Dual Voltage Power Supply

Discussion in 'General Electronics Chat' started by chaseBank, Jul 26, 2014.

  1. chaseBank

    Thread Starter New Member

    Dec 14, 2013
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    Hello all,

    So I plan on building an amp and I started with the power supply. I have a schematic that I'm trying to use but I'm an having an issue. The issue is I keep blowing my diodes on the lower bridge rectifier on the negative voltage aspect. Does this schematic look ok and or do I need to get higher voltage diodes. I'm currently using 1n4001 diodes. I'm using 2 caps for the upper and lower portion of 4700uf each.
    [​IMG]
     
  2. Fdam

    New Member

    Jul 8, 2014
    6
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    What is the transformer voltage on the secondary?

    What is the expected current?

    The 1N4001 only allows 50V reverse voltage and 1A maximum...

    Maybe the load on the negative side is too much.

    Just my 2¢
     
  3. MrChips

    Moderator

    Oct 2, 2009
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    10000μF is like a short circuit to AC. 1N4001 rectifiers cannot handle the peak current which could be as high as 100A!
     
  4. chaseBank

    Thread Starter New Member

    Dec 14, 2013
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    The secondary voltage is 30 +30, I think the transformer states 4.16 amps. It only blows on the bottom diodes, specifically the top 2 of the bridge. This didn't happen until I noticed that I had the polarity wrong and switched things around. I took the bottom caps and had the negative terminals connected to the -0 of the bridge.
     
  5. paulktreg

    Distinguished Member

    Jun 2, 2008
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    Isn't your bridge rectifier configured wrong for your negative supply?
     
  6. Lestraveled

    Well-Known Member

    May 19, 2014
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    chasebank
    First, the turn on surge current is very high due to the electrolytic capacitors. Until they get charged up the diodes will see very high current. The 1N4001s are too small for this power supply. I suggest that you use a bridge rectifier block. A 10 to 20 amp bridge rectifier at 100 volts would work well.

    Also, per paulftreg, the diode polarities are wrong.

    Mark
     
  7. chaseBank

    Thread Starter New Member

    Dec 14, 2013
    14
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    Thanks for the input, and will upgrade. I'm not sure on the polarities. Can you elaborate? Initially I had both secondary's connected the same until I noticed the difference in negative(bottom) voltage. Once I switched the caps around to the configuration in the schematic I started to blow the diodes...
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    oops! I missed the fact that the diodes are backwards. Good eye guys.
     
  9. chaseBank

    Thread Starter New Member

    Dec 14, 2013
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    Backwards? Wouldn't it be the same?
     
  10. Lestraveled

    Well-Known Member

    May 19, 2014
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    If you wired up the circuit per the schematic then the capacitors are in backwards on the bottom section. Electrolytics do not like being reverse voltaged and will often short.

    View this power supply as two identical and separate 30 volt power supplies. How you connect them together sets the output polarity for a dual supply. You could copy and paste everything to the right of the secondary of the top section down to the bottom section and everything would be fine. (Post the schematic before you power up.)

    Mark
     
  11. Lestraveled

    Well-Known Member

    May 19, 2014
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    I missed it also until paulftreg mentioned it.
     
  12. chaseBank

    Thread Starter New Member

    Dec 14, 2013
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    Ohh gotcha. That's how I connected it initially as basically the top portion twice. It worked, nothing burnt, and I had a reading of something around 40v. Then I noticed the polarities, tried it that way and had the issues. That's why I wanted some opinions as the configuration seems odd.
     
  13. Lestraveled

    Well-Known Member

    May 19, 2014
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    Here is an example
     
  14. chaseBank

    Thread Starter New Member

    Dec 14, 2013
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    Awesome ,Thank you
     
  15. MrChips

    Moderator

    Oct 2, 2009
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    Also what is the purpose of the 1R/2W in series with the 0.1μF capacitor?

    As far as I can see this is overridden by the 0.1μF capacitor across the rails.
     
  16. chaseBank

    Thread Starter New Member

    Dec 14, 2013
    14
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    Not sure, I just found a DIY project that included a schematic for both the lm3886 and a power supply.
     
  17. chaseBank

    Thread Starter New Member

    Dec 14, 2013
    14
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    What's interesting is my voltage is now at +-45 from my 30v transformer...
     
  18. MrChips

    Moderator

    Oct 2, 2009
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    That's because transformers should be rated at RMS voltage with full load.
    Remember to multiply by 1.4 and make allowance for no-load voltage,
    and then subtract the diode forward voltage (times two for a bridge rectifier).
     
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