Request help - does the same current flow in all secondaries of a transformer

Discussion in 'General Electronics Chat' started by iinself, Dec 8, 2015.

  1. iinself

    Thread Starter Member

    Jan 18, 2013
    69
    0
    Hi,
    below is the circuit diagram of a old Pioneer receiver SX 1500. I am planning to reuse the transformer. In the picture (the thick black lines on the right top) pin 11 seems to be center tap and 13 and 9 are the other 2 ends of the winding. Between 11 and 13/9 I measure 33 V. But I don't need 33 V, all I need is about 18V which is what I measure between 12/10 and 11 the center tap. Can I use 12/10 with 11 for the PSU for a 50 W amplifier?

    The fuse rating for the old amplifier says 2.5 A and is designed for about 100 W. My doubt is is there any current limitation on the different secondaries of the same transformer or can all of them supply similar currents.

    Thanks

    bando ats-285.jpg
     
    Last edited: Dec 8, 2015
  2. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    Generally, they are different. You'll need to get a datasheet to find out what they are.
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,984
    745
    I would say yes, if you use the two diode full wave rectifier method (D601,602) like on the circuit of IC601 its got 23V input, (top right on circuit,) this method doubles the current of the transformer windings.
     
  4. iinself

    Thread Starter Member

    Jan 18, 2013
    69
    0
    So I won't be able to power a bridge rectifier to get a dual +/- 24 to power about 50 W amp using the 12/10 and 11 the center tap ?
    Thanks
     
  5. umphrey

    Member

    Dec 1, 2012
    39
    1
    If you can figure out the VA rating of the transformer you should be able to take it out of any tap combination you want
     
  6. hp1729

    Well-Known Member

    Nov 23, 2015
    1,951
    219
    How big are the wires going to 10, 11,12? Are they the same size as 9 and 14?
     
  7. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,984
    745
    The 24V tap is probably feeding the preamp and tuner section, so it wont need to be large, the 33V is for the power amp, that winding will have more current capability,

    Look at the transformer windings see if they are the same size.
     
  8. iinself

    Thread Starter Member

    Jan 18, 2013
    69
    0
    Yes.
    Also when I measure resistance between them they all come to almost a short.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,537
    2,369
    You cannot use any single winding in order to utilize the max Va of the transformer without taking into consideration the current rating (gauge) of the winding you intend to use.
    Max.
     
  10. iinself

    Thread Starter Member

    Jan 18, 2013
    69
    0
    I don't in terms of wire gauge but they are all the same size, even on the primary side from the mains the size is the same. Also all of the points 9 - 13 are show up as short ( < 1ohm) on the multimeter.
     
  11. iinself

    Thread Starter Member

    Jan 18, 2013
    69
    0
    Ok. thanks.
     
  12. crutschow

    Expert

    Mar 14, 2008
    13,006
    3,232
    Measuring the resistance of transformer windings is difficult because the resistance values are typically so small.

    A way to more accurately measure low resistances is to do a 4-wire measurement.
    If have a DC voltage supply, put a known current (say 1A) through the winding and measure the voltage drop directly across the winding leads. From that you can calculate the winding resistance using Ohm's law.
     
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