I have two 220V AC power sources and a contactor that switches to another if one is unavailable. But it takes few microseconds to switch and there is a moment when both power sources are disconnected during switch process.

Is it possible to add some electrical circuit (inductors?) after the contactor which will power the AC load during the switching process? Is it possible to make it safe without possibility that two power sources will be connected to each other for some time?

  • This question is probably a better fit for Electronics.SE.
    – LShaver
    Commented Feb 15, 2023 at 12:26
  • @LShaver Maybe, but I can't ask questions here and don't know why.
    – Robotex
    Commented Feb 15, 2023 at 12:29
  • This is an XY question, because you are asking generically and not about a specific critical load, for which other options might be available. Commented Mar 7, 2023 at 2:49

1 Answer 1


I think you probably mean milliseconds, so I'm answering on that basis.

The underying issue is that the two sources are almost certainly not synchronised. You can't get microsecond switching that way, you can't even get millisecond switching. So you need to answer the question "what do I want to happen when things switch?"

The usual answer is a 30ms or so interval when "we don't care, but don't cross the streams" (at 50Hz you have 20ms per cycle). This is how Automatic Transfer Switches work, for example. They are very carefully designed to switch quickly, but always have a gap when both switches are open.

Doing it electronically you could analyse the two waveforms and choose a least-current point to switch over. But that would be very difficult to get approved for grid connection*, so both AC sources will have to be "off grid". For larger systems the usual answer is to synchonise the secondary supply to the primary before connecting them. But unless you're in the price range where "ask your engineer" is the appropriate answer you will not be doing that*

The normal way around this for anything sensitive is to use an AC UPS or a DC power supply that can provide continuous power to the sensitive equipment for the duration. In high load applications this is commonly via an anlways-on inverter where the AC supplies charge the batteries that feed the inverter. Depending on the money involved this can be very robust and also very expensive. The "cheap" option is UPS's which get quite pricey once you want more than a kW or so for a few minutes.

Note that companies like Victron make inverters that can be parelleled or arranged in other co-operative modes so you can DIY them for a reasonable price (by, for example, having three inverters per phase where any two can provide the full required power). Making that setup properly fault tolerant is just a matter of spending money on COTS parts.

  • because certification cost for grid connection is in the millions of dollars
  • So, it's impossible to switch immediately without AC-DC-AC conversion, right? Thank you for your answer.
    – Robotex
    Commented Feb 21, 2023 at 6:33
  • 1
    You wouldn't want an instant switch. A guy tried DIYing a fast switcher. Big surprise! The furnace fan started running in reverse and filled the house with carbon monoxide. Turns out, the auto-switch happened just at the perfect time, and in counter-phase, to cause the motor to kick backwards, and once it's moving backwards it's going to stay going backwards because it's single-phase! When you glue two random AC waveforms together, stuff happens! Commented Mar 7, 2023 at 2:45
  • @Harper-ReinstateMonica Wow, this should be an answer
    – Robotex
    Commented Mar 9, 2023 at 12:21

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