and not emitting EMI that needs to be kept away from mains (to pass EMI regulations), you do not need them.

Its only that if you decide you need capacitors for input filtering, they need to meet those specific safety requirements, to avoid shock in case of failure.

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  • \$\begingroup\$ Ah I see. Okay! I will reassess if we need to include them in the design. The IRM-15-12 does come with an EMI filter. Thanks! \$\endgroup\$
    – the-dstrp
    Sep 20 at 15:49

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