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    Protecting Low Voltage Electronics From 220V AC Voltages

    Hi All,

    I'm a newbie here. I'm currently working on a device that controls 220V AC equipments via an electromechanical relay. The relay and the low voltage electronics are all housed in one enclosure. The connections between my electronics and the relay are VCC, GND and CONTROL line for turning on/off the relay. In this setup, how can I protect my electronics from the AC voltage in cases like there is a faulty relay or that the AC wiring is shorted with the VCC, GND or CONTROL lines? Thanks!

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    Re: Protecting Low Voltage Electronics From 220V AC Voltages

    Hi,

    a relay must be rated for the isolation voltage. Then there is no need to fear a failure connection between contact and coil.

    There are regulations about air gap and creepage distances.
    The regulations are difficult to read because the values depend on a lot of parameters.
    Try to use 8mm of creepage distance on a PCB.
    The wiring of the different voltage domains need to be separated ... or both (even the low voltage ones) need to be rated for the higher voltage.

    A metal housing needs to be connected to earth ground.

    The connections between my electronics and the relay are VCC, GND and CONTROL
    I wonder why three wires, why not just the two relay connections?

    Klaus
    Please don´t contact me via PM, because there is no time to response to them. Thank you.



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    Re: Protecting Low Voltage Electronics From 220V AC Voltages

    Quote Originally Posted by KlausST View Post
    Hi,

    a relay must be rated for the isolation voltage. Then there is no need to fear a failure connection between contact and coil.

    There are regulations about air gap and creepage distances.
    The regulations are difficult to read because the values depend on a lot of parameters.
    Try to use 8mm of creepage distance on a PCB.
    The wiring of the different voltage domains need to be separated ... or both (even the low voltage ones) need to be rated for the higher voltage.

    A metal housing needs to be connected to earth ground.


    I wonder why three wires, why not just the two relay connections?

    Klaus
    I fear for relays that have defects or AC wires accidentally come off and touches the three low voltage lines. Three lines are needed as the MCU GPIO I use can't provide enough current to activate the relay and I'm also not using optocouplers. Here is the relay kit I'm using:

    https://www.sparkfun.com/products/13815



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    Re: Protecting Low Voltage Electronics From 220V AC Voltages

    A circuit that incorporates high and low voltage circuits must be designed with safe clearance. Wires must be attached and fixed in a way that prevents accidental shorts between both circuits. Review the requirements for reinforced insulation.

    The most critical point is that exposed parts of the low voltage circuit can carry a hazardous contact voltage in this situation. It may be possible to clamp the overvoltage to protective earth using current and voltage limiting elements.



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