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I'm using a 5V relay to control wall AC socket to power up whatever AC appliance I could think of. But for the sake of testing I'm using 45W fan. For that purpose an AC/AC transformer (240VAC to 6VAC and later rectified and regulated to 5VDC) and wall socket are fed from the same mains socket.

The problem is that when the relays are engaged or disengaged it causes a voltage spike which can be heard through speakers attached to the same mains socket. What can I do to suppress these spikes, considering that in an extreme situation I might plug-in a 240VAC at 16A appliance going through the relay?

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  • \$\begingroup\$ It's just possible that some research would turn up previous solutions to this problem - have you done any? Have you tried any of the usual solutions? \$\endgroup\$
    – John U
    Commented Sep 6, 2013 at 16:56
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    \$\begingroup\$ I've found some arguments regarding lower voltage and amperage applications, using RC components or even diodes in anti-parallel. But here we're talking high voltage of 240VAC at 16Amps max. So I've some doubts if it can be solved exactly the same way. \$\endgroup\$
    – cvicente
    Commented Sep 6, 2013 at 17:17
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    \$\begingroup\$ Useful search terms : zero crossing switch, or zero crossing SSR, or Solid State Relay. \$\endgroup\$
    – user16324
    Commented Sep 6, 2013 at 18:55
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    \$\begingroup\$ I searched for solid state Relays, but appear that they aren't enable to withstand that much AMPS (in this case 16A). In what regards to the other terms, I'll investigate... thank you! \$\endgroup\$
    – cvicente
    Commented Sep 6, 2013 at 23:30
  • \$\begingroup\$ @Brian I was looking at the wrong place. I end up by finding SSR up to (and more) 16A. Thank you again... \$\endgroup\$
    – cvicente
    Commented Sep 6, 2013 at 23:51

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There are 2 devices that can generate spikes.

  1. The relay coils. The diodes in anti-parallel will help suppress the spikes when the relays are turned off (The stored energy in the magnetic field is converted back into voltage/current causing a spike that may be inductively and capacitively coupled back in to the mains). Assuming these are DC relays, feed by DC this could be a good solution But may still need filtering.

  2. The motor of the fan and the contacts of the relays. Here the motor can be switched on at any point in the mains cycle. If the relays close or open when the maximum current is being drawn you have a similar situation as turning off the relay coils. (The stored energy in the magnetic field is converted back into voltage/current causing a spike that is conductively and may be inductively and capacitively coupled back in to the mains) The solution is to switch at the zero point in the mains cycle. +1 @Brain Drummond comment for devices to search for that can use Zero Point Crossing ( zero crossing switch, or zero crossing SSR (Solid State Relay) )

Keeping the current loops as small as possible and moving cables away from noisy circuits can also help depending on how sensitive the audio equipment is.

After that it is inductors ,resistors and capacitors to construct filters to damp any spikes left.

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  • \$\begingroup\$ Thank you very much spoon... I need some time to investigate, and I'll probably get back for more advises! \$\endgroup\$
    – cvicente
    Commented Sep 6, 2013 at 23:40
  • \$\begingroup\$ +1 but consider the relative magnitude of the spikes. Relay coil : power maybe 1W. Inductive load : 45W fan (startup power maybe 200W!) or other load to 16A. Both may matter but the load is likely to be more. Hence zero crossing SW is likely the cure. But that forces you to SSR which also eliminates the coil... \$\endgroup\$
    – user16324
    Commented Sep 7, 2013 at 8:23
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    \$\begingroup\$ @BrianDrummond, Yes your right, the load is potentially the largest source of noise in this example. I wanted to highlight the possible sources. But if the load was a heating resistor then possibly the relay coil might be the worst offender. \$\endgroup\$
    – Spoon
    Commented Sep 7, 2013 at 11:42

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