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first time in here!

I designed a simple PCB to accomodate a relay, meant to switch a 230VAC(50Hz)- powered lamp using a 24VAC command signal. I used an ABB CR-P024AC1 (properly rated for my application) with a fuse and thermistor in series with the phase line (to protect from overcurrent and inrush).

About 90% of the time, when I switch on the command signal the relay activates properly but produces an audible buzz. This was tested both with and without mains voltage and lamp hooked up to the circuit. I have found that after letting the lamp switched on for an hour or so, the buzzing goes away, and I can switch it on and off a few times without it buzzing, then it starts buzzing again after a minute or two. It seems very likely that the contact arm causes this, however I don't see any variation in the intensity of light produced by the lamp.

I researched this issue, but all situations I found seemed to be either a result of controlling a DC relay with AC, or control voltage being too low. In my case, I'm using a 24VAC, 1A wall wart as a quick way to provide the command signal for testing, and my cheap multimeter gives me a reading of 26V (using the 200VAC caliber). I must clarify that I have no previous experience working with relays.

Is this buzzing normal? If not, could this buzzing be caused by a low quality control voltage?

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  • \$\begingroup\$ What AC frequency are you operating on? \$\endgroup\$
    – Andy aka
    Feb 28, 2020 at 13:51
  • \$\begingroup\$ I am on 50Hz (French mains). Will edit the question to include it, thank you! \$\endgroup\$
    – Hugo Cuzon
    Feb 28, 2020 at 13:52

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AC relays will hum or buzz. Their coil windings are not so very different from the voice coil of a loudspeaker, but without a large diaphragm to drive. They are often dipped in varnish or wax to fix the winding in place and minimise the noise, but that's an imperfect process. If it's not drawing excessive current or getting excessively hot, I would consider it normal.

Poor quality AC waveform will have higher harmonic content, at frequencies where this loudspeaker is more efficient, so you will hear it get louder or change tone at certain times of day as usage of the supply changes.

If the noise was intolerable I'd consider running it off 24VDC instead (if the relay datasheet permits that)

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  • \$\begingroup\$ That explanation makes sense. I couldn't feel any heat coming from it after it ran for two hours. And the buzz is noticeable in relative silence and close proximity (a meter or two), but not loud in any way. Looks like it is normal behaviour then, judging by your answer! \$\endgroup\$
    – Hugo Cuzon
    Feb 28, 2020 at 14:20

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