0-30 Volts variable DC supply without using any transistors

Discussion in 'Homework Help' started by abdullah8391, Sep 8, 2015.

  1. abdullah8391

    Thread Starter Member

    Sep 6, 2015
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    HI everyone, this is my first post so i am expecting a lot from this forum (basically also my first time asking for help on a forum).

    I am given a project to design a dc power supply with
    1. centre-tape transformer (220-V AC to 24*2 V AC).
    2. LM-317 ic.

    I can't use any transistors or any other ic's, so can anyone give me some ideas on how to do it?

    my design works from 1.25-30 volts DC. But i couldn't understand how to drop those 1.25 to 0. since Vout=1.25*(1+R2/R1).
     
  2. be80be

    Senior Member

    Jul 5, 2008
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    You do it like this you put two diodes under the adj pin I'll draw you it and post in second.
     
  3. dl324

    Distinguished Member

    Mar 30, 2015
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    You'll definitely get your money's worth;)
    You need a negative reference. This is from an LM317 datasheet:
    LM317-0to30.jpg
     
  4. be80be

    Senior Member

    Jul 5, 2008
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    Like this.jpg Like this maybe better little better
     
    Last edited: Sep 8, 2015
  5. crutschow

    Expert

    Mar 14, 2008
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    That won't work.
    You didn't move the ground, only the minus input voltage to ground.
    You need to move the bottom of the adj pot to the minus input.
    The problem is that the output current goes through the diodes so the output voltage will vary with current due to variation in the forward diode drop with current and temperature.

    abdullah8391. is it an absolute requirement that the output voltage go to 0V?
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    As indicated in the application circuits on the datasheet, the only way to get the 317 (and most other 3-terminal regulators) to regulate down to zero volts is to fool the chip by connecting its reference potential to a negative voltage. That is not difficult, and there are many ways to get a little low-current negative voltage out of a standard transformer-bridge-filter positive output power supply. The problem is that that negative voltage must be regulated at least as well as whatever the positive voltage regulator circuit is. If it is not well regulated, the overall regulation and stability will suffer. Post #3 illustrates the generl idea.

    ak
     
  7. be80be

    Senior Member

    Jul 5, 2008
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    Im sorry i did that backward i haven't use it like that in years i was in a hurry i fixed.
    it's good way to get LCD to work at 3.3 volts it moves the ground up.
    There no zener diode used in the data sheet it's just a diode and I don't see any poor regulation.
     
    Last edited: Sep 8, 2015
  8. abdullah8391

    Thread Starter Member

    Sep 6, 2015
    53
    1
    yes...
    for 1.25 to 30,i already have designed and working.
     
  9. abdullah8391

    Thread Starter Member

    Sep 6, 2015
    53
    1
    now that's the problem, i am beginner.
    so no idea is pooping out of my mind on how to get this -10 reference voltage...
    can you suggest any ideas?
     
  10. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Not there yet. See post #3 for the general idea. You still don't have the adjustment connected correctly.

    ak
     
  11. abdullah8391

    Thread Starter Member

    Sep 6, 2015
    53
    1
    any ideas on how to get -10V reference?
     
  12. dl324

    Distinguished Member

    Mar 30, 2015
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    Need more information. How did you connect the transformer secondaries? What is their current rating? How much current do you need from the supply?

    As always, a schematic will help narrow choices.

    But, given your constraint of doing it without transistors or other IC's, it seems like an impossible task.
     
  13. be80be

    Senior Member

    Jul 5, 2008
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    Like I said I drawled that up little fast it gives you negative voltage I've used it for Hitachi HD44780 lcd they don't like 3.3 volts but I don't think you'll get a lot of current out of it.
    This video shows it works I just drawled it wrong I was distracted And not thinking good.
    But I'm going to arguing if the op what me to show him Ill draw it out better.
    It shows it going a little lower then 0 and to 9.75 volts I didn't have a power supply that can go 30 plus volts handy just a 12 volt one. And the pot wasn't good LOL took that with my phone I changed the size for pictures and it doesn't do good now.
    The pot wasn't linear you had to turn it very slow but you can still see it drops to below ground.
    This is a little better the pot still not easy to change maybe just the bread board.
     
    Last edited: Sep 9, 2015
  14. crutschow

    Expert

    Mar 14, 2008
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    The solution is that you were given a transformer with a center tapped output.
    Connect the transformer to a bridge rectifier with capacitor filters and ground the transformer center tap.
    The bridge will then operate as a plus and minus full-wave rectifier with output voltages of about +33V peak and -33V peak to ground.
    You use the positive voltage for the regulator input and the minus voltage to power the negative reference as shown in Post #3 (increasing R3 to about 2kΩ.

    Interestingly you could use this to make a plus and minus supply where both outputs can be adjusted to zero if you add an LM337 (and another LM113) for the negative regulation.
     
    Last edited: Sep 9, 2015
  15. be80be

    Senior Member

    Jul 5, 2008
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    Here's a better look at a lm317 going 0 and a diode as a voltage drop is not that bad just need diodes that can handle the needed current of the LM317 I changed the pot.

     
  16. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Since no other IC's are allowed, the least complex option is a full wave bipolar rectifier as in post #14, and a 1-resistor, 2-diode negative reference. The two diodes replace the LM113 in post #3.

    ak
     
  17. be80be

    Senior Member

    Jul 5, 2008
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    This is home work
     
  18. be80be

    Senior Member

    Jul 5, 2008
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    temperature coefficient of a diode is the same for a zener diode as a 1n4007.
    And post 3 isn't like any data sheet I've seen out of 30 I looked at. I did find it in appnote.
    But in the data sheet it's using a diode with resistor to -10 volts.
     
  19. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    The image in post #3 is from the original National Semiconductor LM117-317 datasheet (page 13 of the August, 1999 rev.). ON Semiconductor (Motorola) and other second source datasheets have different circuits.

    ak
     
  20. be80be

    Senior Member

    Jul 5, 2008
    431
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    The zener or two diodes not going to be as good as it could be done with a regulater that can go to ground here little easier way from eeblog
     
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