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Center Tapped and Regular Transformer

Discussion in 'Electronic Components' started by Anand P. Paralkar, Aug 27, 2012.

  1. Hi,

    Could you please explain the following points regarding a center tapped
    transformer (some questions apply to a non-center tapped "regular"
    transformer as well):

    a. Is the center tapped transformer wound differently than a
    non-center tapped transformer? Or is it just a regular transformer for
    which the center point of the secondary winding is "brought outside".

    b. Considering the secondary voltage of a transformer is Vs, the two
    terminals of the secondary are at +Vs/2 and -Vs/2. This implies a
    voltage gradient across the secondary. The gradient passes through a
    zero point which we "tap". What causes this voltage gradient?

    c. Can we say that all the turns in the secondary winding of a
    transformer have the same amount of flux passing through them at a given
    instant or do they have a different amount of flux (with the flux
    depending on the position of the turn)?

    Thanks,
    Anand
     
  2. Daniel Pitts

    Daniel Pitts Guest

    I don't know the answer to these, and I'm interested to know, but they
    are sound very much like homework questions and I wouldn't be surprised
    if the answers you get are along the lines of "do your own homework".

    Now, I would guess that Wikipedia and Google are going to help you in
    your quest for information. Good luck.
     
  3. Phil Allison

    Phil Allison Guest

    "John Larkin"

    ** That would need to be a rather large ( multi kVA) transformer.

    The "rule of thumb" for a laminated iron E-core is 5 to 6 turns per volt,
    for 1 square inch core cross section.

    ( 5 turns if it is for 60 Hz only and 6 turns if it is for 50Hz )

    If the core is a strip wound toroidal, then the numbers is reduce to 4 and 5
    turns.

    So, for 1 turn per volt, the cross section needs to be around 5 square
    inches.


    .... Phil
     
  4. Daniel, John, Phil and Tim, thanks to each one of you for your reply.

    Let me assure you, this isn't a homework question that I am trying to
    push on to the newsgroup. (If it was homework, I wouldn't do it. :) )

    I stumbled on these doubts while trying to develop a negative DC voltage
    source. Amongst the other features in the requirement spec., one
    requirement is that we would like to make this negative voltage source
    as far as possible without using a center tapped transformer. As it so
    "turns" out - its difficult.

    What was irritating is that I could not find (printed or online) any
    standard text which discusses these points:

    a. A transformer with its primary terminals connected to Vp and
    ground (zero volts) will have its secondary terminals as +Vs/2 and
    -Vs/2. (For a small bit of time, I continued to think that the
    secondary terminals would be: +Vs and 0V, mimicking the single ended
    primary.)

    b. If the flux crossing all the coils is the same (as in same
    density), what causes the gradient in the voltage across the coil? You
    see, the gradient on the coil is a change in the voltage magnitude and
    polarity from one end to the other! I have concluded that this happens
    because at one end, the flux is *entering* the coil and *leaving* at the
    other. This *entering* and *leaving* causes the opposite polarity (some
    "thumb rule"), but what about the drop in magnitude? Then again I
    "imagine" that the voltage generated all across the coil is such that
    the positive voltage is completely worn out by the time it reaches the
    other end and vice versa for the negative voltage. The positive and
    negative voltages meet at the mid-point, cancel each other and give us a
    beautiful zero! But why?

    I will continue to be worried that my understanding is a figment of
    imagination until I see some text/formal/mathematical treatment which
    confirms/denies all this.

    I will be embarrassed to see any URLs which discuss these points
    straight away. But then they are welcome anyway.

    Thanks once again,
    Anand
     
  5. To "use a couple of bridge rectifiers.....", you are assuming that the
    bridge rectifiers are galvanically isolated from each other. That is,
    the source for positive voltage regulator and the source for the
    negative voltage regulator should not come from the secondary of the
    same transformer.

    If you do "tie one of the bridges negative terminal...." when the source
    of the positive and negative regulators are the same there is a problem.
    For a full wave bridge rectifier, the capacitor charges to 2Vpeak (no
    load condition) as in: +Vpeak on its positive terminal and -Vpeak on its
    negative terminal. So connecting the positive of one bridge to the
    negative of one bridge is not a solution.

    You could simulate this or build this. Just measure the voltage between
    the two points you want to tie as common before connecting them. You
    will see a 2Vpeak voltage between them. (Obviously, you don't go ahead
    and connect them.)

    If using two separate windings or separate transformers is your
    solution, then I agree. It is easy.

    Thanks,
    Anand
     
  6. I don't see where I have assumed or stated that the secondary is
    galvanically related to the primary. All that I have said is valid with
    or because of the galvanic isolation.
    Sorry I wasn't clear about this, the idea is to make a positive *and*
    negative supply from the same source (as in from the same secondary
    winding) without using a center tap. (In which case "calling the "+"
    terminal ground doesn't help. And flipping the diode's orientation just
    moves which side of the bridge the positive and negative voltage appears.)

    Thanks,
    Anand
     
  7. I am sorry, I haven't been clear. We need to generate a positive *and*
    negative voltage using the same transformer secondary without a center tap.

    (I kind of started writing where I was irritated thinking about the
    negative voltage.....)
     
  8. But you have to examine the whole cost, and it may be simpler to go with
    the centre tapped transformer.

    You could use a second transformer. This works especially well if the
    second voltage is relativley low current, you can use a much smaller
    transformer.

    Got to a switching supply, more complicated but with multiple voltages it
    may work out cheaper.

    Especially if the second voltag is low current, use a dc to dc converter,
    ie connect to the positive supply an oscillator and rectify the output for
    the lower current negative voltage.

    Use a higher voltage supply and split the DC voltage in half. This works
    well if the two voltages are the same but of different polarity, and works
    best if the current is relatively low. You in effect are setting an
    artificial ground at the output of the rectifier.

    Michael
     
  9. OK can I add more components to the none ceter tapped secondary?

    Then make yourself (say) a 30V supply. Add a power opamp rail
    splitter*, and tie the center point to ground. Viola... +/-15 Volt
    supply.

    George H.

    *in it's simplist opamp is a buffer with non inverting input driven
    from two resistors one tied to each rail of the power supply.
     
  10. Fred Abse

    Fred Abse Guest

    Don't forget interwinding capacitance. A 10 meg DMM will almost always
    see something.
     
  11. Phil Allison

    Phil Allison Guest

    "Anand P. Paralkar"

    ** It is ridiculously simple to make equal "+" and "-" voltages from a
    single winding.

    The circuit is called a "full wave voltage doubler". Two diodes and two
    filter caps, each pair wired as a half wave rectifier gives two DC rails of
    opposite polarity from one winding.

    http://electrapk.com/wp-content/uploads/2011/08/full-wave-voltage-doubler-1.jpg

    The centre of the two caps is the zero volts point.


    ..... Phil
     
  12. P E Schoen

    P E Schoen Guest

    "Phil Allison" wrote in message
    Agreed. Here is a simulation of a circuit that I actually built:
    http://www.enginuitysystems.com/pix/120Sine-320DC_Doubler.png

    Here's the ASCII file if you want to play with it:
    http://www.enginuitysystems.com/pix/12V-300V_CT_Doubler.asc

    Paul
    www.muttleydog.com
     
  13. Jamie

    Jamie Guest

    sure that's easy..

    use one leg of the secondary as the common and the other leg will have
    2 diodes branching from it. Each diode will be such that you'll get a
    + and a - source...

    THere is one big problem with this how ever, you need to use a cap
    double or more than what you'd normally would to filter it on the DC
    side. This is because you'll only get 30 hz per diode. Basically, you'll
    have a gap on each side when the opposite side is conducting.

    Of course, if your requirement for the - source is a low current type
    that could be done with a added circuit that operates from a single
    power source.

    Jamie
     
  14. Mark Zenier

    Mark Zenier Guest

    Another way, that I saw in an Elektor project, is to use a full wave
    rectifier for the main supply and use two half wave doublers hooked
    to each end of the secondary. (Or you could call it a capacitivly
    coupled full wave rectifier).


    ------|(-----+----|<---------+----- V-?
    | |
    +-->|--gnd +--|(--gnd
    |
    +-->|--gnd |
    | |
    ------|(-----+----|<---------+


    Mark Zenier
    Googleproofaddress(account:mzenier provider:eskimo domain:com)
     
  15. Hi Everybody,

    Firstly, thanks a lot for your detailed replies. Although this
    started-off as a thread on my doubts on the transformer, this thread got
    (unintentionally) drawn to another topic. One that I was planning to
    post here anyway.

    The reason I said that getting both - the positive and negative source
    from a transformer without a center tap is difficult is that I tried
    what a lot of people have recommended here. I built a circuit that had
    the two diodes connected to the transformer secondary. One diode had
    its anode while the other had its cathode connected to the transformer
    secondary. The other end of these diodes were connected to a capacitor
    each. These capacitors had a common point which we could call the ground.

    I was surprised to find that this ground actually drifted! When I
    measured the voltage across the ends of the capacitors (the end
    connected to the diodes), the voltage measured was constant. However,
    when I measured the voltage across the ground and the other end of the
    capacitors, I saw that this voltage changed. So the V+ source and V-
    source with respect to the ground was not constant!

    I don't know the exact reason what causes this drift. But as a remedy,
    I put a resistor in parallel to each of the capacitors. (P. E. Schoen
    has posted this).

    This stopped the ground from drifting but I don't think one could use
    this solution in a practical circuit. A resistor in parallel at the
    output of a voltage source will not hold up in case of a heavy load (low
    load resistance).

    That's why it seemed difficult.

    There are ofcourse many other circuits that have been suggested here.
    Thanks a ton gentlemen.

    Thanks,
    Anand
     
  16. Jamie

    Jamie Guest

    For some reason that circuit just does not look appealing..

    Not knowing the application it is kind of hard to come up with the
    proper solution however....


    Transformer Bridge
    +-+-++---+--------+
    -. ,+--------+ A A + | |
    )|( +--++ | --- | |
    )|( +--+(-+ --- | |
    -' '+--------+ A A + | |
    +-+-++ + +------------------+
    | .-. | | |
    | | | | + |
    R1, R2 100K | | | | |\| Ilimit |
    | '-' +-+|-\ ___ |
    | | | >--|___|-+---+ Commom
    | +-----+|+/ GND
    | .-. |/+
    | | | |
    | | | |
    | '+' |
    +---+--------+

    Jamie
     
  17. Jamie

    Jamie Guest

    Is that how "Fly Backs" got their name ? :)

    Jamie
     
  18. Jamie

    Jamie Guest

    unbelievable.

    Jamie
     
  19. Jamie

    Jamie Guest

    I think you got that backwards, bud!

    Jamie
     
  20. Fred Abse

    Fred Abse Guest

    Please:

    E = -L di/dt

    Opposing the change that produces it.

    This is .basics, so we need a little pedantry.
     
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