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Switching 2 AC sources to a single load

  1. Oct 14, 2009 #1
    I would like to build a circuit that would switch 2 separate AC power sources to a single load.
    Here's what the circuit should do:
    Circuit has single AC output to a system which utilizes AC current
    Circuit has both a 120 VAC and 12 VDC input. 12VDC circuit is connected to an inverter circuit to produce a separate 120 VAC input.
    In the event AC current is present in the AC input source, switch the circuit to source the load with the AC input
    Switch the circuit to the 12VDC/inverter input if the AC input current is not present
    Power requirement is for the output is 150 watts
    So basically, the 12VDC from the input goes through an inverter circuit to produce the first AC source, a separate AC input connects directly to a 120VAC source to produce the second AC source, and a switch or relay switches the first or second AC source depending on the conditions listed above. I have enclosed a jpeg image to better illustrate what I am saying. In this diagram is a DPDT switch which would allow manual switching but my intent is to use a relay to provide this functionality. My question is if there is a relay that would perform the function of the switch and if so how would it be connected? Specifically, can I use the AC voltage from the second input source to drive the relay? Thanks in advance for your help.

    Attached Files:

  2. jcsd
  3. Oct 14, 2009 #2
    Why can't you just use a UPS? Unless you just want to build one yourself, a $40 unit could easily handle your requirements.
  4. Oct 15, 2009 #3
    That's a good question. An UPS would work in this application if I could supply the 12 VDC externally from the UPS. I don't know if I can purchase one that does not have the 12V battery supply. I would be using this circuit for a marine application so the 12VDC is already supplied. I will have to research this a little bit.
  5. Oct 15, 2009 #4
    The UPS circuit probably controls the inverter output to have the same voltage, frequency, and phase of the ac input when it is present, so that when the ac fails, there is no inductive surge when the circuit switches over to the inverter. Inductive surge could lead to sparking at the switch. Inductive surge is not a problem if you are supplying power only to lights.
    Bob S
  6. Oct 15, 2009 #5
    You may be able to connect the 12v from the marine source to the battery of the UPS. You should make sure that grounding will not cause a problem. Is the marine 12v negative grounded? Is the battery negative of the UPS grounded? You should make sure that the marine 12v will not exceed 13.8v.
    If it's possible to connect this way, it would give you an extra measure of protection. Even if both AC and marine 12v is interrupted, you will still have power from the UPS battery.

    Edit: also, since there will be a .7 volt drop across the diode, you should check that the marine power will not drop so low as to cause the UPS to switch off. Also, most UPS units emit an annoying beep when running on battery power. Further modifications would be need to prevent this.
    Last edited: Oct 15, 2009
  7. Oct 15, 2009 #6


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    If you just wanted to switch to mains power when it was available, you could use something like the attached diagram.

    The mains power would be used to pull in a relay when it was available. This would switch the load to the mains as shown.

    Not shown, but probably necessary, is that the power to the inverter could also be switched with another set of contacts on the relay. This would cut the standby current of the inverter to zero when mains power was being used.

    Attached Files:

  8. Oct 16, 2009 #7
    But beware of current surges if the load is inductive, and the ac phases of the inverter output and mains don't match.
    Bob S
  9. Oct 16, 2009 #8


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    But beware of current surges if the load is inductive, and the ac phases of the inverter output and mains don't match.
    Bob S

    Someone said this earlier.
    If a UPS was running on its internal battery, how could it possibly anticipate the phase of the mains when it came back on?
  10. Oct 16, 2009 #9
    Good question. The UPS clock is synchronized to the ac using a phase locked loop to ensure smooth transfer when power fails, but how does it synchronize wnen ac returns? Some UPS units rectify the ac to the dc battery voltage, and the inverter runs 100% of the time.
    Bob S
  11. Oct 16, 2009 #10


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    I've only ever seen UPS units used as computer power backups.

    Computer power supplies just charge up a big capacitor via a bridge rectifier or voltage doubler, so they wouldn't be too fussy about the exact phase of the AC voltages they were getting.

    I have see inside a couple of them and they just detect the presence or absence of mains voltage and do the swapover with a big relay with healthy contacts on it.
    Nothing too elegant.

    I would be hesitant, like you, to use them on equipment with transformers. Could be big "splats" sometimes.
  12. Oct 18, 2009 #11


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    Are you conerned about transitiom time. If you use a mechanical relay it will not be a very smooth transition fron mains power and inverter output power. There would be a dip,just a guess, of a little over a second. Some equipment that would be running on that would have a fit about lossing voltage for that small amount of time.
  13. Oct 18, 2009 #12


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    Yes, transition times do matter. It depends on the application.

    If it is just to switch some maritime lights on a boat, then it doesn't matter if there is a slight flicker in the lights.

    The relay avoids some complex switching of mains voltages possibly using triacs.

    The original question asked for a relay.
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