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I have a remote controlled switch that's connected to a bathroom fan (220v AC, 50Hz). The switch turns the fan on without issues every time. However, when instructed to turn it off, it immediately comes back on. The switch does NOT do this every time and in order to turn it off, I need to send the off command several times. Sometimes, it works the first time, sometimes on the 5th or 6th attempt.

The same switch at the same location, works with a normal light bulb so I'm pretty sure there is nothing wrong with the switch.

I am told that since this happens only with the fan, it could be an inductive kickback which with this particular type of switch, might be turning it back on.

My question is:

  • Can a small AC 220v bathroom fan generate inductive kickback (I thought that's DC only).
  • Is there a way to prevent this by using something similar to a flyback diode (I understand that on an AC circuit a flyback diode can't work, but I am asking about a similar solution for an AC inductive kickback)

For information, this is the switch: https://sonoff.tech/product/diy-smart-switches/zbmini/

Khash
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    Any inductive load will generate kickback when turned off, even AC loads. – Jonathan S. Jan 16 '23 at 15:06
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    with an AC load, you might use a snubber, or a bidirectional voltage limiter – Neil_UK Jan 16 '23 at 15:16
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    Do you have a *datasheet* for the switch? Not just a product page, but a datasheet? It's quite possible it's not designed to switch inductive loads and doing so, especially without a snubber, may be damaging its internal relay or thyristor. – Hearth Jan 16 '23 at 15:31
  • @Hearth thank you for the comment, unfortunately I haven't been able to find any (the accompanying docs with the switch are just setup instructions). I will try to find a suitable snubber before there is damage to the switch – Khash Jan 16 '23 at 16:01

1 Answers1

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Can a small AC 220v bathroom fan generate inductive kickback (I thought that's DC only).

Any conventional motor (whether AC or DC) will generate a back-emf when the supply is suddenly disconnected. This will generate a spark across the opening contacts that are disconnecting power to the motor. A bathroom fan is no exception.

Is there a way to prevent this by using something similar to a flyback diode (I understand that on an AC circuit a flyback diode can't work, but I am asking about a similar solution for an AC inductive kickback)

The name of the fix is a "snubber". They are usually made from a capacitor in series with a resistor. This should be applied across the terminals of the contact that opens to disconnect the fan. Try 100 nF (appropriate voltage rating) in series with a 100 Ω resistor. They are available like this: -

enter image description here

From ebay or just search images for fan snubber

Andy aka
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  • Thank you! I will fit one as per your recommendation. Is the snubber going to "leak" when the fan is off? – Khash Jan 16 '23 at 16:03
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    @Khash Yes, it will, but unless you use a very large capacitor it won't leak much. – Hearth Jan 16 '23 at 16:09
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    @Khash 100 nF has an AC impedance of 32 kohm at 50 Hz. There's a small risk it might cause your fan to buzz a little with the handful of mA that it will pass to the fan. – Andy aka Jan 16 '23 at 16:14
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    I was going to ask if the additional resistor was needed, seeing how this has an ohms rating. In the listing it says: "Combination 0.1uF (100nF) capacitor internally fitted with 120 ohm series damping resistor" – Krista K Jan 17 '23 at 01:22
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    @KristaK additional not needed. – Andy aka Jan 17 '23 at 07:52