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  1. #1
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    Is this usb relay board safe for high powers?

    I'd like to use this usb device:


    https://www.aliexpress.com/item/3295...64552e0ezng2GS


    to control 220-240V/AC appliances, drawing around 5A. Specifically, to control my central heating unit, which is not programmable itself.

    The device seems easy to program and to wire, however, I am concerned because I found this warning:


    "The USB specification has clearly provided guidelines as to how power should be utilized from the USB port, and surging currents produced by high-current mechanical relays is strictly prohibited."

    (https://ncd.io/choosing-the-right-us...er/#document-5)


    I am wondering whether that warning applies to the device above (whose relay's specs can be found here: http://www.circuitbasics.com/wp-cont...-Datasheet.pdf).


    Thanks!

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  2. #2
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    Re: Is this usb relay board safe for high powers?

    If you read that statement again, it specifically says "...power should be utilized from the USB port...". Unless you're powering your heater directly from the USB port, which you're not, then this is not a problem.

    But it makes me a little concerned that you seem to not know that much about power and you are dealing with lethal voltages.



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    Re: Is this usb relay board safe for high powers?

    Quote Originally Posted by barry View Post
    Unless you're powering your heater directly from the USB port, which you're not, then this is not a problem.
    No, that's not what the quoted text implies to me.

    "surging currents produced by high-current mechanical relays is strictly prohibited"

    Doesn't this mean that it is the relay itself, not the load controlled by the relay, which can possibly produce those surging currents?



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    Re: Is this usb relay board safe for high powers?

    I agree with barry that the text is clearly referring to the USB spec and "how power should be utilized from the USB port". I wonder why you have left out the preceding sentence in your post?

    But I admit that the statement is in some regard missing the point. The most serious problem with switching power relays though USB interface is it's low interference immunity. Relay arcing can easily stop USB operation, you may need to unplug and reconnect the USB adapter to reinitialize communication. Possible counter means are common mode ferrite chokes for the USB cable, electrically isolating the power relay from the USB adapter and suppression of relay contact arcing.



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    Re: Is this usb relay board safe for high powers?

    Relays are inductive, they don't draw surges, the coil current rises gradually. The text is misleading, I can only assume it means that if you use the relay to switch some other load across the USB supply it might 'surge'. If it refers to the reverse voltage spike you get when the relay turns off, there appears to be a MELF diode across the coil which should provide adequate protection.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



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    Re: Is this usb relay board safe for high powers?

    Ok, so the text is misleading, but there are safety problems with that relay, anyway. I think I will consider this, instead:
    https://sonoff.itead.cc/en/products/sonoff/sonoff-basic

    Thank you for your useful answers!



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  7. #7
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    Re: Is this usb relay board safe for high powers?

    "...
    The USB specification has clearly provided guidelines as to how power should be utilized from the USB port, and surging currents produced by high-current mechanical relays is strictly prohibited. While a few small signal relays can be safely powered from the USB Port, the market for such relays is relatively limited.
    ..."

    I would consider your relay "small signal", not a "high-current" relay



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