120V Transformer Replacement Question

Discussion in 'General Electronics Chat' started by zsteve, Jun 27, 2014.

  1. zsteve

    Thread Starter New Member

    Jun 27, 2014
    1
    0
    Broken sewing machine. Found that a transformer inside appears to the be the problem,120V coming into the left side two pins and from what I can interpret, I should have 12V @ 0.3 amps and 10V @ 2amps coming out of the right side. Attaching 120V AC to the left side, I get about 2V AC out of one set of pins on the right and maybe 0.1V AC out of the other two on the right.

    1) DC readings come out at 0. I'm presuming its an AC in and AC out transformer. Is this correct?

    2) I doubt that I'll be able to find an exact replacement as its an old Pfaff sewing machine no longer made. If I use an another power source that puts out more than 0.3a @ 12V will I still be okay? I presume 0.3a is the max output capability.

    3) Why would a transformer have two output voltages so close in range (ie 10V and 12V?) If I want to replace this unit with (1) 12V transformer, can I just add a resistor to get 12v before the resistor and 10V after?

    4) I'm not sure which pins on the right are supposed to be the 10v and which are the 12v. Is there any way I would be able to tell?

    [​IMG][​IMG]
     
    Last edited: Jun 27, 2014
  2. MrCarlos

    Active Member

    Jan 2, 2010
    400
    134
    Hello zsteve

    The data that you mention in your first paragraph seem correct.
    Are those that appear on the face of the image to the right of your post.

    Observed in the same image, a fuse symbol appears.
    And, as seen, fuse protects the transformer primary.
    It says that melts at 109° C.

    It would be very difficult to disassemble the transformer with the intention of replacing the fuse.
    It look as if were some kind of encapsulated in Epoxy.

    If you know the model of your sewing machine you could get the technical manual for your Pfaff machine.
    Research on the website of the manufacturer (Pfaff).

    I made an attempt and find various things. But I do not know the model of your sewing machine.

    Another thing you can do is send to make a transformer with electrical characteristics you mentioned.

    But. . . Apparently this transformer is made with a ferrite core

    I recommend that you first visit the website of Pfaff.
    probably get the original transformer that sewing machine.
     
  3. Alberto

    Active Member

    Nov 7, 2008
    169
    36
    1 : yes is an ac in ac out.

    2: yes if you use same voltage with more power it will be ok. Do not confuse VA ( which is power) with current . To find the current out of a VA value you have to devide the VA by the voltage ( I = VA/ V)

    3:Because two different voltages are needed. No you cannot use a resistor to drop the voltage.

    4: measure the resistance of the secondary coils. The lower in resistance is the 10 volts.

    You can tray @ RS components, it is very likely you can find the replacement.

    Cheers

    Alberto
     
  4. Ramussons

    Active Member

    May 3, 2013
    557
    92
    1) DC readings come out at 0. I'm presuming its an AC in and AC out transformer. Is this correct?

    I don't understand the meaning of DC readings. Yes, its just a transformer, AC in; AC out.

    2) I doubt that I'll be able to find an exact replacement as its an old Pfaff sewing machine no longer made. If I use an another power source that puts out more than 0.3a @ 12V will I still be okay? I presume 0.3a is the max output capability.

    Higher rating of a replacement in terms of Current Capacity is acceptable. No harm done.

    3) Why would a transformer have two output voltages so close in range (ie 10V and 12V?) If I want to replace this unit with (1) 12V transformer, can I just add a resistor to get 12v before the resistor and 10V after?

    The 2 outputs is for 2 different purposes and as such a Single 12 Volts source will not serve the purpose. Further, using a Resistor to drop the voltage is possible Only if the current drawn is constant. In this case we don't know. Any other option will need the electrical schematic.

    4) I'm not sure which pins on the right are supposed to be the 10v and which are the 12v. Is there any way I would be able to tell?

    One way is to measure the resistance of the output windings. In this case, the one with the higher Resistance will be the one with 12 Volt, 0.3 VA - because this will have more turns with a thinner wire than the other output.

    The other way is to feed, say, 6 Volts AC from a known source to one of the windings and measure the voltage across the other. The results will identify the windings.

    In your case, I would be happy with the first option.

    Ramesh
     
  5. tom_s

    Member

    Jun 27, 2014
    285
    333
    just a quick one to throw in, i would assume this old Pfaff machine does have a light besides the motor? hence probably the differing voltages.

    kind regards,

    Tom
     
Loading...