Please explain this smps operation

Discussion in 'General Electronics Chat' started by Mrdouble, Sep 10, 2012.

  1. Mrdouble

    Thread Starter Member

    Aug 13, 2012
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    I found an old instrument with the schematic in the leather sleeve
    It's a low voltage generator (1v and lower). Anyways, it doesn't work but the power section does so I'm gonna use it in a project. Im having a heck of time understanding how this works. Looks like the pair of transistors are cycling on and off but I dont see how the two windings on the primary side do whatever they do in the secondary. Please explain

    Also, I had to substitute the transistors with tip41c , I don't think this makes alto of difference but the output is way higher than what's on the print. I'm wondering if the circuit needs to be loaded to get appropriate voltages on the output

    Thank you
    [​IMG]

    http://pbckt.com/pT.RfBygj
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Think of the power transistors and the transformer primary windings as a "power multivibrator". The winding connected to the bases of the transistors is phased such that the base drive causes positive feedback, and causes the transistors to turn on alternatively. The resulting square-wave current in half of the primary causes a (stepped-up) voltage in the secondary connected to the diode bridge....
     
  3. Mrdouble

    Thread Starter Member

    Aug 13, 2012
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    THanks mike, that's pretty much what I figured :) but let me ask this the output voltages are much higher than should be the ;40v is 100v, the +-15v are like +-38v. Is this because theres no load?
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Yes. These things put out a square wave into the diode bridge. There is a lot of ringing/overshoot on the "square wave" leading edges. Lightly loaded, the filter capacitors charge to the peak of the ringing, but as load is added, the voltage in the filter caps will sag to more like the average of the rectified wave. They have awful regulation, so if you care about it, put a regulator between it and your ultimate load...
     
  5. Mrdouble

    Thread Starter Member

    Aug 13, 2012
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    I haven't got around to throw this thing on my oscope but that was my next idea. Even just to understand the circuit
     
  6. Mrdouble

    Thread Starter Member

    Aug 13, 2012
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    0
    I put it on my oscilloscope and I was bit surprised to see the output coming of the transformer was a square wave. I figured it would be a mutilated sine wave, Mike you were on the money with that one :). Now I wondering how "they" got 40,+-15v without a regulator, is it possible this circuit was designed in a such a manner as to know the entire circuit loading so the voltage would be pulled down to the exact value? This seems unlikely but I don't really see how without a regulator
    Thanks
     
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