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SMPS grounding question

Hithesh
Genius 3290 points

For an SMPS with 2 pin connector (like the old IBM T42 laptop adapters), what is the role of the R and C that are connected to Case ground. Usually R is in Megaohms and C around 0.1uF,2KV.

Are they even required.
Is there a risk of shock when the case is touched, if its a metal case?

  • 0
    TI__Mastermind 34465 points

    These components are part of the EMI reduction circuitry. Essentially they provide a low impedance path from the noise generating source on the primary side (usually this is the main switching FET) to the grounded output side.  Without these components , the noisy signal will find its way back to nose measuring device and register as noise in the EMI spectrum.  You are correct in there being a risk of electrical shock here. For safety reasons , there is a limit to the size of capacitor used here. I believe IEC60950 limits the current here to 0.25mA. Therefore for a 230V,60Hz system the max cap that can be used is about 2700pF. Also these capacitors have high volatge ratings and have several different safety ratings such as Y1,Y2 etc.

  • 0 in reply to John Griffin1
    Genius 3290 points

    John. Thanks for the reply.

    Does the IEC standard have a limit on voltage.

    I am running the power supply in question is a 220VAC to 24VDC Industrial power supply.

    When the Earth pin is not connected, I get a voltage of 100VAC between Earth pin of a wall socket and Neutral point of the power supply. Are Line and Neutral interchanged inside the power supply?

    This seems way too much.

    The soldering station in the lab has 2 pin connector. But I don't see any voltage between Neutral and Earth.

    Even the Lab power supplies have been connected with a 2 pin socket unintentionally. But never experienced shock.

  • 0 in reply to Hithesh
    TI__Mastermind 34465 points

    Because of the Y capacitor, you will measure about half of the AC voltage( 100V ) between Earth and the return of the power supply.

    The lab soldering station is probably powered from an isolated AC source and there is no connection between Earth and the soldering station.

  • 0 in reply to John Griffin1
    Genius 3290 points

    John Griffin1 said:

    Because of the Y capacitor, you will measure about half of the AC voltage( 100V ) between Earth and the return of the power supply.

    The lab soldering station is probably powered from an isolated AC source and there is no connection between Earth and the soldering station.

    Well, in that case, if I touch the case I am coming in contact with Line voltage through the Y cap (assuming the Earth pin is not used in the plug).

    Isn't the Y cap from Line to Case a shock risk.