Dual Mains supply Problem

Discussion in 'The Projects Forum' started by MAS62, Jan 17, 2010.

  1. MAS62

    Thread Starter New Member

    Jun 29, 2009
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    I am powering some low voltage electronics from the Mains Supply via a transformer. I want redundancy in the supply i.e two sources of mains power (not UPS, they must be two seperately wired supplies) and I want to detect when source#1 has disappeared.

    I have been using a relay to switch between the two sources but get so much kickback from the inductive load that I decided to go solid state.

    So now I'm using triacs to switch source#1 whereas source#2 is permanently connected (both sources are in phase).

    However when I disconnect source#1 I can still measure 240V across the terminals and thus cannot detect that it has failed.

    Why then do the triacs allow source #2 to switch them on?
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    This sounds like you are talking a battery backup, which is the norm. Two diodes will allow the device being power to switch between the two power supplies, and you can trickle charge the battery with a little circuitry.

    Your circuit idea is just plain dangerous. If you must have two AC inputs then use 2 transformers. You have connected two lines that should be isolated (and are for a reason) into a potential dead short. The likely hood of a failure of the triacs is very high, plus what happens when power is returned to the 1st line. Very bad idea. If you need 2 independent power supplies then make or use two independent power supplies.
     
  3. MAS62

    Thread Starter New Member

    Jun 29, 2009
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    Bill, a curt reply...I'm asking for help not looking to be told off by someone who doesn't know me from Adam!

    I'm not talking battery backup at all.

    The design has two (fused) independant mains feeds L1-N1 and L2-N2.
    What I really want is to switch from #1 to #2 when #1 fails and then switch off #2 if #1 is restored. I must also detect that #1 has failed to fire up an alarm, for this I need to close a relay contact.

    I'd like the design to be solid state and as a compromise can leave #2 in circuit all the time provided I detect when #1 has failed.

    So, some help please someone, if I knew the answer I wouldn't be asking....
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Interesting, because in the States, we have split-phase power.
    The power company provides L1, L2, and Neutral to residences.
    Neutral is the center tap of a 240vac secondary. L1 and L2 are equal in amplitude, but 180° out of phase with each other. At the breaker panel/power distribution panel/fusebox, Neutral is connected to earth ground.

    If someone in the States attempted the schematic you've posted, they would create a dead short between L1 and L2. This is one reason why Bill_Marsden objected to it.

    Secondly, it would not be a good idea to connect two separate 220v/240v lines together as you have shown, as they likely have two separate breakers/fuses. Connecting them together effectively adds together the current rating of the two breakers. I am not an electrician, however I have a feeling that this would violate your electrical safety codes.

    You need to have your load connected to only one source of 240v at any one time.

    A single-pole double-throw relay would be the easiest and safest way to accomplish this; the coil could be powered by the L1 side, the NO contacts on the L1 side, the NC contacts on the L2 side, and the COM to your transformer. If you used a 350v MOV across the relay coil, it should take care of any transients.
     
  5. MAS62

    Thread Starter New Member

    Jun 29, 2009
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    Thanks Sgt Wookie. I have not explained clearly enough.

    It's not a 3 phase supply, by L2 I just mean mains feed #2. Both supplies are from the same phase. The idea is that should the first feed be unplugged inadvertantly the second feed would keep the circuit running.

    I do currently have a circuit exactly as you described (although I've used a DPDT relay to switch live and neutral) however the two feeds are via RCD's and what happens when feed one is disconnected is that the energy from the transformer jumps (as a spark) across the relay contacts so that a (very) temporary path L1 to N2 is created and both RCD's trip due to the current imbalance and I therefore loose both feeds. I added an MOV across the load (transformer) to prevent HV spikes but it hasn't worked to eliminate the spark I see.
     
    Last edited: Jan 17, 2010
  6. 3ldon

    Active Member

    Jan 9, 2010
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    Would two relays and a 20msec time delay work for you?
    This will also enable you to feed it from two out of phase feeds.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    The problem seems to be that once the mains power is cut, the transformer puts out a huge voltage spike.

    Under-rated MOVs might be a part of the problem. Using MOVs rated for 400+v might help; and wiring them in parallel. The TRIACs would have to be rated for a good bit greater than the MOV rating, or they would probably get shorted when the MOV fired.
     
  8. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
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    Although you've indicated that a UPS is not what you interested in, you may want to check out some of thier schematics as most have a built in transfer switch that switches from lines to inverter. You would just replace the inverter with your second lines.
     
  9. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Try using a 4PDT relay instead.

    [​IMG]
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    The relays would be a good solution, I had visions of the OPs idea feeding voltage to a line that would normally be presumed dead because a breaker was thrown. His idea as originally presented could kill someone.
     
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