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DIY automated power transfer switch

Discussion in 'General Electronics Discussion' started by gsmhack, Sep 14, 2017.

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  1. gsmhack

    gsmhack

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    Sep 13, 2017
    UPDATE
    Original title: uninterrupted switching between 2 sources of 230V AC mains
    I just found out that this is called an automatic power transfer switch. Anyway, do you think I can make one as described down here?

    Hello there, just registered, in hope to find expert advice. I just crafted a circuit, and would like your opinion about it. I have built its DC version, it works OK, but I have no experience with triacs, so this might be totally off. Let me describe the problem, what the circuit does (at least what I believe it would do), and provide a link to az easyeda schematics. Thanks for your help in advance!
    Now the problem: The house is provided with two electricity meters. One is working 24/7, but is more expensive, the other one is working nighttime, *sometimes* during daytime too, and is cheaper. I want to use the cheaper meter when it is available, and switch back to normal, when the switched turns off. I normally do this with a relay. NC goes to "constant on" supply, NO goes to the nighttime-switched supply. The common goes in the house, powering everything with the 230V AC. The coil of the relay is powered from the switched supply. The problem is that sensitive electronics can not deal with the gap, the momentary lack of power during the switching. I would like to solve this, and came up with this circuit...
    EasyEDA link: https://easyeda.com/editor#id=a004ef0d79274fd2854d149cdbf04689
    What this is supposed to do:
    - normally "constant ON" powers the output
    - now "nighttime" turns on
    - this should trigger the TRIAC, and power the coil of the relay
    - now, as the contact lever of the relay moved from the NC contact, the current goes through the D1 and D2 bridge rectifiers (DC pins shorted), that drops ~0.7V×4=2.8V.
    - Now we have 2.8V AC, D3 rectifies it, and through R1 starts to charge C1
    - By the time the LED in the MOC would be powered, the relay hopefully switched all the way to the NO contact. When it does, no more voltage drop through D1 and D2, the MOC will not trigger, the TRIAC continues to trigger.
    - the house is now powered from the nighttime meter.
    - later the nighttime turns off.
    - The current now comes from the "constant on" supply, and starts to flow through D1 and D2 again
    - The current also flows back from the output (relay pin 3) to the NO contact (pin 2), from there to the triac, so the triac will not turn off. (Now we will need the MOC)
    - the rectified 2.8V across D1 and D2 now starts to charge C1, and soon it should turn on the LED in the MOC.
    - the MOC should short between Gate and MT1, and this is supposed to turn off the TRIAC
    - with the triac turned off, the coil of the relay releases, and until it arrives back to the NC contact, current continues to flow through D1 and D2.
    - when the relay sets onto the NC contact, the whole cycle completed

    I am absolutely not sure if this could work at all, or how to select the capacitor and resistor values. Could you please help me?
     
    Last edited: Sep 14, 2017
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,419
    2,790
    Jan 21, 2010
    If you can't withstand any delay the answer is to rectify and filter the mains then run an inverter from the DC. This arrangement generally has a battery and is called an online UPS.

    Once you have this you can take your time switching the AC source that is operating to the input of the UPS.

    If you can handle a cycle or so, you can do the same with a regular (standby) UPS.

    I would steer away from a home brew solution unless you're really certain you know what you're doing. A short between 2 phases is a bad thing when power is present in both, but it is even worse when one phase is out.
     
  3. gsmhack

    gsmhack

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    Sep 13, 2017
    Thanks for the answer! You are soooo right! An online UPS is surely a reliable way, but we are talking 20amps, and a low budget. :) Phase short it not possible, the supply lines come from the same phase. Any notes about the circuit?
     
  4. kellys_eye

    kellys_eye

    4,275
    1,146
    Jun 25, 2010
    This is the crux of the matter - this isn't about switching phases or supplies, this is about disconnecting the 'on peak' meter when 'off peak' tariffs are available. You need to take care over this as the off peak meter usually has a lower current carrying capability (not always).

    It may also be 'illegal' in that the suppliers usually state that the off-peak power is strictly reserved for heating purposes only (water, storage heaters etc) and is wired as a separate circuit precisely for that reason.

    The simplest method would be to use a change-over contactor with the activation coil connected to the off-peak circuit.
     
  5. gsmhack

    gsmhack

    9
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    Sep 13, 2017
    Yes, that is simplest, and that is how it is implemented just now. But during switching there is a momentary lack of power supply, and this question is about trying to get rid of this problem.
     
  6. kellys_eye

    kellys_eye

    4,275
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    Jun 25, 2010
    Look for a make-before-break set of contacts (not that I think they exist.... but they might :D)

    You could use one relay (double-pole) to short the contacts momentarily before the contactor operates and additional relay powered from the contactor output to break the first relay.
     
  7. gsmhack

    gsmhack

    9
    0
    Sep 13, 2017
    See, the problem is that you can not go back in time to do that. The offpeak supply just turns off, nothing to warn you "before"... (When offpeak turns ON, you can do that, yes...)

    Any comment on the circuit?
     
  8. kellys_eye

    kellys_eye

    4,275
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    Jun 25, 2010
    Your on/off peak supply sounds a lot like what's available in Scotland. The off-peak supply is available for a total of 8 hours in a 24 hour period, 6 of which are overnight (1am to 7am) and the other two hours at some (un)specified time during the day - Scotland does it around 1pm-2pm and 7pm-8pm (times vary). Control is via radio signal on 198kHz.

    The off peak supply feeds the storage heating system and hot water. The two 'daily' hours are used to top-up the hot water for post-meal times (and augment heating during cold periods).

    Given that the highest consumption (power) devices in most properties actually ARE the heating supplies (rooms and water) there's very little to be gained from utilising the off-peak supplies during the day for domestic purposes since those supplies are of limited power usage and limited time. The only changes most people make are to power their washing machines (maybealso tumble driers) from the off-peak and arrange to do all their clothes washing overnight.

    The cost-benefit ratio just doesn't justify itself to modifying the system for the two hours that 'cheaper' energy is avaliable.
     
  9. gsmhack

    gsmhack

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    Sep 13, 2017
    Thanks, got it, but I chose an electronics forum instead of a financial one on purpose... ;)
     
  10. kellys_eye

    kellys_eye

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    Jun 25, 2010
    Ahhh.... yes, I can read between the lines too.....;)
     
  11. gsmhack

    gsmhack

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    Sep 13, 2017
    :) so do you have any thoughts about the TRIAC in the circuit? Shorting its gate to A1 in particular for example?
     
  12. Bluejets

    Bluejets

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    Oct 5, 2014
    Given that any changes to supply authority equipment will result in very serious offences, not to mention the dangers of inexperienced unqualified players, I'm surprised this has not been locked out.
     
  13. gsmhack

    gsmhack

    9
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    Sep 13, 2017
    "changes to supply authority equipment"
    OMG, I must have failed to express myself clearly.. Which part made you think I plan to manipulate meters, or do any modifications/"changes to supply authority equipment"?
     
  14. kellys_eye

    kellys_eye

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    Jun 25, 2010
    The 'authorities' legal position ends after the meter. All cabling and distribution equipment AFTER the supply meter is the ownership and responsibility of the property owner.

    No one ever suggested that there was any 'irregularities' with the OP's request or the methods used to achieve it.
     
  15. Bluejets

    Bluejets

    4,290
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    Oct 5, 2014
    . This is one aspect of several that make it illegal. Go check...
     
  16. gsmhack

    gsmhack

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    Sep 13, 2017
    It is NOT illegal. You CAN connect *anything* to this meter, as long as they are . (We have another tariff for water heaters, another for heatpumps. THOSE are for specific uses.) You *come* and check our regulations before making claims. Can you please list any other aspects that you think you have found? Or maybe you can try and be really helpful here, and answer questions, not make me come here on a notification, only to find your off-topic irrelevant comments. (Please don't feel the need to reply, you will not be missed here! If that is what you need, I admit to you: you are ssssoooooo right! Happy? Bye!)
     
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