Dual Voltage Power Supply

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

  1. chaseBank

    Thread Starter New Member

    Dec 14, 2013
    13
    0
    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
    0
    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
    14,238
    4,173
    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
    13
    0

    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
    664
    134
    Isn't your bridge rectifier configured wrong for your negative supply?
     
  6. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,224
    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
    13
    0
    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
    14,238
    4,173
    oops! I missed the fact that the diodes are backwards. Good eye guys.
     
  9. chaseBank

    Thread Starter New Member

    Dec 14, 2013
    13
    0
    Backwards? Wouldn't it be the same?
     
  10. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,224
    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
    1,957
    1,224
    I missed it also until paulftreg mentioned it.
     
  12. chaseBank

    Thread Starter New Member

    Dec 14, 2013
    13
    0
    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
    1,957
    1,224
    Here is an example
     
  14. chaseBank

    Thread Starter New Member

    Dec 14, 2013
    13
    0
    Awesome ,Thank you
     
  15. MrChips

    Moderator

    Oct 2, 2009
    14,238
    4,173
    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
    13
    0
    Not sure, I just found a DIY project that included a schematic for both the lm3886 and a power supply.

    What's interesting is my voltage is now at +-45 from my 30v transformer...
     
    Last edited by a moderator: Aug 12, 2017
  17. MrChips

    Moderator

    Oct 2, 2009
    14,238
    4,173
    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).
     
  18. snav

    Active Member

    Aug 1, 2011
    104
    10
    Assuming the correct polarity of diodes and capacitors on negative side would a CT xfmr work in this application?
     
    Last edited by a moderator: Aug 12, 2017
  19. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
    2,009
    348
    That's almost like the power supply in my amp. 4x35vac @ 3A yielding two each +-50 Vdc supplies,

    Filtering is about the same as well. Current is a bit low.

    I put ZNR's across each capacitor. I needed them for another reason.

    I developed a crude slow turn-on circuit some 40 years ago. I put a flameproof resistor in the AC line and used each of the caps up to 2/3 the final voltage fore I'd ramp up the audio logarithmically and turn on the speakers.
    I don't disconnect the AC line if it takes too long. The resistor just pops which is a good thing.
     
  20. crutschow

    Expert

    Mar 14, 2008
    16,166
    4,315
    With a CT transformer you can get plus and minus voltages with just one bridge as shown below:
    upload_2017-8-11_22-52-2.png
     
Loading...