30v & +-15vdc psu suggestions

Discussion in 'The Projects Forum' started by Plummer, Nov 23, 2013.

  1. Plummer

    Thread Starter New Member

    Nov 18, 2013
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    New here and not an electronics professional , so please bear with me if you don't mind. I came here, as it seems the best forum for a safe reliable project.

    My project requires 3 voltages, filtered clean and 2amps each. 30,+15 & -15vdc. I have successfully built the 15's using 78 & 79's with a step down transformer. The transformer has 3 2ndaries. A 5vac, c-tap 16vac, and a 5 tap 19vac & 42vac.
    I used a LM317 for the 30vdc. Currently I am current limited....haha and I don't know if I can use the one transformer for both regulated voltage boards I built and share a common ground. I'm thinking I cannot use just one secondary for both voltages properly. The transformer is 8-10amps.

    Is there a better way?, devices, or suggestions. I have been searching for weeks, and have found two very similar psu's but the to did not have any more current than what the 78 & 79 & 317 provides.
    I will post some pics of the modulation board and plans of what I have done so far. Thanks for all the great reading so far.
     
  2. Plummer

    Thread Starter New Member

    Nov 18, 2013
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    Pics of all the salvaged parts I have.
     
  3. Plummer

    Thread Starter New Member

    Nov 18, 2013
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    Still learning the pic upload
     
  4. #12

    Expert

    Nov 30, 2010
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    It's the 5 tap transformer that needs a drawing here. There are several ways to do this. Show me a drawing of the secondaries and I'll guide you to an answer.

    Apparently you want +30V compared to the center of the +/- 15V supply. Entirely doable.
     
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  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    But it would take an isolated winding...
     
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  6. #12

    Expert

    Nov 30, 2010
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    Did you look at the photograph? He has 2 completely separate transformers. I'd guess that means the secondaries are isolated from each other.
     
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  7. #12

    Expert

    Nov 30, 2010
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    And while we're on the subject of, "you can't get there from here", this is a drawing of the 30 volt supply added to the split 15's with only one transformer...as in, "not isolated".
     
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  8. Plummer

    Thread Starter New Member

    Nov 18, 2013
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    Yes there are 2 transformers, the larger one also has 3 secondaries 5vac=2 taps, 14vac=3 taps and 48vac=5 taps. On the AC side there are 3 taps, but nothing was connected to the loop or third tap. I get slightly less output when connected to the loop....couple volts.
    Many thanks for the quick replies. Ask away, I'm standing by
     
  9. #12

    Expert

    Nov 30, 2010
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    Dreadful math results. The 19 volt taps will only achieve about 25 volts after they are rectified and the 42 volt taps will achieve about 58 volts, which means a lot of waste heat. Stacking the 5 volt winding on the 19 volt winding will achieve 32.5 volts under no load conditions, but it won't hold up under load. Stacking a 16V on a 19V gives about 48 volts instead of 58 volts, but it could only be half wave rectification.
    Your second transformer sucks for this application.

    Maybe the answer is a variation of the drawing I already posted, or are you sharp enough to do a fast switching design?
     
    Last edited: Nov 23, 2013
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  10. Plummer

    Thread Starter New Member

    Nov 18, 2013
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    Not sure on the fans witch, but if I have a schem I can build it No problems. Building the drawing you posted is no problem, if that means anything. I'm not stuck on the transistors I listed either. I have two NTE suppliers local, and a shop full of junk electronics to salvage from. Right now I am looking for a quick and dirty for getting my audio set up. Then building a permanent supply. Plans change depending on what I have on hand.

    This is the board I'm feeding, the 24vdc is actually the 30v tie. Some are running as low as 22v-28v. Same with the 15's, it works with 13.8. It's just easier to get a clean asymmetrical wave with 15 & 30
     
  11. #12

    Expert

    Nov 30, 2010
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    Considering that your first transformer is rated for 8 to 10 amps, I think I'll go with this design.
     
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  12. Plummer

    Thread Starter New Member

    Nov 18, 2013
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    Ok, thanks, that's real easy. I assume I will still be limited to 1amp each on the 15's and 1.5a on the 317 30v supply. This might be enough current. I have mje3055t's & SD188 but every plan I've seen calls for the 2955. We will see.
    Your help is much appreciated 12.
     
  13. #12

    Expert

    Nov 30, 2010
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    Your current limits are determined by the regulators and the filter capacitors. You can buy regulators that provide more than 1 amp. The basic difference is the package they are in so they can get rid of the heat.
     
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  14. Plummer

    Thread Starter New Member

    Nov 18, 2013
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    What would I add to increase current
     
  15. #12

    Expert

    Nov 30, 2010
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    LM338T or LM338K has a 5 amp limit, but you will never get there without a complete study of heat sinks.

    LM350T is a 3 amp chip

    L78S75CV by ST Microelectronics = 2A

    That's the best on the first of 5 pages. Go to a vendor site and do a parametric search for voltage regulators to find over a hundred that can do 2 to 5 amps.

    http://www.mouser.com/Semiconductor...0w8p8Z1z0vlw7Z1yzryzwZ1z0wd5eZ1z0wbywZ1z0wbdj
     
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  16. Plummer

    Thread Starter New Member

    Nov 18, 2013
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    Thanks, I have plenty and many types of heat sinks. I use a sink on each transistor anyways. I also have plenty of fans, AC and dc.
     
  17. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Connecting the 42 volt to the closest 19 volt lug will net 23vac. After rectification you are about 31.3 volts. If the OP can tolerate a bit under 30VDC after regulation it becomes an option (depending on the drop out of the regulator.
     
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  18. #12

    Expert

    Nov 30, 2010
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    I shied away from that because the measurements are unloaded and you can bet the peak voltage will fall 5% or more under load.
     
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  19. Plummer

    Thread Starter New Member

    Nov 18, 2013
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    Can't thank you gents enough for your input. Really made my project easy. Now I just need enough uninterrupted time to complete.

    My next project will be making this work in the mobile off 12v or inverter.
     
  20. #12

    Expert

    Nov 30, 2010
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    Arrrgh! The only inverter I bought had a 2 step square wave pretending to be an AC voltage and it will not work for this circuit because the peak voltage of a sine wave is not provided.

    It looked like this:
     
    Last edited: Nov 24, 2013
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