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Combining dual secondaries of a toroidal transformer

Discussion in 'Electronic Basics' started by [email protected], Feb 18, 2007.

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  1. Guest

    Hi, I have a fairly good sized toroidal transformer that has two pairs
    of primaries and two pairs of secondaries. The inputs are both 115V
    and the outputs are both 33V, at 7A. I'd like to combine the outputs
    in parallel, to give me 33V at 14A.

    I have some questions.

    1. The label on the tranny says PRI1 = blue/white PRI2 = black/grey
    SEC1 = orange/yellow SEC2 = red/green I assume that to keep the secondaries in phase, that "orange"
    should be tied to "red" and "yellow" should be tied to "green" --
    since that's the way they are ordered, on the label? (i.e., do I
    trust the label to indicate phasing?)

    If I shouldn't trust the label, is there any sort of test I can do? I
    don't have an oscilloscope, unfortunately.

    2. What's the math equation to figure out what my ultimate DC voltage
    will be after my 33VAC goes through full-wave rectification and

    It's been a long time since high school electric shop, as you can
    tell...thanks for any help you can give me...

  2. That is what I would try first, assuming that the first
    color was the start of each winding, and the second color
    was the end of each winding.
    All you need is a volt meter. First connect the the assumed
    end of the two primaries together (lets sat, white and gray)
    but connect 120 volts to only one of them (blue to white).
    Then check that the voltage between the remaining open end
    (black) to the one you expect to connect it to (blue), (to
    parallel the primaries, later) has very little voltage
    between them. I would think that this would be less than a
    volt. If you measure 240 volts, the two wires you connected
    together are a start and an end.

    Once you get this low difference voltage condition, it is
    safe to connect the second pair wires together to complete
    the primary parallel.

    Now you can perform the same test on the secondaries.

    Say you connect orange and red together, and measure voltage
    between yellow and green. If you see very low voltage, it
    is safe to connect yellow and green. If you see about 70
    volts, you will have to repeat the test the other way.

    Even if you get this all right, it is possible that there is
    a slight turns miscount on one or more of the windings, and
    the transformer will circulate current between windings of
    slightly different voltages, even with no load. If this
    happens (you can insert an AC amp meter in the loop formed
    by the two windings to check) you may have to add a turn or
    two to the low winding to get a perfect match. But this is
    rare. At least it is easy to do with a toroid.
    The peak of a sine wave is the square root of two times the
    RMS value, so neglecting diode drops, I would expect the
    unloaded DC to be about 33*1.414=47 volts. It may be a bit
    higher since the transformer's rated voltage applies at
    rated current load.
  3. Connect the two secondary commons together. Now check across the other two
    leads with an ammeter for difference current. If it's only a few mA you are
    probably OK.


  4. The Phantom

    The Phantom Guest

    If he only gets a few mA, then the secondaries are phased properly. But what
    if they're *not* phased properly. Whay do you suppose is going to happen when
    he connects an ammeter across the other two leads?
  5. Nothing much should happen if you get them backwards. The MMF is just going
    to push the currents in such a way that they oppose each other and you get
    no current. It might heat up but if you don't push much through it then you
    should be able to test. What you can do is hook up a fairly large load and a
    volt meter and then check both phases and see which one gives you the larger
    33VAC usually means RMS but your filtering will start at the peak so you'll
    need to convert this into peak value and that will be your DC(So you'll get
    more). Ofcourse it will not be a true DC as there will be ripple but if you
    have good filtering then this would be approximately frequency independent.
    You can simply use an volt meter to measure if you like. I get about 45VDC
    that you'll get (minus a few volts because of the diode drops and stuff).
  6. John Fields

    John Fields Guest

    Absolutely, positively, definitely NOT.

    You're a licensed electrician, where???

    In the first place, the secondaries don't have "commons", they only
    have starts and ends. Two leads from the secondaries should be
    twisted together and one of the primaries energized (with the leads
    of the other kept separate and, preferably, taped up to avoid danger
    of shock or short) then the voltage across the unconnected
    secondaries should be measured with a _voltmeter_. If the reading
    on the voltmeter is 66 volts, then the start of one secondary
    winding is connected to the end of the other and they're wired in
    series. In order to connect them in parallel they should be
    disconnected and each connected to the other secondary lead.

    Once that's done, the primary leads should be twisted together in
    pairs and those pairs connected to the mains while monitoring the
    secondary voltage. If the voltage is zero the primaries are wired
    out of phase and should be connected in reverse. That should cause
    the secondary voltage to rise to 16V.
  7. John Fields

    John Fields Guest

  8. Jamie

    Jamie Guest

    but then again, one could read the wire tags looking for the
    H1, H2, H3, H4 and X1, X2 , X3 and X4 leads.

    H1+H3, H2+H4 for the primary.

    X1+X3, X2+X4 for the secondary.
    But i know your aware of this, this is only for other readers.
  9. If they are not phased properly he doesn't know what he is doing and
    shouldn't be doing this.
    The current will be high.

  10. NO!!!

    If the secondaries are parallelled incorrectly, he will have a short
    circuit, and huge currents will flow through the windings.

    As others have said, connect one end of SEC1 to one end of SEC2 (say,
    yellow to green), and measure the voltage between the free ends - if
    the voltage is near zero, the windings are correctly phased, and the
    two free ends may also be connected together. If the voltage between
    the free ends is about 66 volts, the phasing is wrong, and one winding
    must be reversed - connect yellow and red instead of yellow and green.

    Peter Bennett, VE7CEI
    peterbb4 (at)
    new newsgroup users info :
    GPS and NMEA info:
    Vancouver Power Squadron:
  11. This may work for "electrical" transformers (ie: those intended to be
    installed by electricians), but I have rarely seen an "electronics"
    transformer (one an electronics tech would install inside equipment)
    labelled like that.

    The best way, regardless of the transformer marking or colour code, is
    to get a datasheet for the transformer - it should indicate which
    wires or terminals are the start of the winding, and may even indicate
    the corrects connections for serial or parallel operation.

    Peter Bennett, VE7CEI
    peterbb4 (at)
    new newsgroup users info :
    GPS and NMEA info:
    Vancouver Power Squadron:
  12. hmm, thats why I said to add a load. If the two secondaries are out of phase
    and have exactly the same reactance then there will be no current flow with
    or without a load.

    If you take the two secondaries and put them in series the same effect will
    happen but you will need a load incase they are in phase. (or you could just
    measure the voltage, you will either get twice the voltage of one
    secondary(assuming they are equal) or 0.)

    If he doesn't think they are equal then its still easy to tell. You either
    get +-(V1 + V2) or +-(V1 - V2). He can tell quite easily if they are in the
    correct phase by using series or not. (because if they are in the same phase
    then they can only add there voltages together. Ofcourse this assumes that
    one of the secondaries are not to much greater than the other)

    If he wants to use test using parallel then he should add a load to both
    secondaries and then, say, use a current meter to measure the current. The
    same stuff applies above but with current. (or he could measure the voltage
    on the resistor).

    The fact is that either they will work with each other or against each
    other. As always you have to be careful about shorting things out.
  13. Actually no current will flow if these are perfectly identical. The current
    in one of the secondaries will attempt to flow the opposite way and there
    will ultimately be no flow because of cancellation. Obviously if one
    secondary has more turns than the other it will matter but adding a load to
    it will prevent any larger currents from circulating. The magnetic flux will
    induce a current in both secondaries equal and opposite direction.
    "shorting" the secondaries together will not result in any current flow in
    the ideal case. (Although adding a load will only help).

  14. A 60 watt light bulb at your line voltage, wired in series with the
    primary winding(s) will prevent any damage to the transformer during
    testing. If it is wired properly, the lamp will not light. If it
    lights at close to full brightness, something is out of phase. It is
    referred to as a "Dim bulb" tester, and has been in use on a lot of
    workbenches for about 75 years.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida

  15. A load across a short circuit? Its back to "Electricity 101" for

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida

  16. You still don't get it, do you?

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  17. The lesson would stick a lot better if he took his own
    advice and started his own fire.
  18. jasen

    jasen Guest

    if you're lucky the transformer will survive the short-circuit created
    if you get the primaries connected out of phase.

    better to measure with a voltmeter, possbily while applying a low voltage AC
    signal to the secondary or to use a current-limited supply.

  19. John Fields

    John Fields Guest

    SERIES(assuming no load).

    Jesus christ. What happens when you hook up two identical batteries up in
    series out of phase? in phase? (ABSOUTELY NOTHING BECAUSE THERE IS NO
    CIRCUIT) What happens when you hook them up in parallel out of phase?
    ABSOLUTELY FUCKING NOTHING! (but there is a circuit)

    What happens when you hook them up in parallel in phase? THEN YOU GET A
    Ofcourse you fucking neglect that part because you want to start some shit.
    I guess your life is so boring that the only way you can have some excitment
    is to get on sci.electronics.basics and start arguments by being
    illogical(ignoring half the sentence so that the other have then becomes

    Did you guys actually ever goto school or just pretend? Cause I can't
    imagine that your professors would pass you if you only did the parts of the
    tests that you wanted.
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