Newb Power Supply Question

Discussion in 'General Electronics Chat' started by stingray_65, Sep 20, 2009.

  1. stingray_65

    Thread Starter New Member

    Feb 20, 2009
    3
    0
    First time post

    I've built several tube amplifiers from kits and want to venture into building one of my own design.

    I thought i would start with a HV transformer I had salvaged.

    it has 2 HV taps 300 vac @ 200ma and 215 @ 175ma.

    When I hooked up a bridge rectifier to each tap I expected to get about 405 VDC on point A and about 290 VDC on point 2

    what I'm reading is 303VDC on point A and 205VDC on point B. (read to ground)

    I checked each diode with my fluke and each is good.
    I checked to see that I routed the diodes correctly to bridge and it is.
    I had my son double check the orientation just to be sure.

    what am I missing?

    Thank you
     
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  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The rectifier produces pulsing half-waves. The voltage rises to the peak then drops to 0V at double the mains frequency. Your DC meter is reading the average voltage.
    When you add a filter capacitor then the capacitor will hold the voltage at the peak voltage.
     
  3. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Try adding capacitors (making sure their rated). This will boost the DC voltage by 1.4 volts. This is basic power supply theory, the caps try to turn pulsating DC into pure DC. If they are too small there will be hum.
     
  4. stingray_65

    Thread Starter New Member

    Feb 20, 2009
    3
    0
    I've got a couple 10uf 500V electrolytics

    I'll pop em in and see how it changes.
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Your schematic is incorrect. The diodes are shorting out the transformer. You need to rotate the bridges clockwise 90° in the schematic.
    You are reading the average DC values of the full-wave rectified sine waves. To get the peak voltages, you need filter caps from the outputs to ground. Be sure to add bleeder resistors in parallel with the caps. Otherwise, you could be in for some nasty surprises or worse.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    I think you meant to say "by a factor of 1.4".;)
     
  7. stingray_65

    Thread Starter New Member

    Feb 20, 2009
    3
    0
    422 VDC at point A
    303 VDC at point B

    Right in the Ballpark!

    THANKS !
     
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