Need ideas on switching power supplies

Discussion in 'The Projects Forum' started by Dynaman, May 17, 2011.

  1. Dynaman

    Thread Starter Active Member

    Jan 17, 2008
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    Greetings Folks:

    I have no experience working with switching power supplies. I was wondering if anyone had any schematic links that that could show functional, switching powers supplies in their simplest form.

    What i am looking to do is to get rid of a giant transformer in lieu of a switching power supply to drive all the voltage to vacuum tubes, and related solid state circuitry... so I assume, i will need a 12VDC@3 amps (filaments), 350VDC@100ma (b+), and +-15VDC@ 250ma (Op-amps, LED's etc.

    I have never designed anything like this, and there may be stuff off the shelf already ready to go. Any suggestions?

    A..
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,795
    951
    Two words

    WALL WARTS



    Or Three words

    COMPUTER POWER SUPPLY

    To get the 350 VDC B+ supply you could use an AC wall wart that supplies AC voltage. A common voltage most people find is 9 volts. Any low voltage AC wall wart would do, but here is the problem. You need to feed the AC voltage to a transformer. Put the voltage in on the secondary and use the primary as your output. This will 'step up' the voltage and also provide safety because you are isolated from the AC main by the Wall wart. You then have to rectify and filter the output.
     
  3. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    I've used a 5 volt switching supply for the rectifier filament. Problem is, the brick is floating at 350 DC! Of course you won't need the 5 volt supply to heat a rectifier tube for the 350 volt supply, will you?

    A wall-wart with AC output can get you +/- 15V for opamps and filaments. The problem I haven't fixed is getting the 350 volts DC for the plates without using a big obnoxious transformer. I think I saw high voltage switching bricks for sale somewhere, but I can't remember. All Electronics? Electronics Goldmine? Somebody help me out here.
     
  4. Dynaman

    Thread Starter Active Member

    Jan 17, 2008
    46
    0
    Yes.. I have run into the same problem you are both discovering. There is something about wall warts I hate. The only thing good about them, is they are safety listed off the shelf, and in some cases you can avoid sending a product to UL to avoid getting reamed nicely, if you do your design right.

    So here is what I want to power up (grossly stated):

    1) B+ voltage of anywhere from 350-450VDC @ maybe 150ma
    2) Say 4 op amps, fets, some transistors, leds +-15VDC @ 200ma
    3) Filament supply for 12ax7's, 6BQ5's -- 12volts and 6 volts respectively @3A

    It would be nice to design all of this in a small switching power supply circuit using an I.E.C. 120VAC outlet. This topology versus the old fashioned a large multi-tap transformer. I am not sure the cost would be any cheaper going to a switching power supply though.

    A...
     
  5. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    That is about 75W of power. Try to find a pdf of Switching Power Supply Design by Abraham L Pressman, and read into it. Also look at http://www.smps.us for the pros and cons of each topology.
    The transformer will be multi-tap anyway, but it will be much smaller. At this power level the mosfets even might not need to be on a heatsink.
     
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  6. ifixit

    Distinguished Member

    Nov 20, 2008
    639
    108
    Hi Dynaman,


    Questions:
    1. Would it be possible to get away with ±12V for the 4 op amps, fets, some transistors, leds etc.?
    2. Would it also be possible to wire the 6BQ6 filiments in series and supplied from +12V so a 6V supply would not be required?
    3. Does the 350V at .15A need to be regulated?
    4. If 350V is unregulated, would 450V no-load and 350V full-load be okay?
    If the above can be managed then you only need to buy an off-the-shelf, UL approved*, line voltage in, ±12 Volt out, SMPS module. Isolation from the line voltage is already done so the 350V can be supplied from a simple, 12V in, 350V out, SMPS flyback or boost design.

    Regards,
    Ifixit

    * No reaming required:)
     
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  7. Dynaman

    Thread Starter Active Member

    Jan 17, 2008
    46
    0
    Hmm.. That's pretty clever. I see what your are getting at. Consolidation! I like it. Are these available off the shelf??? or must be customized? I suppose I could rip one apart, mod it, then implement the circuit onto a main PCB. Any link to who sells what you are recommending?

    What do you think?

    A...
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You know that the 12AX7 tubes have a center tapped heater, right? That way you can run them on either 12v if not using the center tap, or 6v at twice the 12v current if operating them on 6v.

    The "ends" of the heater filament are on pins 4 (triode 2) and 5 (triode 1); the center tap is on pin 9.

    [​IMG]
     
  9. Dynaman

    Thread Starter Active Member

    Jan 17, 2008
    46
    0
    Thanks..

    Very much aware of that.. I just threw out 12V as a number. I usually like to use a center tap on the common heater element to balance out and reduce noise. In my last design several years back, I went to DC, and and got rid of the issue altogether. Having said that Sgt., yes... I could up the amperage and run all heaters at 6 volts.

    A..
     
  10. ifixit

    Distinguished Member

    Nov 20, 2008
    639
    108
    Hi Dynaman,

    For smps 120vac in, 12v out, Google: smps open frame

    You would still need a 12V to 350V smps which is not as simple as I thought. To get a few milliamps at 350 is easy but 150mA regulated is not trivial. What are your electronics building skills like?

    This site http://www.audiohobbyist.com/projects/smps.htm has a kit for a 350VDC supply that looks like it might do the job. It is an old site and I'm not sure if the kit is still available. You could still build from scretch however.

    Regards,
    Ifixit
     
  11. Dynaman

    Thread Starter Active Member

    Jan 17, 2008
    46
    0
    Middle aged EE I suppose.

    The kit is interesting. I also found a good link to ON semiconductors guide on switching power supplies. I need to do some reading.

    I'll see if I can put something together on paper. I did see some things I already liked SMPS's, but I still cannot justify it being any cheaper than a linear power supply.

    Thanks for all help...

    A...
     
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