Bipolar supply +/- help (Single transformer)

Discussion in 'General Electronics Chat' started by oidium45, May 25, 2010.

  1. oidium45

    Thread Starter Active Member

    Apr 24, 2010
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    I am trying to get both a positive and negative supply from one transformer. The bigest problem that I am having is that the positive needs to be +24-48v and the negative needs to be -10v. Will the below drawing work (raw power supply for regulators)? If not can someone please recommend an alternative?


    I have included the basic circuit that I am trying to build as well.
    Thanks
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    I would take a separate small transformer for the - 10 Volts and a large transformer for the power section.
    The standard LM317 can only handle upto 40 Volts at its input.
    There is also a high voltage type of the LM317, the LM317AHV.
    That one can handle upto 60 Volts.
    (see datasheet for more info).

    Bertus
     
    oidium45 likes this.
  3. oidium45

    Thread Starter Active Member

    Apr 24, 2010
    130
    8
    Thank you Bertus! I meant 32-40v input. My mistake... Actually I will probably be using a maximum of 36v for the input. I am still having difficulty figuring out how to get the negative supply voltage, but a small transformer is definately something that I do not mind adding to the circuit.
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The "help" transformer for the - 10 Volts can be really small.
    A transformer that handles 100 mA would probably be enough.
    Something in the range from 7 to 9 Volts will do.
    The Fets act as current sources, the voltage is not that critical.

    Bertus
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Ask yourself - what circuit might you want to power from less than 1.25 Volts?

    You really don't want that large of a voltage differential across an LM317 anyway (I'm speaking of 36v in, and a lot less out) because for a given current, power dissipation in the LM317 will get much higher as it's output voltage goes lower.

    For a 36v input, tell me what the power dissipation in the load and the LM317 will be, when the load requires 1 Ampere of current; first at 33V (output of the LM317, input to the load), second at 18v, and thirdly at 3V.
     
  6. oidium45

    Thread Starter Active Member

    Apr 24, 2010
    130
    8
    Bertus, I thought about using a smaller transformer for the negative but I still run into the problem of not knowing how to connect it into the circuit. Let me explain a bit better.
    1) Could I power the second transformer off of a winding from the main transformer? Or do I need to use a 120-10v transformer?
    All information that I have found suggests that if I use a single transformer with two bridge rectifiers they will short each other out. But I have found no information about using one of the windings for a second transformer instead.

    2) How would I connect the second transformer output into the circuit? Would I connect the negative to the transistor, ground to the main ground of the circuit, and leave the positive "hanging"?

    3) Can I create a 36v power supply from one winding on the transformer, and use a seperate center tapped winding on the same transformer to generate the -10v?


    Sgt Wookie,
    I understand the issue with heat dissipation. I believe I have that problem solved for now. "hopefully"... I am planning to try using a variable input adjustable through a rotary selector switch. This will allow me to adjust the input voltage to the LM317 to more closely resemble the output voltage requirement.
     
    Last edited: May 26, 2010
  7. Wendy

    Moderator

    Mar 24, 2008
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