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Transformer wiring

  1. Jul 4, 2005 #1

    I have a 230V to 2x12V (2.3VA) transformer. There are 4 pins in the secondary coil 0(a)-12(a)-0(b)-12(b). When I measure the voltage across 0(a) and 12(a), I get about 23+VAC.

    I shorted 12(a) to 0(b) and measured 14VAC across 0(a) and 12(b).
    I connected a W05 bridge rectifier and 1000uF capacitor to convert it to 14VDC.

    However, when a 12VDC, 60mA relay is connected. The relay make and broke contact, capacitor voltage is <1VDC.

    Anybody can tell me whats wrong?

    Thanks alot!
  2. jcsd
  3. Jul 4, 2005 #2
    First of all, don't short anything. Just worry about one winding for now. One winding is plenty for pulling the relay in.
  4. Jul 4, 2005 #3


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    You should be able to connect the secondary winding in series to get 24VAC. You may want to check the phase of the individual windings, to ensure that you do have them connected correctly.

    Given 14VDC and a bridge rectifier you will not be able to produce 14VDC. IIRC, you will at best get something around 80% of the input AC voltage as DC out of a bridge rectifier.
  5. Jul 4, 2005 #4
    Integral, you will get MORE than the AC voltage. The the cap will charge to PEAK voltage which is more than 14 volts AC.
  6. Jul 4, 2005 #5


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    Not in any DC power supply I have ever worked on.

    While the caps will charge to peak voltage with no load, as soon as you draw current to the DC load the voltage will fall to, I believe, the AC RMS value or lower if there is substantial current draw. This is why you will need a DC regulator, which should regulate to a voltage 70-80% below the max possible DC Voltage. If you want stable DC, that is.
    Last edited: Jul 4, 2005
  7. Jul 4, 2005 #6
    So which is it? 70 to 80 percent below max possible DC voltage (peak) or 80% of the AC input voltage like you said in the previous post? For the record, I am talking about this case concerning what the voltage would be. Obviously the voltage will develop ripple and the average output will drop as you draw current from it. Just how much it drops depends on how much current is drawn and the size of the filter cap. This is pre-regulator I am talking about.
  8. Jul 4, 2005 #7


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    You want to regulate the DC voltage to at least 80% of the max DC output of your bridge rectifier. The Bridge will produce ~%80 of the input AC as DC. So if you want a well regulated 12VDC out, with a reasonable current draw, start with ~24VAC.

    These of course are very rough figures you will need to design you PS with consideration of what your current needs will be.
  9. Jul 5, 2005 #8
    I'm using the multicomp dual SEC 12V, 2.3VA.

    I tried to measure directly from the transformer, its about 17+VAC. Its 15+VAC when measured from the AC pins and 15+VDC from the DC pins of the rectifier.

    However, when the 16V 1000uF capacitor is connected across the DC pins. The voltage would increase to 23+VDC! :mad:

    When I replaced it with another 50V 2200uF capacitor, the voltage would hit 48+VDC.

    There is no load connected to the rectifier. This DC supply would be on standby most of the time and used solely to light a LED and trigger the relay.

    Would it be better to use a voltage regulator (fairchild semicon KA78M12TU ) than a zener diode? I guess the transformer will be running at the max capacity if there is no load across the zener diode?
    Last edited: Jul 5, 2005
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