Wiring primary of this transformer?

Discussion in 'General Electronics Chat' started by TheLaw, Jun 25, 2011.

  1. TheLaw

    Thread Starter Member

    Sep 2, 2010
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    Hi,

    I (hopefully) have a quick question regarding wiring up the primary side of one of my transformers. I never quite understood how to read the transformer diagrams, so if you could help me out....I'd appreciate it.

    From my understanding, I'm hooking up the top black wire to AC, bridging the middle two connections, and hooking up the bottom red wire to AC. Is that correct?

    Thank you.

    [​IMG]
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    That is correct for 120 volt

    if it is 240 volt then you hook both blacks together on one line, and both reds together on the other line.
     
  3. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    I think you have that the wrong way around.

    With Blacks & Reds in parallel you have 120 V operation. In fact you may only need one (Red/Black) primary side connected for 120V operation.

    For 240V operation you need (Neutral)Black------Red-link-Black-------Red(Active).
     
    Last edited: Jun 25, 2011
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    for 120VAC, it is necessary to parallel the windings to handle the power rating of the TX.

    for 240VAC, the windings are always in series

    Kermit's suggestion is wrong
     
  5. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    I thought that as well until I did some thinking/research. Look at the drawing again. It is a 240 volt transformer! The drawing shows how to connect it using 120 volts instead of 240.


    For any of us to be SURE though will require a PART NUMBER from the transformer.
     
  6. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    It would be an unusual transformer indeed in which the primary windings were placed in series to accommodate a smaller rather than larger primary voltage.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Sorry Kermit, but I'll have to agree with the naysayers.
    You've been voted off the island. ;)
     
  8. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Let's hope that this consensus was reached before TheLaw required the services of TheFireBrigade!

    It is not difficult to see how this works: voltage transformers work on the basis of a nominal voltage per turn, so a higher voltage requires more turns. Accordingly, the windings go in series for the higher voltage: Two windings of N turns each in series are equivalent to one winding of 2N turns.

    For the lower voltage, the windings are connected in parallel. Two windings of N turns in parallel are equivalent to a single winding of N turns, but with higher current capacity.

    Incidentally, it is imperative to connect the windings the right way around: dot to dot for parallel, dot to non-dot for series. Getting this wrong will result in massive current drain, and a ruined transformer unless a fuse or circuit breaker saves the day.
     
  9. PackratKing

    Well-Known Member

    Jul 13, 2008
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    OK >>>>> as drawn, the secondary is center-tapped for 2 6v or one 12 v circuit.........center tap grounded for the 12v

    For 240v on the primary, wouldn't the middle red/black tie for neutral, and one hot lead - L1 & L2 - on each of the others, producing 12 / 24 volts on the secondary ??
     
  10. TheLaw

    Thread Starter Member

    Sep 2, 2010
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    Kermit,

    I'm writing this on life support from my hospital bed....just kidding.


    Everyone,

    So do we have a consensus on the correct way to do this. I'm on 120V USA mains. I need to put the primaries in parallel? So that means black attached to black, red attached to red?

    Thanks.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Just for clarity, here's how the primaries should be wired: [eta] Drawing updated

    [​IMG]

    You might use the 240v connection on 120v to obtain a lower voltage, lower current output from the secondary winding, but you cannot use the 120v wiring on 240v without damaging the transformer.

    Please note that I have included fusing for the Line lead, which I consider to be a minimum requirement. Don't fuse the Neutral lead, as if a fault occurs, the transformer will have mains voltage on it, causing an unsafe condition.
    [eta] Note that in the States, 240v is split-phase, and L1 and L2 each have line voltage on them, so you need to fuse both L1 and L2 inputs. However, you only need to switch one of the phases, as that will interrupt current flow.

    You can add a switch if you'd like. The switch should be between the fuse and the transformer primary windings, NOT on the neutral line.
     
    Last edited: Jun 25, 2011
  12. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Yes

    I'm wrong.

    Things look a little different this morning than they did at 3:30 a.m.

    :) Just a little bass ackwards is all.
     
  13. Adjuster

    Well-Known Member

    Dec 26, 2010
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    No, we need to be really careful about this. It all depends on what kind of "240v" service you have. In some locations, the supply might be two-phase 180 degrees, with a neutral line and two separate 120V hot lines. If (a BIG if) such a supply were in perfect balance it might then be OK to take the neutral to the junction of the two half-primaries, but I cannot see any benefit from doing so. Any serious imbalance in the phase voltages could lead to the transformer circulating current and overheating. If the two phases were not 180 degrees apart, you would also see smoke.

    The best way in his situation would be to ignore the neutral connection, and just take the opposite hot lines to the opposite ends of the two series connected primaries.

    In my country, standard house current is single-phase 230V, with one ground-tied neutral and a 230V live (hot) wire. Here you would wire the neutral to one end of the two primaries in series, and the live to the other end. You would connect nothing to the joint between the half primaries.
     
  14. TheLaw

    Thread Starter Member

    Sep 2, 2010
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    Thank you everyone for making it VERY clear. SgtWookie in particular for the diagram.

    Also thank you for the informative mains wiring lessons from a few of you.

    Have a nice day!
     
  15. SgtWookie

    Expert

    Jul 17, 2007
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    I updated the schematic; the 240v wiring has changed a bit to indicate L1/L2 instead of Line/Neut, a fuse was added, and GND was added to both schematics.

    Don't forget to use the GND!
     
  16. TheLaw

    Thread Starter Member

    Sep 2, 2010
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    My project is in a wooden chasis, but I'll be using a bolt to connect up everything that needs ground via wires.
     
  17. TheLaw

    Thread Starter Member

    Sep 2, 2010
    228
    2
    I have another question regarding wiring. I'm debating making another thread. I think here will be fine.

    This center tapped 1A trafo is going to be in my new amplifier. However, this 1A 6V-0-6V is only going to be powering the amps headphone amp and the fans.

    I also have a 10A 25V/25V torroid that is powering the power amp section.

    I just want to verify that I can wire it up with a standard DPST switch and 10-11A fuse. I'll hook up IEC to the switch, and then line to the fuse holder.

    I'll then take both transformers and hook them up as regular from there....

    I just want to make sure I don't need something weird like a 4PST switch or something...

    Is there anyway I can do this with two fuses? Separate for each transformer? (4PST??)

    Thanks.
     
  18. #12

    Expert

    Nov 30, 2010
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    Using 2 fuses is the easy part. Figuring out why you think you need a fancy switch is difficult.
     
  19. SgtWookie

    Expert

    Jul 17, 2007
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    Well, if you are powering multiple transformers, then it certainly would be easier to place the fuses after a single switch.

    The concern is that any connection can come loose and short out somewhere. If you have a main fuse, you can then use a single switch and fuse the individual transformers separately if you wish - or simply use one fuse up front for the entire load.

    Keep in mind that your 10A 25v-0-25v transformer won't be drawing 10A from the mains side; basically take the VA rating and divide that by the input voltage to get close to your max mains current draw, then increase that by perhaps 30%-50%. The idea isn't to blow fuses all the time; it's to protect the equipment (and especially the users) from an electrical failure inside the unit.

    Also, keep in mind that the switch needs to be rated for both the current and voltage it's switching. You may need to use a relay to switch the power.
     
  20. TheLaw

    Thread Starter Member

    Sep 2, 2010
    228
    2
    So it's alright to use the fuses after the DPST switch? In that case, that simplifies things a lot. Well of course I can. I'm acting a fool right now, so excuse me.

    I think I'd like to use two fuses, for some reason. I have a crapload of nice panel mount fuse holders, so I think I'll do two.

    Thank you for clarifying the fuse value, because I really never knew how to do that correctly. I'll go with a 5A fuse on the power amp, and I guess 1A for the other transformer.

    Thanks.
     
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