I am designing a changeover circuit that will switch the power of attached appliances between two electricity providers. The two AC mains are of 220V RMS voltage and 50Hz frequency sine waves. The two supplies are independent and out of sync. I will be using a microcontroller to control dpdt relays for switching. I may need to switch between grids every hour or every half hour.

During one cycle of sine wave, what is the best time to switch the load? I did some research and found out that in the UPS, when it's time to switch over from inverter to mains, the controller first syncs the inverter with the grid by varying the frequency of the inverter before switching. But in my case I can not sync the two supplies.

So my question boils down to: What are the things that I should worry about during switching? I want appliances to remain alive during the switching and I do not want to damage my appliances (due to some weird harmonic) during the switching.

It would be much appreciated if someone points in the right direction.

  • \$\begingroup\$ From the point you make the decision to switch, how long can you hold off implementing the switch? Also what are the loads? \$\endgroup\$
    – Andy aka
    Sep 27, 2015 at 20:18
  • \$\begingroup\$ If you look at some pure sine ups switch curves, they don't look all that awesome as shape, but apparently they are good enough for most loads: youtu.be/KJj3H00idh0?t=592 I think that works fine because spike/noise/crap like that is non uncommon on the power lines. Also you should look earlier in the video where he compares the utility supplied sine wave shape with the one of the UPS: the latter is much better while the one from utility co is rather obviously distorted (surely has harmonics). So I wouldn't sweat it on those issues much. \$\endgroup\$ Sep 27, 2015 at 21:14
  • \$\begingroup\$ @Andyaka, Loads include fridge and washers etc. I would like the switching to be as soon as possible, I do not have a number in mind, but should definitely be in orders of ms. \$\endgroup\$
    – Hassan
    Sep 28, 2015 at 5:31
  • 1
    \$\begingroup\$ Fridge doesn't need millisecond switchover. Minutes would be OK. Washer doesn't either, provided it doesn't need a manual restart after a power blip. What's the real problem? \$\endgroup\$
    – Transistor
    Dec 27, 2015 at 0:11

1 Answer 1


Your approach will cause a gap in your power to the appliances ,in fact if you program the thing to have no gap the relays will blow up ,remember that the load in general is not entirely resistive .You must think of the relays as a safety component which would be essential for product approval.If you need seemless switching which good UPs do then solid state devices are needed to back the relays.Wave point switching like what you say is valid but it is unfortunately load dependant ,Not a show stopper with a micro BUT your relays will be too slow and innacurate for this to work anyway .

  • \$\begingroup\$ Does do you mean by solid state devices backing the relay? \$\endgroup\$
    – Hassan
    Sep 28, 2015 at 5:46
  • \$\begingroup\$ Use relay is serirs with solid state so the relay will cold switch and never arc .Also a mechanical contact is better from a product approval viewpoint. \$\endgroup\$
    – Autistic
    Sep 28, 2015 at 19:58

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