Ferroresonant power supply resurrection

Discussion in 'General Electronics Chat' started by equalizer700, Jun 28, 2009.

  1. equalizer700

    Thread Starter New Member

    Jun 28, 2009
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    I am trying to safely (i.e. without magic smoke) power up an old power supply with a ferroresonant transformer circuit. It is in a vintage computer I am restoring. There is a schematic at http://96.0.36.155/parasitic.pdf. Since the supply is loaded with old, big electrolytic capacitors and has not been in a climate-controlled environment or powered on in 30+ years, I decided to be very cautious. I hooked the power supply up to a 0-120 vac variac with a 60 watt light bulb in series with the output, figuring that if the light lights there is a short somewhere. When I turned it on and got the variac up to about 25 vac, the lamp started glowing. I disconnected all the rectifiers from the power supply transformer and tried again with the same result. I disconnected the big 3 uf 330 vac capacitor that is across one of the power supply transformer secondaries and got no glow up to about 60 volts, where I stopped. I discharged the power supply 3 uf capacitor and measured across it with a vom. That showed no short in either direction (it looks open).

    My questions are:
    - Is it safe to power up the supply all the way with the 3 uf capacitor out of the circuit? Will the transformer voltages be abnormal in any way, or will there just not be any regulation or transient suppression?
    - Can a low input voltage cause the ferroresonant circuit to draw abnormal current until the supply voltage gets up to normal?

    Power supplies are NOT my thing :). Any help or advice is greatly appreciated!
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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  3. equalizer700

    Thread Starter New Member

    Jun 28, 2009
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    Thanks for the advice and the link. I read the article, but I still don't understand enough to know the answers to my questions about whether I can leave the 3uf capacitor out of the ferroresonant circuit and whether a low input voltage would cause the ferroresonant circuit to draw a lot of current until the voltage comes up. Can you help there?

    I went to Mouser to order replacement parts, and have another question: Most of the old existing capacitors are aluminum shell electrolytics about 2 cm diameter and 3-4 cm long. When I look at new parts, the ones I find are much smaller (more modern I guess), and have higher voltage ratings. Are these OK to use? What capacitor types are not interchangable? Are all "electrolytic" capacitors of the same value interchangable, as long as the voltage rating is as high as or higher than what is called for?

    Thanks again for the info!
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    Yes about the electrolytics - as long as the voltage rating is the same or greater, the sub should work.

    As far as leaving out the 3uF cap goes, the only effect is to remove the resonant feature from the transformer. The transformer will act like a "normal" transformer. If your line voltage is clean, you probably won't miss the feature.
     
  5. equalizer700

    Thread Starter New Member

    Jun 28, 2009
    3
    0
    I ordered all the caps today. Only the 440VAC will take 10 days, the rest are on their way. I'll be able to work on it this weekend, and will report progress,

    Thanks!
     
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