Regulated Power Supply 24V to 3V,4.5V,6V,9V and 12V

Discussion in 'The Projects Forum' started by kaname08, Feb 23, 2012.

  1. kaname08

    Thread Starter Member

    Sep 22, 2010
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    Good day, guys,,
    I made a regulated power supply of 24V to 3V,4.5V,6V,9V and 12V, but my resistors always burnt, and even though the resistors is not damaged I can't produce the output that I want to have.
    I have produced 8.9V for 9V,and for the others i produce less than what I wanted to(very far),

    I hereby attached the schematic:
     
  2. KJ6EAD

    Senior Member

    Apr 30, 2011
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  3. kaname08

    Thread Starter Member

    Sep 22, 2010
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    oops, i forgot to say that i used rotary switch for that,,,,
     
  4. #12

    Expert

    Nov 30, 2010
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    You need to do the math and it goes like this:
    12V RMS rectifies to 12 radical 2 minus 1.7 volts of loss in the diodes.
    That comes out to 15.27 volts. But you are better off to measure the rectified voltage because your power line might not be the 120.0 volts that the transformer is expecting, transformers tend to have higher voltage when they aren't being used to their full capacity, etc.
    For the 12 volt output, you have 3.27 volts to use up with a resistor.
    That resistor should provide enough current to amount to the zener diode running at 50% to 100% of its wattage rating.
    Suppose you bought 1/2 watt zener diodes.
    A half watt at 12 volts requires 41.67 milliamps. (P=IE)
    The resistor will let less than 41.67 ma through when there is 3.27 volts across it. (E=IR) 3.27 Volts / .04167 Amps = 78.48 ohms
    The next higher standard resistance is 82 ohms.
    The power of the resistor will be (P = Esquared/R) P = .13 watt. Use a 1/2 watt resistor.

    You continue to do this for each output and the result will be steady voltages with no smoke.
     
  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Never afraid to state the obvious: resistors only burn when they dissipate too much power.
     
  6. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    For my own curiosity, can the outputs be cascaded, so that subsequent resistors have to dissipate less voltage? E.g., can the 12v output be fed to the input of the 9v circuit, and the 9v output be fed to the 6v circuit, et.al.?
     
  7. #12

    Expert

    Nov 30, 2010
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    Yes. This especially simple to do on this circuit because the rotary switch forbids concurrent (multiple simultaneous) loads.
     
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  8. peranders

    Well-Known Member

    May 21, 2007
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  9. #12

    Expert

    Nov 30, 2010
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    Shunt regulators are perfectly OK if they are appropriate to the load. The problem here is that it is difficult to get 12.0 volts, 9.0 volts, etc when you've installed all the wrong values of resistors. Then there is the typical 5% tolerance rating for zeners that can knock a 12V zener off by as much as .6 volts.

    The 317 chip is an excellent choice to do this. The accuracy can be made very nice and the rotary switch only needs one pole.
     
  10. kaname08

    Thread Starter Member

    Sep 22, 2010
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    how am I able to cascade it?? I'm still a student so i'm not so good about this,,,
     
  11. #12

    Expert

    Nov 30, 2010
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    Well, I got the 24 volt part wrong in that first example, but it was an example. It's supposed to show you how, and it was good enough for that.

    As for cascading...You think in terms of current. If I leave out the 4.5 volt requirement, (4) 3 volt zener diodes in series, fed by a resistor that will supply maybe, 100 milliamps from the rectified 24 volts, and it gets real simple. One resistor, 4 zeners, and a switch provides 3, 6, 9, and 12 volts.

    The 4.5V is more difficult because there is no such thing as a 1.5 volt zener. You could put (2) 1N4001 diodes in series with the 6 volt tap, and a resistor to draw some current through them (to produce a voltage drop) to get about 4.7 volts. I think Diodes Incorporated has a variable zener chip, but this is a "quick and dirty" design. If a couple of tenths of a volt was important, you wouldn't be using 5% zeners in the first place.

    Anyway, that's how to cascade a bunch of zeners.
     
  12. peranders

    Well-Known Member

    May 21, 2007
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    I think it's a better idea to use the LM317 because then you can have a adjustable voltage from 1.25V and up.
     
  13. kaname08

    Thread Starter Member

    Sep 22, 2010
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    I also don't understand about this,I mean we don't need to think about the output current of each voltage?? so does it mean that my output current can be 41.67mA?
     
  14. kaname08

    Thread Starter Member

    Sep 22, 2010
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    yup #12 its not 24V, the specific measurement of the transformer is 26V,,so 13square root of 2 minus 1.7,(is that right?),
    hope you can also give a schematic for the cascade,,thank you,,,
     
  15. #12

    Expert

    Nov 30, 2010
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    resistor, zener, zener, zener, zener, ground. Apply the rectfied and filtered supply voltage to the resistor.

    26 radical2 minus 1.7 = 35.06v
    (35V - 12V)/.1 amps = 230 ohms
    make that 220 ohms @ 5 watts
    or maybe
    23V/.167A = 137 ohms
    make that 150 ohms @ 7 watts
    (3V, 1/2 watt zeners can survive up to 167 ma.)

    and yes, you do need to think about the output current of each voltage.
    That's one of the limitations of a shunt regulator.
    The fact that I can do the math doesn't mean I can make a shunt regulator have the quality of an LM317 chip, and even with a regulator chip, the wasted power is getting big enough to need a heat sink at 7 watts. (You'd be better off with a 12V or 15 volt transformer because you'd have less power to get rid of as heat.)
     
  16. kaname08

    Thread Starter Member

    Sep 22, 2010
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    #12, I became confused, why are we using 26radical2 - 1.7?
    in 26radical2,are we getting the rms?? if we are getting rms, isn't it 26over radical 2?? sorry for too many questions,,,,
     
  17. kaname08

    Thread Starter Member

    Sep 22, 2010
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    now I get it so we are getting the Vpeak of the 26Vrms(is that right?),, now why are we using 1.7 and not .7 only??
     
  18. #12

    Expert

    Nov 30, 2010
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    1.7 volts assumes a full wave rectifier bridge as shown in the first post. Also, 1.7 volts is my guess as to how much voltage a pair of those diodes will use up.

    and yes, Vrms times radical2 = Vpeak.
     
  19. kaname08

    Thread Starter Member

    Sep 22, 2010
    37
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    what if I will still use the 24V transformer,is there anyway that i can lower the voltage for that to atleast 13-14 V? just like for example the clamping,,,
     
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