Transformer with centre tap help.

Discussion in 'General Electronics Chat' started by Guinness, Sep 22, 2012.

  1. Guinness

    Thread Starter Member

    Dec 31, 2009

    At work I had a transformer, primary was 110V, secondary was 18-0-18.

    I had information that the 0v on the secondary maybe should be connected to mains earth. But others said it didn't need to be, so I didn't as it didn't seem right to connect a secondary winding to earth...

    The problem is, when I powered up, the secondary windings melted, the insulation burnt off and destroyed the transformer. This was only on for 30 seconds.

    No components on the motor driver it was connected to got hot and after testing there was not short circuit anywhere on the input, so excessive current should not have happened.

    My question is, did I destroy the transformer by not connecting the 0V to earth, or could the transformer have been faulty anyway?

    If it depends on the device the transformer is connecting to, is there a way of telling if the 0V needs to be connected to earth or not?
  2. t06afre

    AAC Fanatic!

    May 11, 2009
    You could perhaps have overloaded the secondary windings by drawing to much current compared to the current rating. Perhaps by a short. Hard to say more without any more information
  3. ramancini8

    Active Member

    Jul 18, 2012
    The center tap connection has no relationship to load current if it is left open, and it is load current or a shorted secondary winding that caused the failure. The center tap should be left unconnected if unused, and if it is connected to either side of the secondary it causes a short that can burn up the secondary. You must provide a schematic to obtain a complete answer.
  4. MrChips


    Oct 2, 2009
    18-0-18 is 36v rms and that is a lot of voltage.
    What are you connecting this to?
  5. t06afre

    AAC Fanatic!

    May 11, 2009
    Also do not confuse/mix ANY "ground" wire on the primary/secondary side of the transformer. With each other at all. Any "ground" on the mains side. Has nothing to do with any "ground" on the secondary side
  6. Guinness

    Thread Starter Member

    Dec 31, 2009
    I cant provide a schematic as its connects to the control circuitry of a industrial stepper motor controller. Big and complicated unit, I know its not the same, but I did put DC voltage onto the inputs of the unit and didn't draw more than 11mA.

    The centre tap is connected to the unit, All three outputs ( 18V-0v-18v ) are connected to the unit.
  7. crutschow


    Mar 14, 2008
    So it's likely ether a short in the transformer or a short in its load caused the failure. It has nothing to do with not having the center-tap connected to ground.
  8. SgtWookie


    Jul 17, 2007
    Your post is rather confusing/confused.

    In the States, earth ground and Neutral are connected together only at the breaker panel/fuse box; they must not be connected together anywhere else.

    It is a good idea to have at least one wire of a transformers' secondary winding connected to ground; that way if a short develops between the primary and secondary windings, the ground connection should cause the primary side fuse to blow due to excessive current. Without that ground, a shorted secondary could be carrying mains power, which would be quite dangerous.
  9. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    The center tap might work as a return path.

    Without it connected you might double the current across the windings.

    Most circuits this should not happen. The currents on each side are normally going to be mirrors and create a natural division.

    But some types of switching circuitry for example will create an instantaneous current and voltage across the full 36 volts where it was intended to go across only the 18 volt side with the center tap as the return path for the sense circuits.

    Doubling the voltage doubles the current and double the voltage and the current means roughly 4 times the power which matches the meltdown you experienced.

    Would not seem likely to me without that evidence.

    In fact your evidence makes me think it might be another even more bizarre case. The sense circuit path might be from +18 through -18 due to the disconnected ground so that it thinks it has 0 volts. Hmmm, not seeing any voltage, better turn the current up.

    Is this a common Stepper controller that others have used for years without needing any ground connections? Is it a new model? Is it possible that other ground connects are in place?

    I don't see why you were so concerned about grounding that Center Tap wire. It could have safety reasons as well as operation issues.
  10. Guinness

    Thread Starter Member

    Dec 31, 2009
    Thanks for all the replies.

    The last place I worked, I grounded a centre tap and the board exploded. Apparently it was the way the pcb was designed, so im always careful about doing that now.

    I always make sure the load has no short on the input side before powering up, after trying another transformer, it turned out the first transformer was shorted on the secondary side.

    Thank you all again for your help, was more confident with the connections I done where right after reading your posts.
  11. cork_ie

    Active Member

    Oct 8, 2011

    I am not sure what you are doing. Your transformer 1s 18-0-18 i.e. 36V AC

    Connecting DC to your load is a totally different ball game. You need to check the power supply requirements of your load before anyone can offer you any further advice.
    1) Supply AC or DC?
    2) Voltage?
    3) Load current?
  12. t06afre

    AAC Fanatic!

    May 11, 2009