Check out my new PSU!

Discussion in 'General Electronics Chat' started by magnet18, May 4, 2011.

  1. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Got it from my chem teacher... kinda
    he said it didn't work and that he wouldn't notice if it went missing, good enough for me

    I have a hunch that it's the capacitor, those old cardboard ones didn't last.
    2000uF, 25V, and the 'lytics I'm replacing it with are like 1 twentieth the size.
     
    Last edited: May 4, 2011
  2. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,172
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    Looks like rectifier is an old selenium one, check. Is it wired as a half wave rectifier? Is the transformer for line isolation, 1:1 ? Nice find.
     
  3. tom66

    Senior Member

    May 9, 2009
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    Wow, is that a tiny variac for the variable output?
     
  4. Jack Puddin

    New Member

    May 4, 2011
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    ooo nice. You going to upgrade it at all?
     
  5. hgmjr

    Moderator

    Jan 28, 2005
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    Magnet18,

    The finned component appears to be a selenium rectifier. Please be careful with it since it can give off toxic fumes if it overheats and fails.

    hgmjr
     
  6. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
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    As I recall, when the selenium rectifier goes, it makes RED smoke!
     
  7. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Yes, I was informed that I should replace it with diodes as necessary, I'm not sure how it's wired, but it has four wires going into it and a 250 ohm power resistor.

    and yep, that's a tiny variac, 0-120 volts, 2.5A (wish it was more current, can these be paralleled?) It also controls the DC.

    I'm going to upgrade the cap and the finned thingy, other than that I'll try to keep it original unless I can get more out of it, I like the classic look :)

    and I honestly can't tell at a glance for sure if that's an isolation transformer, but I think it is.

    [EDIT]
    I think I might drill out all the rivets and replace them with bolts... make things easier.
     
    Last edited: May 4, 2011
  8. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    So the selenium is definitely wired fullbridge, any special rules for replacing it or will any high current, low voltage diode do?
     
  9. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    What is the rated current?
     
  10. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
    1,232
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    0-2 Amps
    0-12 V
     
  11. debe

    Well-Known Member

    Sep 21, 2010
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    The front panell says 12v @ 2Amp max, By the look of the size of the transformer i wouldnt be exceeding that. Silicon rectifiers will certainly be more efficient.
     
  12. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Wait, I thought they were old, inefficient, and prone to failure?

    [EDIT]
    nevermind, you said silicon, for a second I thought you said selenium :rolleyes:
     
    Last edited: May 5, 2011
  13. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Strip it and get ready to rebuild a more effective one.
     
  14. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
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    Looks like the 12V is isolated and the 120V isn´t, but be sure to check anyway.
     
    magnet18 likes this.
  15. tom66

    Senior Member

    May 9, 2009
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    It looks like the 120V output is variable. Neat.

    It would seem that the variac controls the 0-120V, and the transformer is a 1:10 step down for the 12V output.

    That explains the "use only one variable output at one time"; otherwise, you could overload the variac.
     
  16. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    I have three now!
    one works fine, so I'll leave it as is

    the second doesn't work at all, so I'll update it quite a bit, and the third (the one I posted on here) I'll see if it works with the replaced caps, if not I'll replace the silicon.

    unless I can find some chokes so I can parallel them...

    Right you are.

    Spot on, the top 120V outlet is a direct mains outlet, 10 A breaker
    the bottom is variable, 1A

    the transformer steps it down to the selenium, which leads out to the 12VAC and 12VDC

    Ima make a schematic here in a bit.
     
  17. tom66

    Senior Member

    May 9, 2009
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    Very cool. I'm jealous.

    I wonder what devices would do on 60 VAC instead of 120 VAC. I heard some small TVs work down to 30 VAC!

    Nah, but it needs an over-range mode, 0-240VAC! Actually, around me that would probably be dangerous.
     
  18. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Well, I found out that the low voltage ac will put out 5 amps before it kicks the breaker, bit more useful than 2 amps...
     
  19. tom66

    Senior Member

    May 9, 2009
    2,613
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    Maybe true, but the variac is rated to 2.5A. Don't exceed that for too long, or you could burn out one of the windings in it, and that would make for a difficult, if not practically impossible repair.
     
  20. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
    1,232
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    That was with one of the working ones in chemistry
    Using a ammeter to measure the current and measuring the time it takes to create 50ml of gas, from that were supposed to determine the faraday and the charge of an electron... finally, labs I understand.

    It's fun because everyone just looks at me when they look at the lab handout :rolleyes:

    [EDIT]
    I also got a analog ammeter, I think I'm going to put it on this for quick reference, and maybe a volt meter.
     
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