My power supply project

Discussion in 'The Projects Forum' started by jaygatsby, Dec 21, 2011.

  1. jaygatsby

    Thread Starter New Member

    Nov 23, 2011
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    Here is what I have so far -- mains hooked up to this transformer: http://tinyurl.com/bnpryd4 with a 200mA fuse between one side and the mains. I am using a figure 8 cord and I don't know which side is hot and which side is neutral from the wall. I assume that it's important that I figure this out to wire the primary side the right way? Or is it not important?

    Secondary side, there are 4 leads. I take from the outer 2, hoping to get 12.6V. I put each of the two leads into two diodes, 1N4001, facing opposite ways. The output is an ugly, jagged waveform that bounces above 0V. Not a nice half sine wave.

    I added an elecrolytic capacitor to the output of my rectifier and now there is no voltage at all on the output. Maybe I soldered something wrong.

    That's where I'm at so far. I'll correct any mistakes in my circuit as your abuse comes in.

    Thanks
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    1,728
    That's a 2.5VA transformer; if you're using it in a 115v/120v outlet, that will be about 21mA current. A 200mA fuse would guard against a severe overload, such as a short to neutral. You should have the fuse installed in the "hot" wire from the plug.

    You really should be using a 3-wire grounded plug, not a lamp cord. Lamp cords typically have one blade that is wider than the other. The wide blade is Neutral, the narrower blade is Hot.

    However, with no ground, you would not be protected if there was a fault (more like when there is a fault, because eventually it will fail.)

    The fuse should be in the Hot lead. With a 3-wire plug, this is very easy to figure out. Green is ground, white is Neutral, and black is Hot.

    It sounds like you have wired it up incorrectly, but I can't tell if you are trying to get a +12/-12 supply out of it, or a single positive supply.

    If a single +12v supply, you really need a bridge rectifier, or make a bridge rectifier from discrete diodes.

    Here is how you should have things connected up:

    [​IMG]

    Note that the ground from the wall outlet is represented by an inverted triangle, and that it is connected to the bridge rectifier's negative side.

    Due to the low output current of your transformer (~200mA) you could use a 680uF to 1000uF electrolytic capacitor instead.

    A likely possibility. However, without a schematic diagram, all we can do is shrug and say "Well, you must've done one or more things wrong. Where's your schematic?"
     
  3. jaygatsby

    Thread Starter New Member

    Nov 23, 2011
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    Thank you Sgt. It'd be more appropriate to use a 3 prong plug, in order to have the ground wire? I thought that ground wire was for chassis... which I do not have yet, and which will eventually be plastic.

    Thanks again
     
  4. jaygatsby

    Thread Starter New Member

    Nov 23, 2011
    185
    23
    Also, I see in your schematic, and others, that the two middle prongs of the secondary side of the transformer are tied together. I have mine attached to nothing. Should I tie them together?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Yes.
    You connect the ground to the secondary side of the transformer in case the insulation between the primary and secondary windings fail. That way, the fuse blows instead of placing potentially lethal voltage on the output of the supply.

    Since you will be making a positive output supply, you ground the negative side of the bridge. If you were making a negative output supply, you would ground the positive side of the bridge - but that would also require a different regulator; the LM337 or a newer version.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    I do not have the two middle prongs connected; that is how you would wire it if you were connecting the transformer for 230/240v.

    I have pin 1 connected to pin 3.
    I also have pin 2 connected to pin 4.

    If you don't connect them like I have shown, your output current capacity will be reduced from ~200mA maximum to ~100mA maximum.

    Here is the relevant section from the datasheet:

    [​IMG]
     
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