Dual voltage Regulated power supply

Discussion in 'Power Electronics' started by Bigbizkit, Sep 30, 2016.

  1. Bigbizkit

    Thread Starter Member

    Oct 29, 2015
    44
    1
    Hello guys...
    I am planning to make a dual voltage regulated power supply 6A +35 0 -35V
    I was searching google and i stumble with this design…
    My question is…
    1. Whats is the purpose of transistor?
    2. Is this design effective?
    Interesting design, makes me want to use this in a dual voltage.
    3. Can anyone pls give me a dual voltage design?
    Thank you guys..
     
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  2. upand_at_them

    Active Member

    May 15, 2010
    246
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    The transistor is used as a pass transistor to allow the circuit to provide higher current. A voltage regulator circuit with just the 78L05 is limited in how much current it can provide before overheating.
     
  3. Bigbizkit

    Thread Starter Member

    Oct 29, 2015
    44
    1
    Sir, do you have a dual voltage design of this circuit?
    Thank you..
     
  4. upand_at_them

    Active Member

    May 15, 2010
    246
    29
    I don't have any designs. But you'll want to be more specific in your requirement. Do you want a power supply that has two independent outputs?...One 0 to 35V and the other -35 to 0V? Or do you want a dual tracking supply?...One knob that adjusts the positive and negative ends at the same time?
     
  5. dl324

    Distinguished Member

    Mar 30, 2015
    3,243
    620
    It increases the current capability of the voltage regulator.
    No. It's a terrible way to boost current because it adds a non-linear voltage drop to the regulator output.
    You can do something like this with external pass transistors added to boost the current capability. But add filter caps on the input and protection diodes as desired. LM317 datasheet should give examples of current boost and discharge protection.
    upload_2016-9-30_18-57-17.png
     
  6. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    I would not use the 2N3055 or the TIP3055 because their beta (current gain) is a minimum of 20 at 4A. That means the 78L05 would
    go to current limit at about 2A of load current. I would use a darlington NPN like a @2N6284 that has a beta of 750 @ 10A.
    For you power supply you want to build your going to need a physical large transformer and heat sink. If you plan on a Linear supply.
    For the specs you mentioned I would recommend a switching type power supply.
     
  7. Bigbizkit

    Thread Starter Member

    Oct 29, 2015
    44
    1
    Hi sir, can u pls give me a schematic on how to add transistors?
    Thank you sir.
     
  8. Bigbizkit

    Thread Starter Member

    Oct 29, 2015
    44
    1
    Got schematic sir?
     
  9. dl324

    Distinguished Member

    Mar 30, 2015
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    Most datasheets give a circuit. The one in the LM317 datasheet I referenced used LM195 for the power transistor, so I grabbed this from a 78xx datasheet. The concept is the same.
    upload_2016-10-1_8-50-0.png

    If you want to have the regulators safe area protection protect the external pass transistor, you can add a few more components and mount the regulator and pass transistor on the same heatsink; taking care to observe the signal on the cases. LM317 case is Vout, many other regulator cases are ground.
     
  10. hp1729

    Well-Known Member

    Nov 23, 2015
    1,952
    219
    Design 884 plus and minus 30 V at 6 A.PNG
    Here's a start. 2N3055 gain drops as current increases, but even a gain of 6 at 6 Amps will suffice. Big heat sinks all around is suggested. The meters are optional but recommended since current limiting is weak on this design.
    Engineers here could probably suggest improvements. I'm just a technician.
     
    Last edited: Oct 1, 2016
  11. Bigbizkit

    Thread Starter Member

    Oct 29, 2015
    44
    1
    Thank you so much guys!
    I learned a lot with your help.
     
  12. dl324

    Distinguished Member

    Mar 30, 2015
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    If you want to set the current ratio between the regulator and pass transistor, you can use a current divider:
    upload_2016-10-1_9-50-3.png

    If you want to use a less expensive NPN power transistor, you can do this:
    upload_2016-10-1_9-51-29.png
     
  13. dl324

    Distinguished Member

    Mar 30, 2015
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    Don't use the external pass transistor configuration as shown in post #10. It impacts load regulation because the base-emitter voltage of the pass transistor isn't in the feedback loop. It always puzzles me why someone would do that.
     
  14. ci139

    Member

    Jul 11, 2016
    341
    38
    78x05 - - does not hold up to it's specs (output power(some items have higher than 5V output(so you cant fast trim them by 2 resistors))) - - it's like you have to design a power supply and (must) use there an RND generator (78x05) ???? to make it "buzzing" / add a random noise - - to keep your regulator doing something even if there were no need for
     
  15. dl324

    Distinguished Member

    Mar 30, 2015
    3,243
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    You mean they're outside of the typical 4% tolerance? Or the A suffix parts which are 2% tolerance?

    Using the regulator as in the first post has an effect on load regulation. The regulator wasn't designed to minimize quiescent current change (out of the ground terminal) with load. The LM317 was designed to be used in this manner and the maximum delta Iadj is 5uA.
     
  16. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    That circuit will have lousy regulation. The appnotes suggest using a PNP pass transistor with a positive regulator. A current sensing resistor is sliced into the in lead of the 78xx. Decide what current you want in the 78xx for the transistor to start conducting - you want the resistor to drop 0.7V at the current you decide.

    If you can get away with 48V; there are off the shelf SMPSU modules for telecoms equipment.
     
  17. Bigbizkit

    Thread Starter Member

    Oct 29, 2015
    44
    1
    Sir, following the schematic in fig 7.3 can i use a npn transistor for positive output voltage?
     
  18. dl324

    Distinguished Member

    Mar 30, 2015
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    If you want to use an NPN power transistor, you need to substitute the circuit in figure 7.4 for the power PNP.
     
  19. Bigbizkit

    Thread Starter Member

    Oct 29, 2015
    44
    1
    I need a dual voltage power supply for my project amp sir.-35v 0 +35v
    I thought i could use a 3055 and 2955 for each supply rail.
    An npn for positive output and a pnp transistor for negative.
     
  20. hp1729

    Well-Known Member

    Nov 23, 2015
    1,952
    219
    Keep in mind the gain of the transistors. They drop as current increases. The regulator must be able to output enough current no matter which design you use. Regarding the two designs presented (PNP on the input or NPN on the output for a positive regulator) I couldn't say one was significantly better.
     
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