modifying a power supply circuit

Discussion in 'The Projects Forum' started by lokeycmos, Apr 15, 2009.

  1. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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    im in the process of building a homemade lab power supply. i just have a question about modifying an existing circuit. i have kindof narrowed down the circuit to this one

    http://www.rason.org/Projects/powsupply/powsupply.htm

    .my question is: how would i go about modifying it to make it variable from say, about 5 volts to about 30 volts(assuming im using a 40 volt transformer and appropriatly rated components) ive been trying to research on my own for a couple days but no luck. Thnks in advance! :)
     
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
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    My advice is don't try to 'modify' this supply. It was designed for a particular purpose different from yours. Further it is underspecified and underprotected for a general purpose supply.

    If you have a 40v transformer your input rail will be nearly 60 volts. At minimum setting your regulator will be dissapating (60 - 5) * 10 watts. This is a massive half a kilowatt! Not within the range of a pair of 2N3055s.

    Define your own purpose and I'm sure there will be plenty of help available to achieve a better design.
     
  3. bertus

    Administrator

    Apr 5, 2008
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  4. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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    that is an awesome circuit! my next question is, can i use my 40 volt transformer with it? i looked up the specs on all the transistors and they are rated for 100volts. i know the lm723 is rated for 40 volts max, so i was thinking mabey a preregulator or possibly just a small resistor to pins 11 and 12. any thoughts on this? :cool:
     
  5. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    In the datasheet they use a 36 Volts zener connected between V- and (V+ and Vc).
    This is to limit the voltage on the LM723.
    The transformer of 40 Volts is overkill.
    It will give a much to high input voltage.
    This to much voltage has to be dissipated in the transistors that will become very hot.
    When the output voltage is very low the transistors will become even hotter.

    Greetings,
    Bertus
     
  6. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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    my bad, i checked the output of my transformer and it is actually 36 volts rectified.
     
  7. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Mistakes at 10 amps can have serious consequences. You would definitely benefit from posting your exact requirements for discussion, as I suggested.
    Do you really need 10 amps and what is the output of the transformer at max current, 10 amps or otherwise?

    The circuit from Bertus is a league or two above your original.
     
  8. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
    432
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    it is going to be a general purpose lab supply. used for digital circuits all the way to flyback drivers and occasionally charging a motorcycle battery. the transformer is rated for 10 amps. some of the features im looking for are the widest possible voltage range that the transformer and regulator allow for, adjustable current limiting, and good ripple filtering at low voltage/current draw. i liked the circuit i posted cause it was fairly simple,( im capable of more complex circuits too) but i really liked the circuit bertus posted. but im not picky as long as it has the features im looking for. thanks for all your help
     
  9. studiot

    AAC Fanatic!

    Nov 9, 2007
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    I didn't ask what your tx was rated for, I asked what the voltage was at (your) max current output. There will be a considerable difference in output voltage between no (low) load and full load. Have you measured 36 volts at 10 amps? You can do this with just the rectifiers and capacitors (have you got these).

    723 regulators and 2N3772 transistors are old technology and hard to find these days, so check your sources before starting too much work. The low value, high power resistors may also be difficult to find.

    Look out some hardware which will allow plenty of space for air circulation within the box, perhaps think about a fan,, you could run a 24volt one off the tx, which I presume is 24 volt secondary.
     
  10. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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    sorry, i misundertood you. I currently dont have a test load of 3.6 ohms that can handle the wattage.
     
  11. Pich

    Active Member

    Mar 11, 2008
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    Because of the way the 723 is configured in the circuit shown it will give you a minimum of 7 volts output when R8 is 0 ohms, by playing with the value of R10 you can set maximum voltage. As stated the 723 max voltage is 40 volts. The maximum output voltage is ditermined by the voltage at pin 11 and 12 minus the voltage drop across the transistors so if the voltage at pin 11 is 35 volts then I would not expect much more that 32 volts at the output no matter what your tranformer voltage is. I think this circuit would work quite well for you as a 7 to 30 volts output, but i would suggest lowering your current output to 5 amps and install the output transistors on a large heat sink.
     
  12. italo

    New Member

    Nov 20, 2005
    205
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    remove the lm723 it is a good old chip but better is lm117 easy to design 3 to-220 leads and leave the 2n3055 that should go from the input as opposed to the output
     
  13. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The links I gave from tony's website are for a rigid powersupply.
    It is special disigned for use with RF circuits.
    It will not trip if a RF amplifier is connected.
    A circuit with a LM317 will have more problems with it.

    Greetings,
    Bertus
     
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